Structural Guidance in Platform TF 2010

8/3/2019 Structural Guidance in Platform TF 2010

1/54

STRUCTURAL GUIDANCE FOR PLATFORM TIMBER FRAMEUKTFA Special Project, May 2008


2/54

Top ic : Design of Platform Timb er Frame for Dispropo rtiona te Collap se provisionsPurpose: BS 5268-2:2002/ Am d 1 August 2007 includes new c lauses provid ing

guida nc e fo r c om p lying w ith Pa rt A3 of the Build ing Reg ulations 2000

(upd a ted 2004).

This Tec hnica l Note is a imed a t interpreting this guidanc e as it app lies to

Pla tform Timber Frame build ings and suggests me thods for ac hieving

compliance.

Rep ort Date: February 2008

Keywords: Platform Timber Frame, Disproportiona te Collap se

Building Regulations Requirement

Req uirem ent from Part A3 of the Build ing Reg ulations 2000, upda ted 2004 for Eng land &

Wales:

The b uild ing shall be construc ted so tha t in the e vent o f an a c c ident the build ing w ill not

suffer c ollapse to a n extent d isp rop ortiona te to the c ause .

Req uirem ent from Sec tion C3 of the Tec hnica l Sta ndards for co mp lianc e w ith the Build ing

Sta nd ards (Sc ot land ) 6th Amend me nt 2001:

A build ing to w hic h this standard a pp lies must be d esigne d and c onstruc ted so tha t in theevent o f dam ag e o c c urring to any p art of the building, the extent of any resulting c ollap se

will not b e d ispropo rtiona te to the c ause of the da ma ge .

Cod e Req uirem ent relevant c lauses

The fo llow ing extrac ts a re taken from British Sta nd ard

BS 5268-2:2002/Am d 1 Aug ust 2007

1.6.3 Acc idental damage

1.6.3.1 General

In ad dition to d esigning a structure to supp ort loa ds1 from

normal use, there should b e a rea sonable p robab ility tha t

the struc ture will not c ollap se c ata strophica lly because o f

misuse or acc ident. No struc ture c an be expected to be

resistant to the excessive loads or forces tha t c ould a rise

from an extreme c ause, but it should not c ollapse to a n

extent tha t is d isprop ortiona te to the original cause.

The g eneral recommend ations in 1.6.1.1 apply to al

Commentary

The fo llow ing note s provide som e c omm enta ry on the

interpretation of the new clauses:

1. The d isprop ortionate c ollapse loa d c ase (a s a result

of m isuse, acc ident or extrem e c ause) is neve r

de fined . It is a t heoretical event that c auses a fo rc e

or remo val of a load be aring eleme nt as de fined in

the clauses.


3/54

buildings. The mea sures required fo r ea ch Class of build ingas defined in the National Building Regulations o

Standa rds are asfollows:

a) Class 1 buildings: no a dd itiona lrequirements.

b) Cla ss 2A b uildings eithe r:1) Op tion 1. Effect ive a ncho rag e of

suspe nde d floors to load -bea ring wa lls in

ac co rdance w i th 1.6.3.2;o r

2) Op tion 2. The p rovision of effe c tivehor izonta l t ies in a c c ordanc e w ith1.6.3.3.

c ) Cla ss 2B b uilding s eithe r:

1) Op tion 1. Effec tive ho rizont a l ties inac co rdance w i th 1.6.3.3and vert ic a l t ies

in ac co rdance w ith 1.6.3.4;o r

2) Op tion 2. Check for the not ional rem ova lo f load -bear ing e lements in ac co rda nce

with1.6.3.5.

d ) C lass 3 b uilding s: The d esig ne r sho uld c a rry

ou t a risk a ssessme nt a s requ ired b y the

Nat iona l Build ing Reg ulations or Sta nd a rd s.

1.6.3.2 Effective anc horag e of suspe nde d floors

A suspend ed f loor ca n be co nside red to b e

e f fec t ive ly anc hored if the c onnect ion be tween

the f loor and load -bea ring wa l l c omp l ies with

either:

Figure M.32; o r BS 5628-1:2005, Ann ex D for t imb er flo ors

supp orted b y loa d-b ea ring ma sonry.

1.6.3.3 Effec tive horizonta l ties3

All b uild ings should b e effe c tively t ied t og ethe r at

ea ch pr inc ipa l f loor level and at roof level.

Horizont a l t ies shou ld b e p rovide d a s follow s (see

also Figure M.I):

Periphe ral t ies with a de sign c ap ac ity of 0.5Ft should be p rovided around the w hole

pe rimet er of the b uilding . Ties should be

anc hored at external and re-entrant

corners.

2. Effect ive a nc horage is provided by t he ho rizonta l

structural junction strength of floo r to w a ll c onne c tions

as a de eme d to satisfy.

3. Effec tive horizonta l and vertica l ties are the

structural junc tion strengths of floor to wa ll

connections as calculated.

The c a lculation formula for ties is ba sed on similar

force c alculations taken from the light ga uge steel

industry and for the d esign o f restraint straps in

ma sonry design cod es. The tie force is an a c c idental


4/54

Internal t ies should b e p rovid ed in twodirect ions ap proxima tely at r ight a ngles.

They should b e effe c tively c ontinuou s

througho ut their length a nd should b e

anc hored a t the p er iphe ry of the bui ld ing.

They m ay b e d istribut ed even ly througho ut

the f loor o r may b e c once ntra ted a t c o lumn

positions. Internal ties should be designed

for a load of Ft.

External c o lumns and load -bea ring wa l ls should b e t ied in by a t ie perpend icular to

the e dg e of the b uild ing. The tie should b e

de signed for the greater of Ft or 1% of the

ma ximum d esign ver t ic a l dea d a nd

imp osed loa d in the c o lumn a t tha t leve l.

Corner co lumns should b e t ied in tw o

directions approximately at r ight angles.

The b a sic t ie fo rc e Ft shou ld b e c a lcu la ted a s

follows:

For d istribute d ties: Ft = 0.5(gk + qk)L kN/ m

but not less tha n 3.5 kN/ m.

For c onc entrate d t ies: Ft = 0.5(gk + qk)StL kN

b ut n ot less tha n 10kN.

where

gk is the fu l l de ad loa d pe r unit a rea of the f loor

or roo f (kN/ m2).

qk is the f ull impo sed floor or roo f loa d pe r unit

area (kN/ m2).

st is the me a n sp a c ing o f t ies transverse t o t he

d irec t ion of the t ie being c onsidered (m).

L is the leng th of t ie being co nsidered (m).

When assessing the c ap ac ity of an e lement a c ting as atie, in ac co rda nce w ith1.6.3.8or its connections in

ac cordance with1.6.3.9, the tie load ca n be

c onside red as an a lternative load ca se to any other

loads ac ting on that eleme nt.

1.6.3.4 Vertical ties

Eac h co lumn or wa ll carrying ve rtica l loa d should b e

tied continuously from the lowest to the h ighest level.

The t ie should be c ap ab le of resisting a tensile fo rce

load value.

Horizonta l and vertic al tie force s are no t p rac tica l for

most p latform fram e c onstruct ion.


5/54

eq ual to the ma ximum design load rec eived by thec olumn o r wall from any one storey. There should be an

effec tive co nnec tion betwe en vertica l ties and

horizonta l ties at ea c h level.

When assessing the capacity of an element acting as a

vertical tie, in ac co rda nce with1.6.3.8or its c onnec tion

in accordance with 1.6.3.9, the tie load can be

considered as an alternative load case to any othe

loads ac ting on that eleme nt.

1.6.3.5 Notiona l remova l4 of load -bea ring element

The struct ure should b e c hec ked for the e ffect of the

remo val, within e ac h storey, of e ac h supp ortingc olumn, or bea m supp orting c olumn(s) or loa d-be aring

wa ll(s), or any nominal leng th of load -bea ring w all, one

at a time5, to ensure tha t d isprop ortiona te c ollapse

do es not oc cur. The p ortion o f the building a t risk of

collap se should not exceed the lesser of 15% of t he floo r

area of tha t storey or 70 m2. 6

If the area at risk exceeds the limits given then the

c olumn, beam or loa d-be aring w all should be de signed

as a key eleme nt in ac c ordanc e with1.6.3.6.

The nom inal length o f a load -bea ring wa ll should b e

taken as:

In the case of an external wa ll, the length

measured be twe en vertica l lateral restraints7.

In the c ase o f an internal wa ll, the lengthmeasured between effective vertical lateral

restraints but no t exceed ing 2.25h, where h is the

height betwe en ho rizonta l restraints as shown in

Figure M.2.

When c onsidering the residua l structure the loa d ing

should b e a s de fined in1.6.3.7. The c ap ac ity of any

relevant elements should be c alculated in ac c ordanc e

with1.6.3.8and their c onnec tions should be c alc ulate d

in acc orda nce with1.6.3.9.1.6.3.6 Key element8

A key element should be d esigned for the ac c identa l

load ing spec ified in BS 6399-1. Struc tura l element s tha t

provide late ra l restraint vital to the sta b ility of a key

element should a lso b e d esigned as a key eleme nt. The

ac cidental load ing should b e ap plied to the memb er

from all horizontal a nd vertica l direc tions, in one

direc tion at a time, toge ther with the reac tions from

4. Key po int: Notional remo val relate s to t he ima ginary

remova l of a d efined a rea and is not an a ctua l length

of pa nel or length of pa nel to a predetermined wea k

junction.

5. Key po int: One w all at a time is notionally remo ved ,

not a numbe r of w alls tog ether.

6. Som e c ollapse is allowa b le within the limits set by the

Building Reg ulat ions. ie 15% of the floor area o f tha t

sto rey o r 70 m2whic hever is the lesser and doe s not

extend further than the immed iate ad jac ent storeys.

Sto rey a rea is the full building p lan a rea.

