Basements CHG v19- 19 Apr 2016 Basements_The Concrete Centre.pdfGuidance on the design and construction of in‐situ ... • Fire safety ... basement slab load, upward water pressure,

Concrete Basements

ASELB 29 April 2016 1

Charles GoodchildCEng., MCIOB, MIStructE

Principal Structural Engineer

MPA ‐ The Concrete Centre

Concrete Basements

Guidance on the design and construction of in‐situ concrete basement structures

ASELB29 April 2016

The whole lecture usually takes 90 mins.

The talk is aimed at commercial basements but the principles are applicable to domestic basements, water retaining structures, etc.

It is necessary to understand the background before kicking on with the structural design.

Concrete Basements

Introduction/backgroundPlanning a basement• Types & Waterproofing strategy• Site Constraints

Ground movements & Methods of construction

MaterialsStructural design• Loads• ULS• SLS• Example

Specification and constructionCase studies





Concrete Basements

Then: BS 8007:1987

Basements

Then:BS 8102:1990

BS8102

Concrete Basements


Then: CIRIA 139/140 1995 Then: Design & Construction of Deep Basements: 2004

• Eurocodes

• Withdrawal of BS 8110, BS 8007 etc

• Revision to BS 8102

• New information:• CIRIA C660

• CIRIA C580

• ICE Reducing the Risk of Leaking Substructure: A Clients’ Guideuide

• Debate• S Alexander, TSE Dec 06

• B Hughes, TSE Aug 08?

• ICE project 0706 on reinforcement to control cracking (report Feb 2010)

What’s new(ish) in basements (and water retaining structures)? EN1992-3

CIRIA C660 BS8102

Concrete Basements


Basement Information Centre Concrete Basements





Concrete BasementsBasement design requires:

• an holistic approach

• an understanding of both the ground and the structural behaviour

• formal consideration of construction methods

• communication:

Structure

Space Planning

Services

Architecture

WaterproofingGeotechnics

Cost and risk

Construction sequence

Architects want:Dry walls and base – with no impact on space planningSimple shapes – unless it’s shapes they are definingLarge holes – often at points of maximum in-plane stressNo columns – but if you put any in, they will clad them to twice the sizeNarrow beams between holes (not appreciating they will act more like props)

Clients want: short construction timeslow costslow risk and uncertainty

Wants

Contractors want: Bottom up Construction – simplerCantilever walls – no props, simplerNo tanking – simpler, & anyway it always leaks somewhereNo constraint on construction sequence - leaves more options open when things get out of sequence. He will assert that HIS sequence CANNOT have any affect on the design forces

• Get him on-side ASAP - he can be an ally

And Engineers?. . . . .. . . . . . a simple life!

Concrete Basements

1. Establish Clients requirements

2. Site surveys, etc

3. Outline designs, methodology and proposals

4. On approval do detailed design

5. Construction

Outline of the design process

Concrete Basements


BS 8102:2009 Table 2 provides guidance:

Grade of useGrade 1 Some leakage, some damp.

Parking, Plant roomsGrade 2 No water penetration or damp patches.

Plant rooms, workshops Grade 3 Dry environment. Ventilation required.

Residential, Commercial(Grade 4) (totally dry and vapour proof)

Archives, stores …. go to BS 5454As an aid . .

Grades

Planning a basement

Grades BS 8102:2009

Planning a basement

Types BS 8102:2009

Type A

Barrier protection

Type B

Structurally integral protection

Type C

Drained protection

Planning a basement

Types of water‐resisting construction vs risk BS 8102:2009 :

Planning a basement

Combinations possible

Forms of rc basement construction related to site conditions and use of basement space:

Basement excavated after dia-phragm with the floors acting as props in the final condition

Diaphragm walling

Secant piling or sheet piling etc

Medium to high: permanently above lowest floor level

Basement excavated after piling with the floors acting as props in the final condition with/without subsequent concrete facing

Contiguous piling

In open excavation or within temporary works

RC boxLow:generally below floor level

Method of constructionForm of construction

Water Level Costincreasing

Planning a basement

Other subjects

• Surveys and ground investigations

• Precautions near underground tunnels, sewers & service mains

• Working adjacent to existing structures: Party walls

• Tolerance of buildings to damage

• Space planning

• Integrating basement with the superstructure

• Fire safety considerations

• Client approval

Planning a basement

Concrete Basements


Exploratory works

NEEDED EARLY - commission early!desk study

• geological maps, borehole records, • ordnance survey, water courses, utilities.

site surveys• boundaries, adjoining buildings and roads,

liaison with adjacent owners, party walls, • incoming services, tunnels

subsoil investigation • bearing capacity, water level, pile design,

earth pressures, settlements, (modulus of subgrade reaction) contaminants. See BS EN 1997-2

• money well spent!assess

• risk of risk of flooding (EA), likely obstructions, foundation details of adjoining buildings, disposal of groundwater

Planning a basement

• 3m and 6m notices• Distortions cause damage – not absolute movement• 10 mm often used as a trigger

Party Walls/Adjacent buildings - notices

Planning a basement

Space planning:

Check for:

• room for temporary works- clearances for piling rigs. diaphragm wall equipment takes up considerable space.

• restrictions imposed by owners of underground tunnels and utility companies

• dimensions of guide walls for contiguous piles (may be around pile diameter + 800 mm);,

• wall thicknesses : zone for cavity drains if relevant;,

• tolerances for piling and temporary works;

• capping beams

• projecting features of adjoining structures.

• superstructure – follow through into basement,

• Fire – means of escape, compartmentation, access

Planning a basement

Space requirements

Planning a basement

Capping beams

Planning a basement

Guide wall?

Planning a basement

Pile tolerances

Concrete Basements






Concrete BasementsGround movements

Vertical load

& relief

Horizontal

load relief

Portcullis House:

Observed vertical and horizontal movements around the Palace of Westminster car park

Big Ben

Ground movementsConstruction Sequence

• The temporary loads from the construction sequence will probably have an impact on the permanent design.

• For anything other than a very simple basement, the engineer should assume a construction sequence and include it in the tender documents.

• The contractor should be allowed to deviate from the assumed construction sequence; but at least everyone knows what the original assumptions were, and can see if any change will affect the design of the permanent works.

Construction methods

Construction methods:

• Open excavation• Bottom – up• Top – down• Semi-top downGroundwater

Options for basement walls:

• In open excavations: R C walls • Incorporating temporary embedded

retaining walls: o King post wallso Steel sheet pilingo Contiguous piled wallo Secant piled wallo Diaphragm walls Facing walls

Temporary works

Retaining Wall Types


Concrete Basements



Contiguous Piled Wall Contiguous Piled Wall

http://www.oasys-software.com

600 mm diam. Rakers every 2 m


Contiguous Piled Wall


Secant Piles


Secant Piles


Facing wall (courtesy GCL Ltd)


Diaphragm Wall

Concrete Basements


Sheet Piles


http://www.terraingeotech.com/index.html


Propped Excavation


Propped Excavation


Reality

Top Down Construction

Dig

Move

Raise

Cart away

Grouting

Concrete Basements






Concrete BasementsType A construction:

Waterproofing membranes and systems:• Category 1 – Bonded sheet membranes• Category 2 – Cavity drain membranes• Category 3 – Bentonite clay active membranes• Category 4 – Liquid applied membranes • Category 5 – Mastic asphalt membranes• Category 6 – Cementitious crystallisation active systems • Category 7 – Proprietary cementitious multi-coat

renders, toppings and coatings

Selection of materials

Bonded sheet membranes…modified bitumen on a range of carrier films

Liquid applied membranes…generally applied as a bitumen solution, elastomeric urethane or modified epoxy

Mastic asphalt…applied in 3 coats as a hot mastic liquid

Proprietary cementitious multi‐coat renders, toppings and coatings

Concrete Basements


Proprietary cementitious multi‐coat renders, toppings and coatings

Type A construction:



Types B & C construction:ConcreteAdmixtures for watertightness



Concrete:• Benign soils:

RC30/37? Cement IIB-V (CEM I + 21%-35% fly ash)

or IIIA (CEM I + 36% - 65% ggbs).