7. Vertical lateral restraints are structural walls of

minimum width 1200mm a nd a re no t key elements.

8. The p rinc iple of design fo r key elements or

protec ted mem be rs is different from notional remova l.

Leng ths of p ane l either side o f a key e lement a re likely


6/54

other building co mponents attac hed to the memb erthat a re subject to the sam e ac c ide ntal loa ding but

limited to the m aximum rea c tions that co uld rea sonab ly

be transmitted, c onside ring the c ap ac ity of such

mem be rs and their co nnec tions. The a cc ide ntal load s

should b e c onsidered as ac ting with the loa ds given in

1.6.3.7. The c ap ac ity of the element should be

ca lculated in acc orda nce with1.6.3.8and its

co nnections should be ca lculated in ac co rda nce with

1.6.3.9.

1.6.3.7 Design loads for the residua l structure

When considering design of the residual structure the

following loads should be considered, where

appropriate:

the dead load

a third of the impo sed load, exce pt that in the

case of buildings used predominantly

for storag e, or whe re the imposed load is of a

permanent nature, the full imposed load

should be used.

A third of the imp osed roof or snow loa d

100% of any ceiling storag e loa ds a third o f the wind load9

1.6.3.8 Permissible stresses for acc ide ntal load cases

When considering the p roba b le effec ts of m isuse,

ac c ident or pa rticular hazards, or when c omp uting the

residua l sta bility of the da ma ge d structure, the d esigne

should norma lly multiply the va lues rec om mended in BS

5268-2 for a ll long -term permissible stresses by a fac to r of

2.25. 10

1.6.3.9 Permissible fastener load for accidental load

cases

When c onsidering t he p rob ab le effec ts of m isuse,ac c ide nt or pa rtic ular hazards, or when c omp uting t he

residua l sta bility of the da ma ge d structure, the d esigne

should no rma lly multiply the va lues reco mm end ed in BS

5268-2 for all long-term p ermissible loa ds on fa steners

by a fac tor of 3.011. In the c ase o f fa sten ings through

pa rticleboa rd the va lues rec omm end ed for long-term

pe rmissible load s should b e increa sed by a fa c tor of

4.0.

to b e removed which ma y be significa nt lea ving ap ost and be am struc tural fram e to supp ort the

rema ining building.

9. The w ind load ing requirement is to c hec k aga inst

rac king c ap ac ity of the structure a fter remo val of a

load bearing wall.

10. The p ermissible stress inc rease fa c to r ca n b e

assumed to b e a k3 = 2.25 duration of load fac tor.

11. The p ermissible fa stene r loa d increa se fa c tor c an

be a ssumed to b e a k48,52 = 3.00 duration of loa d

fac tor. The va lue is higher for mec hanic al fastene rs

due to the fac tors alread y incorporated in the cod e.


7/54

Referenc es to other Guidance:

1) UKTFA Code of Prac tice for Eng inee red Wood Prod uc ts 1st Ed. Jan 07

2) UKTFA Tec hnica l Bulletin 3 Design Guide for Disproportionate Co llapse - Ma rch 20053) Multi-sto rey timber frame b uild ings a design g uide TRADA/ BRE 2003

4) BS5268-2:2002 (Amd1) August 2007

5) NHBC Tec hnica l Guidanc e Note Part A3 Guida nc e Novemb er 2004

Introduction:

Building Reg ulations Req uirements:

All structures must comply with the minimum UK Building Regulation requirements forrob ustne ss. The build ing reg ulat ions add ress this aspec t o f the design using the te rm

disproportionate collapse. All buildings will be subject to the robustness check as required

by Part A Struc ture, o f the Build ing Reg ulations ame nded Dec em ber 2004.

Building Reg ula tions Tab le 11 ha s c lassified buildings into 4 c lasses based on risk assessments

de pe ndent on the type o f building a nd levels of oc c upa ncy, as follow s.

Class 1, Single oc c up anc y buildings from 1 to 4 storeys e.g . de ta c hed houses, townhouses etc.

Cla ss 2A, Houses or apartments (residential nature), not exceeding 4 storeys. Class 2B, Houses or apartments and other residential buildings, exceeding 4 storeys

but limited to 15 storeys. Educational buildings not exceeding 15 storeys. Hospitals not

exceeding 3 storeys.

Class 3, Sta d iums, sports g rounds, or build ing sub jec ted to high frequenc y of loa d ing(crowd accumulation) etc, subjected to full sensitivity analysis and has no specific

mentioned design c riteria .

Class of Building for Dispropo rtiona te Co llap se:

Req uirem ent A3 as a princip le a pp lies to a ll build ings. Previous reg ulations we re a imed a t

spec ific he ights of build ings. For exam p le, for 5 sto rey a pa rtments d isprop ortiona te c ollapse

provisions app lied but for 4 storey a pa rtments it did not .

The 2004 Reg ulat ions b rings the UK in line w ith Europ ea n Co de p roposa ls in termino logy and

while there a re som e d ifferenc es, the g ene ra l 5-storey o r grea ter rule for disp rop ortiona te

c ollap se-spec ific design still ap p lies. For buildings of 4 sto reys or less the c od e and

reg ulat ions aim a t goo d prac tic e to ac hieve rob ustness.

Som e terminolog y refers to progressive collapse which is a reference to w hat the d esign for

d isprop ortiona te c ollapse is a imed a t avoid ing. This doc ument w ill refe r to the te rm


8/54

d isprop ortiona te c ollap se.

Referenc e should b e ma de to NHBC Tec hnic al Guida nc e Note Part A3 Guida nc e

Novembe r 2004(5) for c om p lex building s w ith multiple uses to assess the releva nt C lass ofbuilding.

Elements of the building with differing uses or numbers of storeys may be classed

independently for disproportionate collapse as long as they can be shown to be

inde pe ndently stab le for wind loa d e ffec ts.

Mee ting A3 req uirem ents:

Disproportionate collapse is instigated by localized failure of one of the elements within the

structure leading to significant failure of several floors in the building. Disproportionate

collapse can be reduced or minimized by providing some structural continuity (ties) within

the elements of struc ture or by e nsuring a de gree of struc tura l redund ancy by c onsidering

notiona l rem ova l of load -bea ring e lem ents.

BS5268-2:2002 (Amd1) August 2007 guidanc e:

The mea sures req uired for ea c h Class of build ing a re a s fo llow s:

Cla ss 1: no additional requirement.

Cla ss 2A: either:

Option 1 p rovision of e ffec tive a nc horage of suspend ed floo rs to loa d -bea ring wa llsas shown in Figure M3; or

Option 2 p rovision o f effec tive horizonta l ties as shown in Figure M 1.Class 2B: either:

Option 1 p rovision o f effe c tive ho rizonta l and vertica l ties as shown in Figure M1.; or Option 2 c hec k for the notiona l rem ova l of loa d -bea ring e lem ents

Class 3: the designer should carry out a risk assessment as required by the Building

regulations.

Cla ss 2A Buildings:

The Build ing Reg ulat ions sta te tha t fo r Cla ss 2A b uild ings, robustne ss will be ac hieve d by

provid ing effe c tive horizonta l ties, or effec tive anc horag e o f suspend ed floo rs to w a lls.

In p roviding rob ustness for ca teg ory 2A, minimum m ec ha nica l fixing spec ifica tions to p rovide

anchorage of suspended floors to walls and notional horizontal tying for platform timber

frame structures are provided in UKTFA Tec hnic a l Bulletin 3 Design Guide fo r

Disproportionate Collapse - March 2005(2) (see Appendix 1), UKTFA CP for Engineered

Wood Prod uc ts 1st Ed. Jan 07(Fig 3.13) (1)and Figure M3 of BS5268-2:2002. (4)


9/54

ForClass 2A buildings, the a pp roa ch is to a dop t go od building p rac tic e o f providing late ra l

restraint to wa lls and c om mo n a nc horag e deta ils of floo rs to wa lls. The design p roc ess will

involve chec king the c apac ity of the c om po nent interfac es (e.g. pane l rail to solep la te,

solep la te to floor de c k, floo r joists to hea d binder and hea d binde r to p anel ra il) ag a inst the

variab le ho rizonta l wind fo rc es. The timber frame designer should the refo re b e p roviding a

rob ust c onnec tion a t ea c h and every junc tion as pa rt of the no rma l design proc ess.

Fig 1: BS 5268 Figure M3 - exploded floor detail showing minimum nailing densities.

Key p oints to no te a re:

The fixings a t ea c h junction interfac e. The b loc kings a t floo r pe rimeters where joists a re pa ra llel to the wa ll.

Where these d eta ils are not a pp lic ab le o r c annot b e a do pte d due to d ifferent fram ing

arrange me nts, effec tive ho rizonta l ties should b e d esigned in a c corda nc e w ithBS5268-

2:2002 Cl1.6.3.3. (4)


10/54

Class 2B Buildings:

The Build ing Reg ulat ions guidanc e sta te tha t fo r Class 2B build ings, robustness will be

achieved by p roviding e ffec tive horizonta l ties tog ethe r with e ffec tive ve rtic a l ties or by

che c king tha t upo n notional remo val of a loa d be aring w all (one a t a time in ea c h storey of

the building) the building rem a ins stab le and tha t the a rea of floor at any storey at risk of

collapse d oe s not exce ed 15% of the floo r a rea of tha t sto rey o r 70sq .m, whicheve r is the

sma ller, and does not e xc eed further than the immed iate a djac ent storeys.

Where the notiona l rem ova l of lengths of w alls wo uld result in an extent of d ama ge in exc ess

of the a bo ve limit, then the use o f a key eleme nt d esign a pp roa c h for an a cc ide ntal

design loa d ing of 34 kN/sq .m a pp lied in the ho rizonta l and vertica l direc tions (one a t a time)

to the key element a nd a ny atta c hed c omp onents (e.g. cladd ing) having reg ard to the

ultimate streng th of those c om pone nts and the ir c onnec tions, should be a dop ted .