• Aggressive soils:

Advise producer of DC Class.

For DC-2: FND-2? (C25/30)?

More aggressive soils: Cement IIIB (CEM I + 66% - 80% ggbs) or IIVB-V (CEM I + 36%-55% fly ash)

• Car Parks: C32/40? + provisos (PAV2?)

cf BS 8007 C35A?: C28/35 (equiv) WCR 0.55 CC 325 CEM I, IIB-V,)BS 8500 RC30/37: C30/37 S3 WCR 0.55 CC 300 CEM I, IIA, IIB-S, IIB-V, IIIA, IVB-V B)

• Fibres? Possibly. Fibres only help once the concrete has cracked.

Admixtures


Admixtures

Concrete Society Working Group on Water Proofing admixtures:

• no conclusive evidence to support their use (- from a material scientist’s point of view).

• from data there is some evidence to suggest that they may reduce drying shrinkage (less permeability) and therefore reduce onset of cracking and reduce crack widths

It’s the cracks that matter – not (usually) the concrete!

Traditional: Engineering, workmanship, supervision issues, risk & possible remedials and upheavals and contractual issues

vs Admixtures: warranties, supervision & possible remedials and upheavals

but few contractual issues


£££vs

££££ ?

Whatever the basement should still be designed properly!

Cost and risk:

Type A construction:



Types B & C construction:ConcreteAdmixtures for watertightness

Water stops • Preformed strips – rubber, PVC, black steel• Water-swellable water stops • Cementitious crystalline water stops • Miscellaneous post-construction techniques

• (Re) injectable water bars • Rebate and sealant


Concrete Basements


Waterbar

Photo credits Watermans

Construction, inspection and testing

HydrophilicsPhoto credit Watermans


Resin injectionPhoto credit Max Frank

Construction, inspection and testingCavity drain membranes…high density dimpled polyethylene sheets

Cavity drain Sump pump

1800

Concrete Basements






Concrete Basements

Ultimate Limit State ≡ ‘normal’ design

Serviceability Limit State ≡ control of cracking

Structural design outline:

Structural design ‐ Loads

Loads to be considered:• Slabs: column & wall loads, basement slab load, upward water

pressure, heave.• Walls, lateral earth pressure, water pressure, compaction, loads

from superstructure, imbalances.

Design ground water pressure• ‘Normal’ and ‘maximum’ levels

Options for basement slabs• Soil-structure interaction• Beams on elastic foundations• FEA

Options for basement walls• Temporary conditions: construction method and sequence• Permanent condition

Unplanned excavations• Allowances for cantilever retaining systems

Consider a 8m x 1m base with 1000 kN loads each end on a very stiff clay (Es = 150 MPa):

≡ UDL 250kN/m2

@ SLS

16.7

mm

se

ttlem

ent

Structural design :Soil-structure interaction:

1000 kN1000 kN

t = 0.5, 0.7, 0.9 or 1.1 m

Settlement

Calculation of lateral earth pressures

Angle of shearing resistance:

• Granular soils:

Estimated peak effective angle of shearing resistance

′max = 30 + A + B + C (A - Angularity, B - Grading, C - N blows)

• Clay soils

In the long term, clays behave as granular soils exhibiting friction and dilation.


Decoding Eurocode 7 Fig 10.8


Surcharge loadings:

• Imposed loads: general, highways

• UDLs, point loads, strip loads, rectangular loads : Boussinesq

• Compaction pressures

Design angle of shearing resistance: tan ′d = tan ′k/(NB according to Combinations 1 and 2)

Pressure coefficients• Active pressure at depth z below ground surface ′ah = Kad ′v + u

• Passive pressure at depth z below ground surface ′ph = Kpd ′v + u• At rest pressure at depth z below ground surface ′ph = K0d ′v + u


Concrete Basements



Pressure coefficients: K0d, Kad or Kpd?


• If the soil has a chance to ‘relax’ it will and Kad is appropriate.

• In some situations, e.g. top down, it can’t and that is where K0d comes to the fore. Sometimes it can move ‘partially’ and some designers will go between Kad and K0d. Some always use K0d.

• Where you have compaction both the ‘soil’ and initially the uncompacted backfill have a chance to move so Kad is appropriate. With compaction, you start at Kad and move towards Kpd – hence the pressure additional to Kad.h.soil. The amount of the addition depends on the size of the design force of the compaction plant.

Design for Ultimate Limit State

EQU – Equilibrium Limit State

STR & GEO – Structural and geotechnical Limit States

• EC7: Combinations 1 and 2

• F for ground water

o Normal F = 1.35

o Most unfavourable F = 1.20 (≡ ‘Accidental)

• Structural design

o As ‘normal’ elements

o 3D nature of design

Structural design ‐ ULS

Design for Serviceability Limit State≡Control of cracking

Structural design ‐ SLSTest for restraint cracking

A section will crack if:

r = Rax free = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] ctu

where K = allowance for creep= 0.65 when R is calculated using CIRIA C660= 1.0 when R is calculated using BS EN 1992-3

c = coefficient of thermal expansion (See CIRIA C660 for values). See Table A6 for typical valuesT1 = difference between the peak temperature of concrete during hydration and ambient

temperature °C (See CIRIA C660). Typical values are noted in Table A7ca = Autogenous shrinkage strain – value for early age (3 days: see Table A9)R1, R2,R3

= restraint factors. See Section A5.6For edge restraint from Figure L1 of BS EN 1992-3 for short- and long-term thermal and long-term drying situations. For base-wall restraint they may be calculated in accordance withCIRIA C660. Figure L1 may be used with CIRIA C660 methods providing an adjustment forcreep is made (See Figure A2 and note).For end restraint, where the restraint is truly rigid 1.0 is most often used, for instance ininfill bays. This figure might be overly pessimistic for piled slabs.

T2 = long-term drop in temperature after concreting, °C. T2 depends on the ambient temperatureduring concreting. The recommended values from CIRIA C660 for T2 are 20°C for concretecast in the summer and 10°C for concrete cast in winter. These figures are based on HA BD28/87[60] based on monthly air temperatures for exposed bridges. Basements are likely tofollow soil temperatures so T2 = 12°C may be considered appropriate at depth.

cd

ctu

=

=

drying shrinkage strain, dependent on ambient RH, cement content and member size (see BSEN 1992-1-1 Exp. (3.9) or CIRIA C660 or Table A10). CIRIA C660 alludes to 45% RH for internalconditions and 85% for external conditions.tensile strain capacity may be obtained from Eurocode 2 or CIRIA C660 for both short termand long term values

Structural design ‐ SLS

CIRIA C660 Cl 3.2

CIRIA C660 Fig 4.1


T1 Difference between the peak temperature of concrete during hydration and ambient temperature °C