Consideration of notiona l panel rem ova l and a Sensitivity Analysis ap proa ch:

In c hec king the rob ustness of timb er frame build ings, Eng ineers a re to app ly judg me nt-

based thinking to the likely 3-dimensiona l struc tural behaviour of a build ing, ba c ked , whe re

approp riate, w ith a 2-d imensiona l struc tura l assessment o f d isc rete elements. The TF2000 full-

size testing ha s show n tha t this app roa c h is c onservative b ut app rop riate to d ete rmining the

rob ustne ss of p lat form frame c onstruc tion in b uild ings suc h a s the me d ium-rise TF2000building (3).

Multi-sto rey timb er fram e b uild ings a design g uide TRADA/ BRE 2003(3)provides guidance

for the design p roc ess for Class 2B build ings where no tiona l rem ova l of loa d bea ring w a lls is

pa rt of the d esign che c k to c om p ly with Reg ulation A3:

A Sensitivity Ana lysis should b e c a rried out on p rima ry sup porting m embers to esta b lish if

their removal, one at a time in each storey, to check that upon its removal the rest of the

structure would bridge over the resulting lack of support, albeit in a substantially deformed

condition, or that the risk of collapse of the remaining structure due to the removal of the

me mb er is within the limits p resc ribed by the Build ing reg ulations.

If it is not possible to bridge over a missing member or to limit the area at risk, the member

should b e d esigne d a s a p rotec ted or key element.

Method s of Deta iling for DC Provision of Bridg ing Elem ents:

The TF2000 test build ing p rovide d p roo f of the inherent robustne ss and ava ilab ility of

sec ond ary loa d pa ths in p la tform timbe r frame . Therefore, shea thed wa lls with no op enings


11/54

designed to BS5268:Part 6.1 ca n be rega rded as dee p bea ms with vertic a l shea r ta ken inthe panel to panel connections and tension taken out through the sheathing material in

continuation with any timber framework across the panel junction e.g. rim boards.

Furthermore, the TF2000 tests dem onstrated tha t the floo r ha s add itiona l streng th through the

transverse spanning capacity of the floor that is supported on the walls parallel to the span

(3).

It is possible to undertake structural calculations to prove that wall panels can be supported

by pa nel ac tion but o ften large o r numerous op enings oc cur, lea ding to a req uireme nt for

ad d itiona l b ridging m em bers to b e p rovided as pa rt of the rob ustness design.

Unfortuna te ly the TF2000 tests we re spec ific to tha t b uild ing floo r and panel sha pe and size.The findings c annot b e used as a g eneral com p liance w ith the regulations and inde pe ndent

structural c hec ks on build ings a re req uired .

The Rim Bea m Me thod :

The provision o f a c ontinuous eng ineered timb er rim be am a t every floo r leve l (not

ge nerally req uired a t roof leve l) a t the end of a ll joist spans ensures tha t structural c ontinuity

is achieved by providing vertical load transfer as a bridging elements and horizontal

continuity by providing a nailing density at all interfaces in accordance with the

recommendations of UKTFA Tec hnica l Bulletin 3 Design Guide for Disproportionate

Collapse - March 2005(2) and BS5268-2:2002 (Amd1) August 200 Figure M3. (4)

Fig 2: Indica tive Rim Beam a rrang eme nt

This me thod a llow s joisted floo r struc tures to be assem b led in the fac tory a s cassettes with a


12/54

rim bo ard used to c onnec t the e nds of the joists tog ethe r for transportation and whichrem ains as a vertic a l loa d t ransfer elem ent in the c om pleted struc ture. A sep ara te rim

bea m , which is usua lly insta lled loo se o n site, spans betw ee n p oints of ve rtica l la tera l

restraint (return wa lls) or key elements and ac ts as a bridg ing me mb er suc h tha t if load -

bea ring wa lls a re no tiona lly rem ove d betw ee n the w a lls or key elem ents, the resulting

collapse will be limited to the m aximum a rea s a llow ed and any rem aining struc ture w ill

rema in in plac e a lbe it with signific ant d eforma tion b eing a c c ep tab le.

An examp le of the Rim Bea m Struc tural Metho dolog y is show n in App endix 2

Intersec ting return wa lls:

Intersec ting return wa lls must be o f 1200mm minimum leng th in tota l (exc lud ing frame dop enings). These wa lls c an be non-loa d b ea ring in the c onve ntiona l sense b ut must be

c apab le o f transferring loads dow n through the struc ture. The use o f lightw eight pa rtitions

built off o f floa ting floors is no t ac c ep ta b le. The Rim Bea ms are supported a t these w a ll

intersections by corner stud groups. It is important that the Rim Beams supporting the

rem aining struc ture ha ve a full bea ring on studs a t the panel junc tions and to ac hieve this,

the w a ll panels shou ld be lapped in the op posite ma nner to the Rim Bea ms. If no stud

c lusters a re p resent below the Rim Bea m b ea ring, hangers or fixings a re to be p rovided off o f

adjacent Rim Beams.

For external panels the minimum length of wall to be considered for notional removal is

2.4m, with no maximum length. Where the Rim Beams cannot be designed to span therequired distance between return walls, Key Element posts will be required to split the span

of the Rim Bea ms.

For interna l wa lls the ma ximum leng th o f wa ll to b e c onsidered is 2.25H where H is the c lea r

height of the p anel betw ee n la teral suppo rts.

Rim Beam Design:

The Rim Bea ms and the ir c onne c tions shou ld be d esigned to w ithsta nd vertic a l loa d ing

com prising the full floo r dea d loa d plus one third of the normal impo sed loa d p lus the w eight

of a single storey of w a ll p lus any c lad d ings or linings. A loa d dura tion fac tor, K3, of 2.25 and

a deflection limit of L/30 should be applied to timber members and a k48,52 = 3.00 formechanical fixings.


13/54

Fig 3: Exam ple Design o f a typ ica l Rim Beam suppo rting a floor and e xternal wa ll only


14/54

Assumptions for Rim Bea m de sign:

In d esigning Rim Bea ms, the assump tions listed below are app lica b le:

a ) Rim be am d esign chec ks are c a rried o ut on the princ ipa l of no tiona l rem ova l of w allpanels, one at a time, between intersecting return walls ie: a wall is notionally

rem oved , the struc ture is c hec ked for ad eq uacy and then the wa ll is rep laced . These

chec ks a re then rep ea ted for othe r notiona lly rem oved pa nels.

b ) For external panels the minimum length of wall to be considered is 2.4m, with nomaximum length. For internal walls the maximum length of wall to be considered is

2.25H whe re H is the c lea r height o f the pa nel be twe en late ra l supp orts (the top of the

structural deck level below to the underside of the structural joist level above). Forcom partment w a lls, only one lea f at a time is to b e c onsidered fo r remova l.

c ) Rim be ams are d esigne d to supp ort the de ad we ight of the struc ture to be supp orted ,1/ 3rd of the imposed loads and a single storey of wall panel with any supported

claddings or linings, following a collapse event of the supporting wall panels below

being removed (one at a time). For this event, a duration of load factor of k3 = 2.25

and deflec tion limit of L/ 30 are a pp lic ab le fo r timb er elem ents.

d ) Rim beams are also to provide a horizontal tying action at all levels through thestructure. The fixings p resented for Ca tegory 2A build ings (BS5268-2:2002 (Amd1)

August 200 Figure M3(4)) are the minimum fixings required for robustness.

The Eng ineer is to dete rmine the d esign a pproa c h for the Rim bea m. The d isprop ortiona tec ollapse design does not req uire the build ing to be servicea ble afte r the eve nt, merely safe

for oc c upa nts to esc ap e a nd e me rge ncy servic es to enter the b uild ing.

A typical Rim b ea m d esign can be seen in Fig 3.

Key Element d esign princip les:

The key e lem ent a pp roa c h is not related to no tional rem ova l. The introd uc tion o f a key

element is an alternat ive de sign app roa ch. The d ifferenc e is ea sily exp la ined w hen a key

elem ent is a c olumn in a length o f wa ll. This key eleme nt c olumn is not c onside red as a

la teral restraint to the wa ll and a c hec k of the no tiona l rem ova l of the wa ll either side o f thec olumn is no t va lid . The d esign o f the c olumn is for 34kN/sq.m in any horizonta l direc tion. The

panels attached to the post on both sides are to be checked to see if they will remain

attached to the column under this loading. If they remain attached, then the column is

designe d to ta ke the reac tion from the wa ll pa nels as we ll as the loa d on the c olumn itself.

Other co nsiderations to a chieve a robust struc ture

The in-service d esign of the b uild ing must not b e c om prom ised by the d isp rop ortiona te


15/54

collapse design.

Key p oints to no te a re:

A building should not be provided with intentional weak points for notional panelrem ova l. The te rm not iona l is deliberate ly used as a me ans of und ertaking a n

ima ginary design situation. The a c tual c ause a nd prac tic a lity of the event is not

defined or to be considered. Disproportionate collapse design is a methodology to

enha nc e a build ing rob ustness.

As always, a check on the differential movement of the in-service condition of thebuilding should b e c a rried o ut.

The support of external clad d ings should a lso be c onsidered during a n event .


16/54

App endix 1: Minimum rec omme nded nailing d ensities to provide nominal horizontal tying for

Class 2A& 2B buildings:

For co nvent iona l timb er frame build ings of c ellular p lan form the UKTFA ha ve rec om me nd ed

that the effec tive a nchorage o f floo rs to w a lls will be ac hieved with a m inimum d ensity of

nails as shown below.

Fig 4: Diagramm atic details of typica l nailing d ensity at all interface s in ac co rda nce with the rec omm enda tions

of BS5268: 2002 and UKTFA g uida nc e d ate d Decem ber 2004

For mo re informa tion reg a rd ing rob ust junction c onne c tions for Pla tfo rm Timber Frame

build ings, refe r to UKTFA Technic al Note: Rob ustness of pla tform timber fram e and

connec tivity of the fram ing mem bers.