e.g:T1 for a 400 mm wall, 350 kg/m3

CEM I using 18 mm ply removed after 7 days ≈ 30oC

Table 1 – Values of restraint factor R for a particular pour configuration

0,8 to 1,0Infill bays, i.e. rigid restraint

0,2 to 0,4Suspended slabs

0,3 to 0,4 at base 0,1 to 0,2 at top

Massive pour cast onto existing concrete

0,1 to 0,2Massive pour cast onto blinding

0,6 to 0,8 at base 0,1 to 0,2 at top

Thin wall cast on to massive concrete base

RPour configuration

BS EN 1992-3 Annex L

Beware: effects of creep included

usually 0.5


Restraint factors

Concrete Basements


CS TR 67

Short term load strength

Long term load strength

Stress due to early thermal –allowing for creep

. . . . . .plus drying shrinkage

. . . . plus seasonal

SLS Design vs time

Structural design ‐ SLSTest for restraint cracking



where K = allowance for creep= 0.65 when R is calculated using CIRIA C660= 1.0 when R is calculated using BS EN 1992-3

c = coefficient of thermal expansion (See CIRIA C660 for values). See Table A6 for typical valuesT1 = difference between the peak temperature of concrete during hydration and ambient

temperature °C (See CIRIA C660). Typical values are noted in Table A7ca = Autogenous shrinkage strain – value for early age (3 days: see Table A9)R1, R2,R3

= restraint factors. See Section A5.6For edge restraint from Figure L1 of BS EN 1992-3 for short- and long-term thermal and long-term drying situations. For base-wall restraint they may be calculated in accordance withCIRIA C660. Figure L1 may be used with CIRIA C660 methods providing an adjustment forcreep is made (See Figure A2 and note).For end restraint, where the restraint is truly rigid 1.0 is most often used, for instance ininfill bays. This figure might be overly pessimistic for piled slabs.

T2 = long-term drop in temperature after concreting, °C. T2 depends on the ambient temperatureduring concreting. The recommended values from CIRIA C660 for T2 are 20°C for concretecast in the summer and 10°C for concrete cast in winter. These figures are based on HA BD28/87[60] based on monthly air temperatures for exposed bridges. Basements are likely tofollow soil temperatures so T2 = 12°C may be considered appropriate at depth.

cd

ctu

=

=

drying shrinkage strain, dependent on ambient RH, cement content and member size (see BSEN 1992-1-1 Exp. (3.9) or CIRIA C660 or Table A10). CIRIA C660 alludes to 45% RH for internalconditions and 85% for external conditions.tensile strain capacity may be obtained from Eurocode 2 or CIRIA C660 for both short termand long term values


CIRIA C660 Cl 3.2

Short term(≡ 3 days)

Medium term(≡ 28 days)

Long term(≡ > 10000 days)

9.5 Minimum reinforcement

As,min = kc k Act (fct,eff /fyk)where kc =

=A coefficient to account for stress distribution.1.0 for pure tension.When cracking first occurs the cause is usually early thermal effects and the whole section is likelyto be in tension. If bending involved kc may be calculated and kc < 1.0

k ==

A coefficient to account for self-equilibrating stresses1.0 for thickness h < 300 mm and 0.65 for h > 800 mm (interpolation allowed for thicknessesbetween 300 mm and 800 mm).

Act = area of concrete in the tension zone just prior to onset of cracking. Act is determined from section properties but generally for basement slabs and walls is most often based on full thickness of the section.

fct,eff == fctm

mean tensile strength when cracking may be first expected to occur: for early thermal effects 3 days for long-term effects, 28 days (which considered to be a reasonable approximation)See Table A5 for typical values.

fyk ==

characteristic yield strength of the reinforcement.500 MPa

[1] CIRIA C660 Recent research[61] would suggest that a factor of 0.8 should be applied to fct,eff in the formula for crack inducing strain due to end restraint. This factor accounts for long-term loading, in-situ strengths compared with laboratory strengths and the fact that the concrete will crack at its weakest point. TR 59[62] concludes that the tensile strength of concrete subjected to sustained tensile stress reduces with time to 60–70% of its instantaneous value.

Provision of minimum reinforcement does not guarantee any specific crack width. It is simply a necessary amount presumed by models to control cracking such that Fts ≥ Ftc ; but not necessarily a sufficient amount to limit actual crack widths.


BS EN 1992-1-1 Exp (7.1)Tightness Classes

Crack widths and watertightness


BS EN 1992-3 Cl 7.3

Tightness Classes - notes

Crack widths and watertightness


BS EN 1992-3 Cl 7.3

Crack widths and watertightness – recommendations for basements (TCC)Construction typea and water table

Expected performance of structure

Crack width requirement Tight-ness Class

wk mmFlexuralwk,max

Restraint/ axial,wk,1

A Structure itself is not considered watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure

0 0.30 0.30e

B – highpermanently high water table

Structure is almost watertight

Design to Tightness class 1 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm to 0.05 mm for cracks that pass through the section

1 0.30b 0.05 to 0.20 (wrt hd/h)

B – variablefluctuating water table

Structure is almost watertight

Design to Tightness class 1 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm for cracks that pass through the section

1 c 0.30 b 0.20

B – lowd

water table permanently below underside of slab

Structure is water-tight under normal conditions. Some risk under exceptional conditions.

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structures

0 c 0.30 0.30

C Structure itself is not necessarily considered watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure. Design to Tightness Class 1 may be helpful for construction type C

0

(1)c

0.30

(0.3)

0.30e

(0.05 to 0.20 or 0.20)

Key b Where the section is not fully cracked) the neutral axis depth at SLS should be at least xmin (where xmin > max {50 mm or 0.2 × section thickness}) and variations in strain should < than 150 × 10–6.


Concrete Basements


Crack width calculations

Crack width, wk = sr,max crwhere

sr,max = Maximum crack spacing = 3.4c + 0.425 (k1k2 /p,eff)

cr = Crack-inducing strain = Strain between cracks = Mean strain in steel – mean strain in concrete, (sm - cm ). . . . . .

wherec = nominal cover, cnomk1 = 0.8

(CIRIA C660 suggests 1.14)k2 =

==

1.0 for tension (e.g. from restraint)0.5 for bending(1 + 2)/21 for combinations of bending and tension

= diameter of the bar in mm.p,eff = As/Ac,eff

Ac,eff for each face is based on 0.5h; 2.5(c + 0.5); (h – x)/3 whereh = thickness of section and x = depth to neutral axis.


BS EN 1992-1-1 Exp (7.8)

NDP’s

S0S0S0S0

(cs - cm ): Consider a crack in a section:

sm - cm

εsm

εcm

ε = 0

εsm

εcm

ε = 0

ctuStrain

Plan (or section)

Strain in reinforcement

Strain in concrete

εc

εs

εc

εs

Sr,max


cm ≈ ctu /2wk = sr,max cr = sr,max (sm - cm)

Test for restraint cracking




Short term(≡ 3 days)

Medium term(≡ 28 days)

long term(≡ > 10000 days)

If r ≡ sm and we assume the section cracks, then to go from r to cr

we allow for the ‘tension stiffening’, by deducting cm ≈ ctu(t) /2

So:

cr = sm - cm = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] - ctu(t) /2

cr = Crack-inducing strain = . . . . . . . . . . . . . . .

a) Early age crack-inducing strain

cr = K[cT1 +ca R1 – 0.5 ctu

b) Long term crack-inducing strain

cr = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] – 0.5 ctu


CIRIA C660 Cl 3.2

CIRIA C660 Cl 3.2

cr = Crack-inducing strain = . . . . . . . . . . . . . . .

c) End restraint crack-inducing strain

cr = 0.5e kckfct,eff [1 + (1/e ) /Es

d) Flexural (and applied tension) crack-inducing strain

cr = (sm – cm) = [s – kt (fct,eff /p,eff) (1 + e p,eff /Es

cr 0.6 (s)/Es


BS EN 1992-3 Exp (M.1)

BS EN 1992-1-1 Exp (7.9)

See Concrete Basements Section 9.7

Good practice:

• Crack control without direct calculation: don’t do it!

• Crack widths – keep restraint and flexural cracking separate!