17/54

Appendix 2: Disproportionate Collap se Philosophy Exam ple of Rim Beam Structural Method ology

Key to d iagram : E = exte rnal w a lls, I = interna l load bea ring wa lls, P = pa rty wa ll (single skin), J = joists, H =

c lear height o f the p ane l betw een late ral supp orts.

Design Chec k/ D.C. Event 1: Notional removal of Internal wall p ane l I0 of ma ximum length 2.25H

Continuous joist spans J1-J5 avoid the need for rim beams on internal supports. On removal of the supporting

wall I0 the joists act in double span at each subsequent level and support the floor loads plus a single storey

height of (no w non-load be a ring) wa ll pa nel I1-I5 suppo rted off the doub le-spanning joists. Ie. J1 supp orts I1, J2

supports I2 etc.

Design Che ck/ D.C. Event 2: Notional rem ova l of External wa ll pa nel E3 between intersec ting return walls or

de fined key e lem ents. (Party w alls P0 to P5 simila r)

Following removal o f wa ll pa nel E3, unless the joists a re top-hung ove r the rim bea m, joists J4 a re assume d to

collapse or cantilever and a check should be carried out to ensure that the resulting floor collapse will

constitute less than 15% of the floor area of that storey or 70sq.m, whichever is the smaller. Rim beam R4 is

designed to support panel E4 and floor joists J5 by 'bridging' over the notionally removed wall panel.

Subseq uent rim b ea ms R5 supp ort wa ll pane ls E5 and roof joists J6. The rim be am s are t ied ba c k to t he floo r

dia phrag m w ith the minimum na iling de nsities.


18/54

Appendix 3: Disproportionate Co llapse Philosophy - Cantileve red Joists Method

Notional remova l of External wa ll pa nel E3 (Party wa lls similar) between intersec ting return walls or de fined ke y

elements.

Following remova l of wa ll pa nel E2 co ntinuous joists J3 c ant ileve r and supp ort panel E3 (inc luding a ny

supp orted c ladding ). Subseq uent joists also c an tileve r and suppo rt a storey height o f wa ll pa nel. A c hec kshould be carried out to ensure that there is sufficient holding down resistance at the backspan of the

c an tileve red joists to resist up lift (espec ially a t t op storey level).

Continuous joists a re d esigned to supp ort a single storey o f wa ll pane l plus the full dea d load p lus 33% of the

imposed load s on tha t floor. A duration of load fac tor of k3 = 2.25 and de flec tion limit of L/ 30 are a pp licab le for

ac cidental load ca se.


19/54

Topic: Floor Serviceability Limits

Purpose: Deflection of floors is classified as a serviceability issue.

There have been investigations carried out by the

industry into the acceptable limits of floor deflection.

In 2006 the NHBC changed their recommendations for

allowable deflection limits for I-joist engineered floor

systems and BS 5268-2:2002 + Amd 1 updated 2007

has now also presented additional deflection criteria.

This report provides background to the changes and

additional recommendations for floor serviceability

that Engineers may wish to follow.Report Date: February 2008

Keywords: Platform Timber Frame, Floor joists, Deflection,

Serviceability

Code Requirement relevant clauses

The following extracts are taken from British Standard BS 5268-2:2002/Amd 1 August 2007

2.10.7 Deflection and stiffness

The dimensions of flexural members should be such as to restrict deflection within limits

appropriate to the type of structure, having regard to the possibility of damage to surfacingmaterials, ceilings, partitions and finishings, and to the functional needs as well as aesthetic

requirments.

In addition to the deflection due to bending, the shear deflection may be significant and

should be taken into account.

For most general purposes, this recommendation may be assumed to be satisfied if the

deflection of the member when fully loaded does not exceed 0.003 of the span. For domestic

floor joists, the deflection under full load should not exceed the lesser of 0.003 times the span

or 14mm, where:

, = 0.86 for floors whose transverse stiffness is provided by the decking/ceiling.

= 1.00 for floors where there is additional transverse stiffness to that from the decking/ceiling.

This additional transverse stiffness may be provided by herringbone strutting or by blocking1

of depth at least 75% of the depth of the joists or, in the case of transverse members which

are continuous across the joists (i.e. joists with an open-webbed structure), by timbers of

depth at least 30% of the depth of the joists.

NOTE The 14mm deflection is to avoid undue vibration under moving or impact loading.2

Commentary

The following notes

provide some

commentary on

the interpretation

of the new

clauses:

1. The changes

reflect the trend

for blocking or

strutting to be

omitted on

proprietary joist

products.

Although the British

Standard does not

address I-joist or

open web joists

this clause is in fact

aimed at thismarket.

2. This note refers

to the fact that

maximum


20/54

Subject to consideration being given to the effect of excessive deformation, members may

be precambered to account for the deflection under full dead or permanent load, and in

this case the deflection under live or intermittent load should not exceed 0.003 of the span.

deflection limits

are not to limit

static deflection

but are to controlvibration

performance.

The following extracts are taken from Eurocode 5 (BS EN 1995 -1 -1:2004) + UK National

Annex:

NA to BSEN 1995-1-1:2004:

NA.2.5 Limiting values for deflections of beams [BS EN 1995-1-1:2004, 7.2(2)]

As stated in BS EN 1990:2002, Al.4.2(2), the serviceability criteria should be specifiedfor each project and agreed with the client.3 The values in Table NA.4, which take

into account creep deformations, are given for guidance.

Table NA.4 Limiting values for deflections of individual beams4

Type of memberLimiting value for net final deflections

of individual beams, wnet,fin

A member of span, l

between two

supports

A member

with a

cantilever, l

Roof or floor members with a plastered or

plasterboard ceiling

l/250 l/125

Roof or floor members without a plastered or

plasterboard ceilingl/150 l/75

NOTE When calculating' wnet,fin w,fin should be calculated as ufin in accordance with. BS EN

1995-1-1:2004,2.2.3(5).

NA.2.6 Vibrations in residential floors [BS EN 1995-1-1:2004, 7.3.3(2)] 5

NOTE For the value of the modal damping ratio, ,,in BS EN 1995-1-1:2004,

7.8.1(3), a value of 0,02 has been found appropriate for typical UK floors.

NA.2.6.1 BS EN 1995-1-1:2004, 7.3.3(2) is implemented nationally by using TableNA.5.

Table NA.5 Limits for aand b in BS EN 1995-1-1:2004 expressions (7.3) and (7.4)

3. This allows for

the Client todecide on the

acceptable level

of deflection

appropriate to the

building use and

quality.

4. These limits are

applicable to total

deflection

including creep

deflection,

something whichthe British Standard

already includes

for. The two codes

should not be

mixed.

5. Vibration of

residential floors is

a complex area

and one that

requires a clear

understanding of

the constructionmass and product

qualities.


21/54

Parameter Limit

1,8 mm for l 4 000 mm

16 5OO/ l1.1 mm forl > 4 000 mm

a, deflection of floor under a 1 kN point load

where l =joist span in mm

fora 1 mmb = 180 - 60a

b, constant for the control of unit impulse

velocity responsefora > 1 mm b =160 - 40a

NOTE The formulae for b correspond, to BS EN 1995-1-1:2004, Figure 7.2. With a value of

0,02 far the modal damping ratio, , the unit impulse velocity response will not normally

govern the size of floor joists in residential timber floors.

NA.2.6.2 The recommended limit on a may be compared with a corresponding floor

deflection calculated as:

(NA.1) 1000 kdist leq3 kamp a mm48 (EI)joist

where

kdist = proportion of point load acting on a single joist

leq = equivalent floor span in mm

kamp = amplification factor to account for shear deflections in the case of solid timber and

glued thin-webbed joists or joint slip hi the case of mechanically-jointed floor trusses

(EI)joist= bending stiffness of a joist in Nmm2 (calculated using Emean)

where

kdist = max kstrut [0,38-0J08ln[14EIb / s4] ]

0,30

kstrut = 0,97 for single or multiple lines of strutting, installed in accordance with reference

NA.4.1, otherwise 1,0

(EI)b = floor flexural rigidity perpendicular to the joists in Nmm2/m

s = joist spacing in mm

leq = span, t, in mm, for simply supported single span joists = 0,9 1 for the end spans of

continuous joists = 0,85 ffor the internal spans of continuous joists

kamp = 1,05 for simply-supported solid timber joists= 1,10 for continuous solid timber joists

= 1,15 for simply-supported glued thin-webbed joists

= 1,30 for continuous glued thin-webbed joists

= 1,30 for simply-supported mechanically-jointed floor trusses

= 1,45 for continuous mechanically-jointed floor trusses.

(EI)b is calculated as the flexural rigidity of the floor decking perpendicular to the joists, using

EmeanforE. Discontinuities at the edges of floor panels or the ends of floor boards may be

ignored.

(EI)b may be increased by adding the flexural rigidity of plasterboard ceilings fastened


22/54

directly to the soffit of the floor joists, assuming Eplasterboard = 2000N/mm2.

(EI)b may be increased for open web joists with a continuous transverse bracing member

fastened to all the joists within 0,1l of mid-span, by adding the bending stiffness of the

transverse member in Nmm2 divided by the span l in metres.The fundamental frequency f1 should not be less than 8 Hz unless a special investigation is

made. In BS EN 1995-1-1 expression 7.5 the mass of the floor should be the permanent actions

only without including partition loads or any variable actions.

In calculating the equivalent plate bending stiffness (El) of floors, in which the decking is

adhesively bonded to the joists, no allowance should be made for composite action unless

the floor is designed in accordance with 9.1.2 and with adhesives meeting the requirements

of 3.6 and the detailing and control provisions of 10.3.

Introduction

The change in BS 5268-2: 2002 clause 2.10.7 Deflection & stiffness reflect changes by earlier

NHBC guidelines. The NHBC changes occurred before in depth research. The BS 5268-2

changes followed summary recommendations by Trada and the Code Committee.

Previous research into floor serviceability:

It is known that Trada Technology Ltd. were commissioned by the NHBC to develop a simple

design approach for lightweight floor systems using engineered timber joists that would ensure

an in-service performance for properly constructed floors comparable to that of traditional

floors made with solid timber joists. UKTFA have discussed the history of research carried out

into deflections of floor joists with Trada and this guidance provides a summary of thisresearch.

Research was based mainly on Eurocode 5 and on five overseas sources of information,

which related design methods for I-joisted floors to user satisfaction. It was possible to

compare the results of the design methods studied with the results given by BS 5268 and EC5,

and to adjust the latter two where it appeared that they deviated from the consensus view of

other researchers, while still maintaining overall performance levels similar to those which have

proved acceptable in the UK for floors made with solid timber joists and strutting up to 4

meters in span.

As a result two sets of design recommendations have been made, one for designs based onBS 5268, and the other for designs based on EC5.

Design recommendations and advice:

For design to BS 5268:

For designs to BS 5268 it was concluded that:

(i) When strutting is omitted from floors in which it would normally be fitted in


23/54

accordance with current best practice, the 14 mm deflection limit under dead +

imposed load should be reduced to 12.6 mm.

(ii) In addition, the stiffness required for all forms of timber joist should be increasedfor spans from 4 m to 8 m by a factor increasing from 1.0 to 1.4

across this range.

(iii) No other special treatment for I-joists is required.Hence, while BS 5268 continues to be used, the following recommendations are made

for the design of floors made with solid timber or prefabricated glued I-joists:

The combined instantaneous bending and shear deflection of a single joist measured

in mm should not exceed the lesser of 0.003 and ulimwhere:

= joist span in mm

ulim = 18 L mm for joists with strutting in accordance with current best practiceulim = 0.9(18-L) mm for joists without strutting

where L = joist span in m.

For design to Eurocode 5:

Under Eurocode design protocols, deflection limits are advisory and should be agreed

by the designer and client at the beginning of the design process. The guidance given

in the UK NA to EC5 is therefore advisory. In the light of this project(3) TRADA recommends

that timber floors in the UK should be designed to Eurocode 5 and its National Annex, but with

the following changes.

(i) The point load deflection limit for spans above 4000 mm should be tightened to131030/1.35.

(ii) While I-joist designers may prefer to use the final deflection limits given in theEC5 NA when calculating span tables in order to be able to claim that their

designs are in accordance with the UK NA, for everyday office design it is

recommended that curvature deflection limits on domestic floors be calculated

as in BS 5268 in order to relate the limits more closely to research results, and to

reduce design time and the possibility of errors. The effect on floor joist stiffness

will be very small.

(iii) For floors in which one end of the joists is supported on a beam the frequencyof vibration of the floor system as a whole should be calculated as:

f1,system= ( f21,joistx f21,beam/ f21,joist+ f21,beam)

Where the frequencies of the joists and beam f1,joistand f1,beam, are calculated as

stated in EC5, but using the stiffness of a joist or beam and the mass of the floor

supported by the joist of beam without imposed load.


24/54

Where a beam is securely attached to a permanent partition above or below it,

it may be possible to regard it as a deep beam which would have a very high

fundamental frequency. In this case it could be considered to be a rigid support

when calculating the fundamental frequency of the floor as a whole.

Other points for consideration:

(i) Increased stiffnessIt is likely that perceived floor performance would improve if the resulting joist bending

stiffness at all spans were increased by an additional factor of 10%.

(ii) Site installationIt is recommended that continuing efforts be made to ensure that floor joists, decking,plasterboard and (where fitted) strutting are installed correctly in properly conditioned

members free from building dust and debris, since squeaks and creaks are one of the

most common causes of complaints about floors.

(iii) Multiple span joistsThere are difficulties in maintaining close tolerances in multiple span joist supports.

Particular care should be taken to ensure that intermediate supports on multiple-span

joists are installed at the correct height, with any necessary packing being of adequate

strength and stiffness. It is recommended that for multiple span joists the span ratios are

kept approximately equal to prevent short span uplift, especially where joists are notbuilt-in but are supported in joist hangers.

(iv) Span table optionsIt is recommended that if prefabricated joist manufacturers wish to publish span tables

giving options for more than one performance level, then the 10% better and 20-%

better spans should be calculated in accordance with the tightened Eurocode 5 point

load limit of 131030/1.35, and then be reduced by 0.90.25 and 0.80.25 respectively, i.e. to

0.974for the 10% better and 0.946 for the 20% better options. These reductions are

broadly equivalent to increasing the joist stiffness by 10% and 20% respectively.

(v) System deflectionIt is recommended that for joists supported by a beam at one end which is also subject

to deflection, then the combined instantaneous bending and shear deflection of a

single joist measured in mm should not exceed the lesser of 0.003 and ulimwhere:

= joist span in mm


25/54

ulim = 18 L (ubeam/2) mm for joists with strutting in accordance with current best

practice

ulim = 0.9(18 - L - (ubeam/2)) mm for joists without strutting

where L = joist span in m

and ubeam = the combined instantaneous bending and shear deflection of the beam at

the connection.


26/54

Top ic : Rob ustness and floor to wa ll connec tivity of Platform Timber Frame

Purpose: To p rovide p la tform timb er frame Eng ineers and Designe rs with

guida nc e o n the p rinc iples of robustness and c onnec tivity of w a lls

to floo rs for a rob ust c onstruction.


Key words: Pla tform Timber Fram e, Rob ustness, Fixing s, Connec tivity , Typ ical

de tails, Disproportiona te Co llapse, stab ility a nd serviceab ility.

Cod e Req uirem ent relevant c lauses

Robustness of design is imp lied in the BS 5268 2- 2002(2007 ed ition) and referred to in the

Euroc od es. However robustness as a d esign p rincip le is no t a lwa ys fo llowed . The fo llowing

p rovides c lause referenc es tha t should b e c onsidered .

BS 5268 2- 2002 c lause 1.6. Design Considerations sta tes the fo llowing :

1.6.1.1 The d esign and de tails of p arts and c om po nents should b e c omp atible, pa rticularly in view o f the

increasing use of prefa brica ted c om po nents such a s trussed rafters and floors. The designer responsible fo r the

ove rall stab ility of the structure should e nsure this c om pa tibility eve n whe n som e o r all of the design and deta ils

are the work of a nother de signer.

To ensure tha t a design is robust a nd sta b le

a ) the g eom etry of the struct ure should b e c onsidered;b ) req uired interact ion and c onnec tions be twe en timber loa d b ea ring eleme nts and be twe en such

elements and other pa rts of the structure should b e a ssured;

c ) suitab le bracing or diap hrag m effec t should be provided in planes pa rallel to the d irec tion of the lateralforces ac ting on t he w hole struc ture.

In a dd ition, the de signer should state in the hea lth and safet y plan any spe c ial preca utions or temp orary

prop p ing nec essary at e ac h and eve ry sta ge in the c onstruction p roce ss to e nsure overall sta b ility of all pa rts

of the structure.

1.6.1.2With rega rd to t he d esign p roce ss, de sign, inc luding d esign for the c onstruct ion durability and use in

servic e, should b e c onsidered as a w hole.

NOTE Unless c learly de fined sta ndards for materials, produc tion, workmanship and ma intena nc e are provide d

and co mp lied with the d esign intentions ma y not b e realized .

1.6.1.3With rega rd to b asic a ssump tions c ove ring durab ility, wo rkma nship and m a terials, the q ua lity of thetimb er and o ther ma terials, and of the w orkma nship as verified b y inspec tions, should b e adeq uate to e nsure

safe ty, servic ea bility a nd durability.

BS 5268 2- 2002 c lause 1.6.3 Ac c ide ntal dam ag erefe r to Tec hnica l Rep ortDesign of

Platform Timb er Frame for Disproportiona te Collap se provisions for interpreta tion of this


27/54

c lause relating to ac cide ntal da ma ge .

BS 5268-6.1:1996 c lause 4.4 Sta b ility refe r to Tec hnica l Rep ortStab ility of p latform Timber

Frame for interpreta tion of the new c od e requirem ents for stability de sign of p la tform timb er

frame struc tures.


1) Multi-sto rey timber frame b uild ings a design g uide TRADA/ BRE 20032) BS5268-2:2002 (Amd1) August 2007

3) Trada Tec hno log y Timber frame housing: UK Struc tural recom me ndations 2006

4) BS 5268-6.1:1996 inc orpora ting amendments Nos. 1&2.

5) BS 5268-6.2:20016) Trada Tec hnology - Timber frame c onstruction: 3rd edition: 2001

Introduction:

Robustnessof p la tform timb er frame is the a bility of the structure to withstand a rang e o f

va riations in the p red ete rmined design a nd c onstruc tion c ircumsta nc es without susta ining

loss of func tion o r req uiring remed ial wo rk.

In this doc ument the te rms Eng ineer and Designe r a re used to m ea n the fo llow ing:

Engineer A suitab ly qua lified Struc tura l Eng inee r/ Timb er Frame Eng inee r who is responsiblefor the num eric a l c a lculat ions assoc iate d with the design o f the superstructure to resist the

applied loadings.

Designer- A suitab ly qua lified Struc tural Tec hnic ian/ Timb er Frame Produc tion Eng inee r who

is respo nsib le fo r the p rod uc tion o f ge neral arrang em ent a nd fab ric a tion d raw ings which

a re req uired to manufa c ture a nd b uild the struc ture in ac c orda nce with the Engineers

design.

Whilst robustness and floo r to w a ll c onnec tivity a re related sub jec ts, this guida nc e is

sep ara ted into tw o p arts. Part 1 c onsiders rob ustness as a design princ iple and philosophy.