• Deflection control - as per ‘normal’ design

• Minimise the risk of cracking:

Materials use cement replacements, aggregates with low c, avoid high strength concretes

Construction construct at low temperatures, use GRP or steel formwork, sequential pours

Detailing: use small bars at close centres, avoid movement joints, prestress


Concrete Basements


• Game changer!• Internal restraint:

T1 becomes large and internal restraint (difference in temperature and stiffness between core and surface) dominates (external restraint still relevant).

(Thick sections > say 750 mm)

CIRIA C660 Fig 4.18


Time

• Game changer!• Internal restraint:

T1 becomes large and internal restraint (difference in temperature and stiffness between core and surface) dominates (external restraint still relevant).

– Try to restrict differential temperature across section.– Insulate (cf steel shutters).– See CIRIA C660 (and HA BA 24/87, HA BD 28/87).


• Game changer!• SLS Analysis:

For rafts especially, all the analysis is a waste of time unless one knows:-

– Precise properties of the concrete being used.– Detailed pour layout.– The ambient temperature of the soil beneath the raft.– Residual strains after the concrete first cracks– Going by experience even large diameter piles offer

little or no restraint to thick slab movement (See C660 Annex A5)

– etc.

• Pass the problem to specialists!






Concrete Basements

Basement example

Slab 300 mmWalls 250 mmGFS 250 mm

C30/37 Class R cement

wk max =0.2 mm

Structural design ‐ Example

Concrete Basements


Basement reinforcement

516 B21091 T2

362 T2 656 B2

Middle strip

H20 @100 B2 & T2 (3140 mm2/m)

631 B21360 T2

789 T22118 B2

Column strip

Span#Support#

SLS: Reinforcement for 0.20 mm crack width assuming endrestraint

ULS: Reinforcement for vertical and uplift cases (mm2 /m)

Asreqd slab (as an upside down flat slab)

# NB Min = 870 mm2/m T and B


Characteristic actions on basement wall and adjacent slabs: LC1 water at ground level

Combination 1 Combination 2


Characteristic actions on basement wall and adjacent slabs: LC2 no water

Combination 1 Combination 2

Compaction pressure


Basement wall moment envelope, ULS

Ground w

ater

No

groundw

ater


Basement wall reinforcement

904(LT edge restraint)


466Inside face cnom = 30 mm



466Outside face cnom = 50 mm

Horizontal (horizontally restrained)

-725i.e. min 1450 /2)(

469Outside faceBottom

-No change907Inside face Middle

-725 (i.e. min 1450 /2)

469Outside face Top

Vertical (vertically unrestrained)

CIRIA C660EC2-1-1, EC2-3EC7, EC2-1-1 etc.

SLS (0.2 mm crack width)ULSLocation

Asreqd (mm2 /m) wall


Lessons:

Slab

SLS dictates – even with uplift

End restraint = lots of reinforcement

Wall - Vertical rebar

Loads and load cases a nightmare but necessary

Minimum steel provides enough moment capacity in most places

Wall - Horizontal rebar

SLS dictates – use CIRIA C660!

Cover critical

Mitigating measures?


Concrete Basements


Mitigating measures? :

Slab – lower strength and/or thinner

H20@90B2 >> H20@125B2 if C25/30, cnom = 40 and kc = 0.8

H20@90B2 >> H20@110B2 if h = 250 mm


Design to EC2-3Original design

EC2-3 CIRIA C660

Concrete C30/37 C30/37 C25/30 C30/37 C25/30

Cement Class R N N N N

Outside reinf. H20@100 H20@160 H16@125 H12@125 H12@150

As,prov 3140 1962 1608 904 753

As,prov/As,prov orig 100% 63% 51% 29% 24%

Influence of T1 on Horizontal rebar in wall -

Cement class

Binder content for a C30/37 (indicative)


Mitigating measures – Concrete strength and Cement class (type)