Part 2 conside rs the c onnec tivity o f joists to wa lls as an example of good prac tice to beac hieved at a wa ll to floor junction.


28/54

Part 1 Princip les for ac hieving a rob ust build ing:

Princip les on building robustness Platform timb er frame

Selec t a structural form which has

low sensitivity to the hazards

considered.

Avoid as far as possible structural

systems, which may collap se

without wa rning.

Provide structural forms that canbe tied tog ether.

Platform timber frame is inherently robust

throug h the interconnec tivity of w a lls and

floor panels. History of use, tests and

research has shown that correctly built

frameworks achieve a significant level of

robustness.

The full rang e of the ha za rds or risks should

be considered as platform timber framecontinues to be used for challenging

struc tures.

As new products for floors and walls get

introduced into the build process the

structural integrity and ability to ensure

robust connections should be questioned

a t a ll sta ges.

Selec t a struc tural form a nd

design that can surviveadequately the accidental

removal of an individual element

or a limited part of a structure, or

rea sona ble loca lised da ma ge .

Disproportionate collapse design

principles have been established andused suc c essfully see Multi-storey timber

frame buildings a design guide

TRADA/ BRE 20031)

Ensure that layouts and plan

arrangements provide returns and

intersec ting w a lls and floors.

Platform timber frame is a structural form

specifically for cellular layouts and plan

aspects greater than 2:1 will require

additional design and robustness to

ensure its suitab ility a s a build ing solution.

Adopt compatible materials usedin the structure and ensure

adequate interaction.

Co nnec tivity of ma teria ls used in the buildprocess is essential and normal timber

fram e c omp onents provide ea sy method s

of fastening tog ethe r.

Rob ustness for service ability:

The guida nc e c onta ined in this rep ort is about the p rovision o f rob ustness for a struc ture


29/54

during norma l use. The struc ture c an b e sa id to be rob ust when it is no t sensitive to slight

va riations in load ing o r as-built d eta ils or as a result of c onstruc tiona l to leranc es. Robustne ssin this c ase is to ensure the struc ture rem ains serviceable. This is in c ont rast to the ne w BS 5268

Part 2 clause on Disproportionate Collapse which is an ultimate limit of robustness; i.e. the

build ing exhib its a d eg ree o f rob ustness aga inst collapse b ut can be unservic ea b le a fter the

eve nt c ausing the loss of support.

A typ ica l examp le of robustness is the ab ility of the structure to c ont inue to func tion if, say

10% of the na ils req uired to fix a junc tion ha ve bee n inco rrec tly insta lled .

The fa c tors of safe ty ap plied in the engineering d esign ha ve long b een c laime d to b e the

proc ess whereb y rob ustne ss is ac hieve d . However, while this is pa rtia lly correc t, there are

other eleme nts of rob ustness tha t fa ll outside of the design c od e fa c tors of sa fety. One o f thefunda me ntal p rinciples of robust engineering and d esign is to ensure prac tic al, b uildab le

solutions are a pp lied , ta king into ac count the p otent ial risks in the build p roc ess (Note tha t

risks a re know n or feasible events and not the unspec ified eve nts app lic ab le to

d isproportiona te c ollap se d esign). An e xamp le o f a rob ust d esign solution w ould b e to d eta il

a ll elem ents so that they c an o nly physica lly b e insta lled in the c orrec t o rienta tion or loc a tion

so tha t errors in interp reta tion can not b e m ade. One of the ma ntras of robust design is tha t

if a d eta il c an be interp reted in mo re tha n one wa y it is not robust.

Platform Timb er Fram e de sign and build proc ess and the p otential for errors:

To a c hieve robustness of a structure the p la tform timb er frame design a nd build p roc essneed s to b e und erstood and an ap preciation of go od prac tic e is also required .

The p latfo rm timb er frame p roc ess c an be c onsidered as c om prising the follow ing stages:

A. Engineer to d esign the fram ew ork and assem bly ba sed on Cod e c riteria and go odpractice.

B. Designer to transla te the Eng ineers solutions and guid anc e into fabrica tion drawingsand erec ting p lans.

C.Fabric a tor to co nstruc t the a ssem b le c om ponents and pac kage for transpo rta tion.D. Erec tor to a ssem b le the c om me nts into a frame wo rk on site tha t forms the structural

shell of the build ing.E. Follow -on trad es c om p lete the build ing by d ressing the struc tural shell with services

and clad ding.

At ea c h stage in the p roc ess a lac k of clarity of informa tion and pote ntia l for

misinterpreta tion c an c ause e rrors in the build which in turn c an be c onsidered a lac k of

rob ustness. At eac h sta ge o f the p roc ess the lea der, a t tha t point of the proc ess, has to

dete rmine if the solution b eing p resented is rob ust. Rob ustness is not a lways a c a lc ulated

eng ineered va lue o r deliverab le. Rob ustness is mo re to do w ith prac tica l solutions tha t ha ve


30/54

minima l risk of no t being a chieved in the fina l assem bly and build of the structure.

The responsibility for robust solutions rests with the Eng inee r for the struc tura l conc ep ts and

frame work solution and w ith the Designer for the a pp rop riate d eta ils, c la rity of informa tion

and ab ility to identify errors in the virtua l on-sc ree n b uild p roc ess.

Eng ineering Rob ustness:

The Engineer will und ertake c a lc ulations for the struc ture a nd deta il junctions to transfer the

app lied forc es with a pred ete rmined fa c tor of sa fety.

The Eng ineer will che c k a struc ture and its com pone nts und er four co nd itions:

1) Standard de sign: to withstand the a pp lied forc es and to a ttain the ag reed level ofservic ea bility in ac c orda nc e with the co de req uirem ents by ad op ting the fac tors of

sa fety w ithin the c od es.

2) Construction period de sign c hec k: to ensure tha t the structure is c apab le o fwithsta nd ing the c onstruc tion loa d ings and to b e sta b le d uring va rious stages of the

construction process.

3) Robustness for Serviceability: to ensure tha t the deta il design o f the a ssem b lies andfram ewo rks are c om pa tible with eac h element a nd that the junctions of m emb ers

ca n b e fitted and sec ured safely and prac tic ally.

4) Design a gainst Ultima te robustness or Disprop ortiona te Co llap se: to ensure tha t thebuilding has suffic ient streng th in ac c orda nc e with the rules for the typ e and sca le of

building.

Co nd ition 3 is where the rob ustness of the prop osed eng ineering solutions is to be

c onside red by the Eng ineer.

Rob ustness Checking for the Eng inee r:

Eac h p rojec t will be spec ific to its ow n c riteria but a s a guide the fo llow ing a re e xamp les of

the checking proc edure to be ad opte d:

a ) Are the fixing requirements c lea r and approp riate fo r the p rojec t?b ) Are the junc tion d eta ils c hec ked for the applied forc es eg sole p la te junc tions,

floo r to w a ll junc tions, roo f to wa ll junctions?

c ) Has the c om pa tibility of elem ents been c onsidered eg c ladd ing to frameinterface.

d ) Are the deta ils for assem b ly of c om pone nts presented ? e.g. a re fixings andassem b ly instruc tions clea r.

e) Are m inimum requirem ents for mec hanica l fixings ac hieved and whe readd itiona l fixings are req uired , is this c lea rly marked e.g . area s of high rac king

forc es c lea rly note d?


31/54

f) Are a rea s of high risk of fa ilure note d and the nec essary informa tion translated

to the designer and c lient for possible designing-out of these risk area s, e.g.whe re multip le trad e c om ponents c om e tog ethe r to form the struc ture without

ad eq uate c oordination or chec king by a structural eng ineer.

g ) Ma teria l spec ifica tion for durab ility e.g . is the c orrec t p reservative trea tment o rdetailing to avoid moisture present?

h) Comp onent c om pa tibility for shrinkage e.g. w ill differential move me nt c rea teadd itiona l stresses to the m em bers and frame wo rk?

i) Comp onent c onnec tivity to othe r mem bers and finishing trades e.g. is theresuffic ient w idth of stud for the c ladd ing fixings?

j) Is there mo re than one wa y of interpreting the c onstruc tion d raw ings?Design Robustness:

The Designer will ta ke the Eng ineers informa tion a nd transla te the informa tion o nto

fab rica tion and erec tion ge neral arrang em ent d rawings. The Designe r has the unique ab ility

to review the build ing in a virtua l build seq uenc e. Structural co nnec tivity of the frame wo rks

should be c hec ked a t this virtua l build sta ge .

The Designer ma y adop t sta nd ard deta ils but it is essent ial tha t the Eng ineer has app rove d

these d eta ils spec ifica lly for the p rojec t.

The Designer should c hec k the deta ils for the follow ing:

a ) Prac tic a l alignm ent of the struc tura l frame s and rep ort on a rea s of m isa lignme nt o rlack of support.

b ) All deta ils a t junctions or referenc es to sta ndard d eta ils a re p rovided .c ) The d rawings have refe renc es to a ll spec ial item s as instruc ted by the Eng ineer.d ) Ensure c o o rd ination o f informa tion from d ifferent eng ineering te ams wo rking o n

d ifferent a spec ts of the struc ture a nd build items.

e) Review d eta ils to e nsure tha t the re is no a mb iguity or lac k of informa tion for thefab ric ato r and erec tor.


32/54

Part 2 - Connec tivity o f the Framing mem be rs floors to walls:

Introduction

This sec tion refers to a numb er of standard go od p rac tic e c onnec tivity de ta ils tha t a re

req uired to c om p ly with the upda ted BS 5268 Parts 6.14) and 22),2007. In particular, Figure M3

in BS 5268 Part 2 provid es a deem ed to sa tisfy deta il for robust ho rizonta l tying of floo rs to

walls.

Fora ll p la tform timb er frame struc tures, the a pp roa c h is to a dop t go od build ing p rac tic e of

providing late ra l restra int to w a lls and c om mo n anc horage d eta ils of floors to wa lls. The

design p roc ess should involve c hec king the c ap ac ity of the c om pone nt interfac es (e.g.pa nel rail to solep la te, solep late to floo r dec k, floo r joists to hea d binder and hea d binder to

panel rail) aga inst the va riab le horizonta l wind force s. The timb er frame designer shou ld

therefore be p roviding a robust co nnec tion a t ea ch and eve ry junct ion a s pa rt of the

norma l design proc ess.

Examples of Good Prac tice to a ssist robust Platform Timb er Frame

The follow ing figures provide typ ica l co nstruc tion de ta iling betw een timb er platform frame

c om ponents for projec ts from 1 to 7 sto reys to ensure minimum levels of robustne ss. The

Eng ineer ca n spec ify mo re o r less fixings dep end ing on the spec ific p rojec t c riteria.

Index to Details:

Figure

1. Softwood joists Wall/Floor intersec tion: explod ed view based on Figure M 3,BS 5268 2: 2002

2. Softwood joists a lternative deta ils a t internal wa ll supp orts app lic ab le to loo se floo rconstruction.

3. Softwood jo ists a lte rna tive de ta ils a t loa d -bearing/ Rac king w a lls.4. Softwood jo ists deta ils a t Bea ms.5. I-Jo ist Floors Typica l wa ll/floo r intersec tion: exp lod ed view6. Open-web Floor joists Typic a l wa ll/ floo r intersec tion: exp lod ed view


33/54PDF created with pdfFactory trial version www.pdffactory.com
http://www.pdffactory.com/http://www.pdffactory.com/


39/54

Top ic : Tec hnic al Rep ort - Stab ility of Platform Timb er Frame

Purpose: To p rovide g uidance a nd a wo rked examp le for the use of the

Sta b ility c lauses conta ined within the revised BS 5268-6.1:1996-

Struc tura l use of timb er. Code o f p rac tic e fo r timb er fram e w a lls.

Dwe llings not exceed ing seven sto reys (AMD 9256) (AMD 17381)

reissued Novem ber 2007.

The new c ode sup ersed es BS 5268-6.1:1988. Amend ment 9256

da ted June 1996. Amend me nt 17381 da ted November 2007.


Keywords: Platform Timber Frame, Racking Overturning chec k, Sliding

resistance chec k

Background: The British Sta nd ard for the design o f p la tform timb er frame

build ing s is c ove red by BS 5268-6.1:1996 and BS 5268-6.2:2001. The

Novemb er 2007 ed ition of BS 5268-6.1:1996 has bee n up da ted to

take ac c ount of expe rience with this type o f co nstruc tion a nd the

issue of releva nt Europ ea n sta nd ard s. The sc op e ha s bee n

extend ed from 4 sto reys to 7 sto reys. The c od e c onta ins significant

c lause c hange s a ffec ting how the stab ility of timber frame rac king

wa lls a re to be c onsidered for buildings in exce ss of three storeys

tall.

This doc ume nt p rovides guida nc e o n the use o f the new BS 5268-

6.1:1996 Clause 4.4 Sta b ility inc luding a worked examp le for a

typ ica l four sto rey b uild ing.

The c hang e in de sign ap p roa c h to stability req uired by the new

c lauses is significa nt fo r dwelling s above three storeys in height.

In p rinciple, the d esign a pproa c h for dw ellings of three storeys or

less and with a ma ximum height to wid th ratio o f 2:1 is as the

previous ed ition o f the c od e a nd it is ac c ep ted that whole b uildingstability c an b e a d op ted . How ever, to keep in line w ith Europ ea n

c od es and othe r ma teria l standards, an increa sed fac tor of sa fety

of 1.4 against building overturning and sliding resistance is required.


40/54

Building Reg ulations or Cod e Req uirem ent

relevant c lauses

The follow ing e xtrac ts a re ta ken from British

Sta nd a rd BS 5268-6.1:1996 (+ Am nd No.1 & No .2)

4.4 Stability

4.4.1 General

The d esigne r should e nsure overa ll building stab ility

by c hecking tha t it has ad equa te rackingoverturning1 and slid ing resista nc e to la teral loa ds.

These c hec ks should b e mad e a t c ritica l levels for

the completed building and for the various

co nstruction stag es, when subjec ted to d ead load,

zero imposed load, and both horizontal and

vertical co mp onents of the wind load . 2

Sta b ility is ge nerally ob ta ined from racking wa lls, set

in two orthogonal directions. Unless demonstrated

othe rwise, w alls with signific ant op enings, for

example doors, should be considered as separate

d isc rete w a lls. The rac king resista nc e o f ea c h wall

should be c alculated in ac c ordanc e with 4.7 for

each direction.

4.4.2 Overturning

4.4.2.1 General

Subjec t to t he limitations in 4.5, it ma y be a ssume d

that floor diap hrag ms are c ap ab le of d istributing

the wind load to ea c h rac king wa ll in prop ortion to

its rac king resista nc e. Due acc ount should b e ta ken

of a ny significa nt ec ce ntricity be twee n the

centroids of the wind load and the ag gregated

wa ll rac king resista nc e. 3

The sta b ility of ea c h rac king w all should be

c hec ked a t the b ase a s follow s:

a) The overturning moment is the p rod uc t ofthe ap portioned wind load and the vertica l

distanc e be twe en its c entroid a nd the wa ll

base. 4

b) The o verturning resista nc e o f a wa ll is theproduct of the dea d load (reduced by any

Commentary

The fo llow ing note s provide some c omm enta ry

on the interpretation o f the new c lauses:

1. Rac king ove rturning is a d eliberate

sta tem ent to instruct the Enginee r to c onsider

the sta bility aspe c t of the w alls tha t arede signed to c arry lateral loa ds through the

building (ie the chosen racking walls). Previously

w hole b uilding stab ility c onsidered the rac king

forces to be evenly distributed through the

structure suc h tha t the ove rturning a nd slid ing

forces are shared througho ut the assumed b ox-

type structure

2. Engineering chec ks should b e c arried o ut a t

va rious build sta ge s. The m ost c ritica l loa dcase is

likely to be at full fram e c omp letion but without

roo f finishes, plasterboard we ight a nd floating

floo r or ce iling c onstruction.

3. It is common sense that Engineers need to

c onsider the d istribution of lateral loads throug h

asymmetric b uildings to ind ividual rac king wa lls.

The Eng ineer is to c onsider the d iap hrag m

ac tion of the floors and roo fs to d istribute the

forc es to t he racking wa lls. Sec tion 4.4.2.1 a llow s

for the c eiling/ de c king of stand ard Platform

Fram e c onstruction to provide this diap hragm

ac tion without further chec king for a m aximum

aspe c t ratio of 2:1.

4. Key point: the lever arm fo r overturning

c hec ks is the vertica l distance to the c entroid of

the lateral loa d a nd not the height of the w all

panel.

5. Enginee ring sta tics are used to ca lculate the

inherent p ane l overturning resista nce. Ad d itiona l


41/54

vertica l comp onent of the wind load ) and thehorizonta l distanc e betw een its centroid and

the leewa rd c orner. 5

Add itional dea d load from return walls, where

present, can be utilized but should be limited to

an outstand distanc e eq ual to the pa nel height

or the distanc e to a d oor or window o pe ning,

wh ichever is the lesser (sma ll op ening s as

de fined in 4.9.4 ma y be ignored ). The

co nnection bet wee n the return wa ll and the

rac king wa ll should b e d esigned to t ransfer the

shea r loa ds ba sed on the resultant a pp lied

design Forces.6

Tension fixings may a lso b e usedto m ob ilize d ea d loa d from the und erlying

co nstruction and their ca pa city add ed to the

de ad load as a c ontribution to the overturning

resista nc e.

c) The fac tor of safe ty of a rac king wa ll ag ainst

overturning is defined as the overturning

resistanc e d ivide d by t he o verturning mo ment.

For each racking wall, under its apportioned

wind loa d, the fa c tor of safe ty should b e > 1.2.

7

The fac tor of safe ty of the to ta l rac king w all

resistanc e, und er the to ta l wind loa d, should

b e> 1.4. 8

4.4.2.2 Ove rturning for dwellings of three or less

storeys

For d we llings of th ree o r less sto reys, and with a

ma ximum height to width ratio of 2:1. the

overturning resistanc e o f the b uilding ma y be

de termined a s the produc t of its tota l dead loa d

(reduc ed b y any vertical com pone nt of the wind

loa d) a nd the ho rizonta l distanc e b etwe en the

load ce ntroid and the leewa rd edg e. The fac tor of

sa fet y, as def ined in 4.4.2.1c ) should b e > 1.4. 9

4.4.3 Slid ing

The d esigner should ensure tha t the re is a fa c to r of

safety of 1.4 against sliding at the top and bottom

of each racking wall, and at sole plate level.

Friction, under dead load only, may be used in

conjunction with metal fasteners when calculating

the resista nc e to slid ing10. The c oe ffic ient of fric tion

bet ween timb ers in conta c t or on the und ersid e of

restoring forces can then be c onsidered as inthe remainder of the clause.

6. The weight o f wa lls pe rpend icular to the

racking w all ca n be used within the limits given

and when fixed ap propriately to the rac king

wall.

7. Alll rac king w alls require a minimum fa c tor of

safety of 1.2 against overturning.

8. The inc reased fac to r of safe ty of 1.4 for the

ag grega ted overturning resistanc e of all rac kingwalls b rings the BS in line w ith Euroc od es.

Key p oint: The fac tor of safe ty fo r rac king

resista nc e itself (ie FOS=1.0) rema ins unc ha ng ed

with the fa c tor of safe ty inherent w ithin the

design values for basic racking resistance Rb

from Tab le 2.