Concrete Basements

Specification:• BS EN 13670• NSCS / NBS• ICE specification for piling and embedded retaining walls

Joints• Construction joints• Water stops

Miscellaneous• Kickers• Formwork ties• Membranes & coatings• Admixtures & additives• Service penetrations• Drainage• Underpinning

Inspection, remedials & maintenance

• Preformed strips –PVC, black steel• Water-swellable water stops • (Re) injectable epoxy water bars


Specification –National Structural Concrete Specification(NSCS)


Materials

Inspections

Waterstops

Ties

Concrete Basements


Kickers

Contractors’ choice of materials

Inspections

Performance Spec

Guidance

Additives

Ties

Joints

Waterstops

NSCS Max pour sizes

Table 1: AREAS AND DIMENSIONS FOR DIFFERENT TYPES OF CONSTRUCTION.

1040 Walls

30500 Slabs with little restraint in any direction

20250 Slabs with major restraint at one end only

13100 Slabs with major restraint at both ends

10100 Water – resisting slabs

525 Water – resisting walls

Maximum Dimension (m )

Maximum Area (m2 ) Construction

“Unless otherwise agreed”. . . . . .and designed.


Workmanship is key

Supervision?

Forms of contract

Risk vs £

Discuss with client!

Construction

Specification and construction





Concrete Basements Concrete Basements

Case study - institutional

Concrete Basements


Salient features

• 4 m deep basement (depth of excavation about 5 m)

• Use – archives, exhibition and public spaces

• Soil – gravels

• Water table about 1 m below ground level

• Propped secant piling to facilitate excavations

• Concrete box designed to be inside the secant piles

• Vapour barrier membrane sandwiched between the piles (faced with polystyrene) and the concrete box

• Drained cavity inside walls and above floorBy courtesy Clark Smith Partnership

GA

GA & details: Temp works

Sections Construction methods

Concrete Basements


Concrete Basements

Pictures!The Fusion Shoreditch

The Fusion ShoreditchConcrete Basements

Case study - residential

Concrete Basements

Domestic case study

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Concrete Basements


Specification:

Figured dims

CDM Regs

Imposed Load

Concrete

Steelwork

Steel Paint Spec

Welds, bolts & bolting.

Fab. drawings

Timber

Doubling joists

Masonry

Padstones

Dry pack mortar

Formations

Temp. Works

Extg structure

Drains

Bed joint reinf.

Straps

Ties.

A London basement My house (I wish!)

Concrete Basements






Having done it once go back and refine it, again,

and again preferably in league with the constructor

Concrete Basements: Summary Concrete Basements

This guide covers the design and construction of reinforced concrete basements and is in accordance with the Eurocodes.

The aim of the guide is to assist designers of concrete basements of modest depth, i.e. not exceeding 10 metres. It will also prove relevant to designers of other underground structures. It brings together in one publication the salient features for the design and construction of such water-resisting structures.

The guide has been written for generalist structural engineers who have a basic understanding of soil mechanics.

Basement issues.

• Clients don’t understand Grades 1, 2 and 3.

• Many specifications and designs looking for two or even 3 types of water resisting construction (membrane + integral + drained cavity). Within BS8102 but £££?

• NHBC looking for combination of 2 types

• Members of the WG convinced that water resisting construction is a lot to do with workmanship. Admixtures are a load of ********– which are just insurance policies.

• Designers saying ‘we’ve designed it properly now any cracks will be down to workmanship’. Concentrates minds!

CS meeting 8/3/16

Charles GoodchildCEng., MCIOB, MIStructE

Principal Structural Engineer

The Concrete Centre

Concrete BasementsGuidance on the design and construction of in‐situ concrete basement structures

Thank you

Documents

Basements CHG v19- 19 Apr 2016 Basements_The Concrete Centre.pdfGuidance on the design and construction of in‐situ ... • Fire safety ... basement slab load, upward water pressure,