9. For dwellings of three storeys or less the

w hole b uilding ap proac h for chec king

ove rturning is ac c ep ta ble (see No te 1).

Note: The UKTFA rec om me nd tha t for dw ellings

exceeding three storeys, a whole building

c hec k for ove rturning should still be c arried o ut

and the fa c tor of safety should b e >1.4.

10. Note: The BS allows friction a nd mec han ica l

fixing c ap ac ity to be c onsidered toge ther.

When c onsidering the use o f mec hanica l fixings,

the British Sta nd ard pub lished va lues for timb er-

to-timber fixings alrea dy inc lude for a fa c tor of

sa fety o f 1.4. The UKTFA c onsidered tha t it is no tap propriate to ap ply an add itional factor of

sa fety fo r the me c hanica l fixings. Alterna tively

the ultima te me c hanica l fixing c ap ac ities c ould

be used with a fa c tor of safet y ap plied . For

fixings into found at ion ma terials, the Engineer

should ensure tha t a fac tor of safe ty > 1.4 is

achieved.

11. The UKTFA c onside r tha t the c oe ffic ient o f

friction is 0.4 as an unfac tored va lue.


42/54

the soleplate may, in the absence of otherinforma tion, be taken as 0.3. 11

4.5 Horizontal diaphragms

The d esign me thod for timbe r fram e w alls given in

this British Sta ndard a ssume s tha t, fo r the rang e of

d wellings covered, t he no rma l co nstruc tion of

floors and roofs provide s ad eq uate diap hrag m

ac tion, provide d t hat, in the c ase of intermed iate

floors, a floor de ck or sub-d ec k is fixed d irec tly to

the top fac es of the joists, or the floo r is b rac ed by

som e ot her mea ns. In the c ase o f pitc hed roofs it is

assumed that the p lasterboa rd c eiling under theroof, together with the roof bracing recom mended

in BS 5268-3 is suffic ient to transfer app lied wind

forc es to the resisting wa lls. 12

Due ac co unt should b e ta ken of the ecc entric ity of

the loading in relation to the w all pane ls providing

resista nc e. 13

12. The Eng ineer is to ensure the effe c tivene ss of

diaphrag ms for different co nstruction types to

transfer the horizontal loads to the racking walls.

13. This c lause refers to the nee d fo r Eng ineers to

c onsider the d istribution of rac king w a lls

througho ut a building (see Note 3) ie t he

rac king wa lls c anno t b e p ositioned on o ne side

of a building w ithout p roviding rac king w alls at

right a ng les to resist the resulting t orsiona l

effects.


1) The Institution of Struc tural Eng inee rs/ TRADA Ma nua l for the design of timb erbuild ing structures to Euroc od e 5 Dec em ber 2007

For a w orked exam p le o f ove rturning , sliding, rac king and roo f up lift che c ks for a

dwe lling of three storeys or le ss referenc e should be ma de to 2)Trada Tec hnolog y

Timber Frame ho using: UK Struc tura l rec om mendations 3rd ed.2006 Sec tion 7.3 OverallSta b ility c a lc ulations, excep t tha t a n increased fac tor of safe ty of 1.4 for overturning

and sliding should be ad op ted in ac c orda nce with the new c od e requireme nts.

Overturning and Sliding wo rked examp le for 4 storey dwelling:

The follow ing w orked examp le indica tes the p roc ed ures to b e adop ted for chec kingtimb er frame build ings of mo re tha n three storeys in ac c orda nc e with BS5268-6.1:1996

(AMD 17381) Cl 4.4.2.1.


43/54

Overall Stability of Platform Timber Frame

Consider the example of a residential building shown below:

Roof

Third

Second

First

Ground

Fig 1. Typical Section

Wall 2

Wall 1

Wall 1

Wall 2

Fig 2. Plan on typical storey

4-Storey Building Worked Example to BS5268-6.1:1996 (incl Ammendments 1 & 2)

November 2007 edition.

Width b (m)

Wind Direction 2

l/2

Spanl1

l/2

Wind

Direction1

JoistSpan

direction

Wall3

Spanl2

l/2

Wall3

Spanl3

l/2

h2

h4

h3

hstorey/

Floor Loads

Roof Loads

Floor Loads

Floor Loads

h1

hstorey/2

hstorey

hstorey

hstorey


44/54

Calculation of Vertical Loads

Typical Unit Loads

Assume the following typical unit loads for the worked example:

Permanent Temporary

Gk Gktemp(see note 3)

Roof Uplift(see note 4) -0.20 -0.10

Roof 1.50 0.50

Floors 1.00 0.50Ext Walls see note 2 0.40 0.10Int Walls 0.60 0.05

Notes:

Loads to individual walls

Summary of 'Loading-Down'

In the example, the loads calculated at Ground Floor Level were as follows: Perm TempGk Gktemp

a) Internal Wall 1 = 28.90 10.20

b) External Wall 2 = 12.50 4.60

c) External Wall 3 = 6.09 1.77

d) Total Building weight = 540.00 194.40

Estimated Unit Dead Loads (kN/m2)

4/Roof uplift pressures have the effect of reducing the effective dead load for resisting overturning and

should be considered.

Item

(kN/m)

For floor joists parallel to external walls it is assumed that a load equivalent to half the joist spacing is

carried by that wall.

2/ The 'Permanent' Dead Load Gk refers to the in-service applied dead loads. In the case of external wall

panel self weight, this should include for the weight of any supported cladding type.

1/ The above unit loads are provided as an example. Unit loads should be calculated for each individual

building, taking into account the construction and minimum imposed loads from BS6399-1:1996 and BS6399-

3:1998 or Eurocode BS EN 1991-1-1:2002 as a ro riate

In such cases the internal support reactions will be increased due to the continuous nature of the joists.

Taking into account the effects of pattern loading, it can be shown that the maximum internal reaction is

approximately 1.25wL and the end span reactions are 0.45wL, where L is one span and w the UDL on that span.

3/ The 'Temporary' Dead Load Gktemp refers to the expected dead loads during construction and shouldgenerally exclude the weight of roof tiles, cladding, plasterboard, floating floors and ceiling constructions

(the weight of plasterboard packs may be considered when it is specified that a building is to be 'loaded-out'

durin construction.

In the example, the joists are indicated as 12m long continuous span floor joists.

(kN)


45/54

Calculation of Horizontal loads

Wind Loads acting on walls to BS6399-2:1997

BS6399-2:1997 Cl 2.1.3.6 Permanent overall pressurefinal = 1.00 kN/sq.m

For simplicity, assume no masonry shielding is applicable:

BS5268-6.1:1996 Cl 3.2.3.1 Assume no masonry s ie ing

k100 = 1.00

BS6399-2:1997 Annex D

BS6399-2:1997 Cl 2.1.2.1 Temporary overall pressure

temp = 0.7492 x 1.00

= 0.56 kN/sq.m

Wind Loads acting on Roofs to BS6399-2:1997

The governing horizontal loadcase for timber frame buildings in the UK will generally be

wind loading, however it may also be necessary to check for the effects of notional

horizontal loading, accidental loading and imposed horizontal loading (from balcony forces

for example).

For temporary wind loads during construction, a factor of sd = 0.749 may be considered

for wind loads with a probability of not being exceeded during a period of 12 months

duration. Overall wind pressures are reduced by the square of this factor.

Overall wind loads Pe should be calculated using BS6399-2:1997 Cl 2.1.3.6 for stability

design. For simplicity, the worked example assumes that the overall wall wind pressures

calculated is:

For an example of how to calculate the effects of roof wind pressures, reference should

be made to Trada technology: UK Structural recommendations: Section 1.2.2

Roof uplift pressures have the effect of reducing the effective dead load for resisting

overturning and should be considered. (For the worked example the overturning effects

of uplift presures on the flat roof are small and have been ignored for simplicity).

For certain pitches of roof, BS6399 gives two sets of external pressure coeficients cpe,

and it may be necessary to consider different combinations of coefficients to identify

the worst loadcase for stability and racking checks.

Wind loads acting on the inclined faces of a pitched roof (refer to BS6399 - 2:1997 )will

also contribute to racking, sliding and overturning forces acting on the building and

should be considered.


46/54

Dimensional Checks

A/ Floor Diaphragm Check

BS5268-6.2:2001 Cl 6.5Wind 1 direction:

Lengt /wi t = /w = 4 6 = 0.7

Wind 2 direction:

Length/width = l/w = 6 12 = 0.5

Trada Technology ' Timber Frame housing: UK structural recommendations': 2006

B/ Panel height Check to BS5268-6.1:1996 Cl 1.1

Storey height hstorey = 3000 mm

Subdeck thickness = 15 mm

Joist depth = 300 mm

Headbinder thickness = 38 mm

Soleplate thickness = 38 mm

a pane eig t panel = 2609 mm

4/ the perimeter of the diaphragm is attached to the walls with fastenings of equivalent

strength.

1/ the diaphragm span: depth ratio does not exceed 2:1 in either wind direction.

The floor and roof diaphragms distribute wind loads acting on the elevations to the racking

walls and can be assumed to be simply-supported between racking walls.

2/ the span does not exceed 12m between supporting walls.

For diaphragms outside of the ranges given or in areas of high wind load (e.g. with a dynamic

pressure exceeding 1500N/sq.) the required fastener spacing should be checked in

accordance with Section 1.1.2 of Trada Technology ' Timber Frame housing: UK structural

recommendations': 2006

3/the fixing around the edges of the panels complies with standard recommendations (e.g.

3.00mm diameter ringed shank nails @ 150mm c/c for plywood or 3.35mm diameter ringed

shank nails @ 300mm c/c for wood particleboard and OSB, with a length equal to 2.5 times

the board thickness)

It can be assumed that conventional floors and flat roofs, in which a wood based panel

product is fastened to timber joists, have adequate strength and stiffness as horizontal

Documents

Structural Guidance in Platform TF 2010