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
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
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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
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Basement Information Centre 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 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
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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
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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
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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 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
Construction methods
Concrete Basements
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Construction methods
Contiguous Piled Wall Contiguous Piled Wall
http://www.oasys-software.com
600 mm diam. Rakers every 2 m
Construction methods
Contiguous Piled Wall
Construction methods
Secant Piles
Construction methods
Secant Piles
Construction methods
Facing wall (courtesy GCL Ltd)
Construction methods
Diaphragm Wall
Concrete Basements
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Sheet Piles
Construction methods
http://www.terraingeotech.com/index.html
Construction methods
Propped Excavation
Construction methods
Propped Excavation
Construction methods
Reality
Top Down Construction
Dig
Move
Raise
Cart away
Grouting
Concrete Basements
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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 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
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Proprietary cementitious multi‐coat renders, toppings and coatings
Type 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
Types B & C construction:ConcreteAdmixtures for watertightness
Selection of materials
Selection of materials
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
Selection of materials
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
Selection of materials
£££vs
££££ ?
Whatever the basement should still be designed properly!
Cost and risk:
Type 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
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
Selection of materials
Concrete Basements
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Waterbar
Photo credits Watermans
Construction, inspection and testing
HydrophilicsPhoto credit Watermans
Construction, inspection and testing
Resin injectionPhoto credit Max Frank
Construction, inspection and testingCavity drain membranes…high density dimpled polyethylene sheets
Cavity drain Sump pump
1800
Concrete Basements
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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
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.
Structural design ‐ Loads
Decoding Eurocode 7 Fig 10.8
Calculation of lateral earth pressures
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
Structural design ‐ Loads
Concrete Basements
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Calculation of lateral earth pressures
Pressure coefficients: K0d, Kad or Kpd?
Structural design ‐ Loads
• 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
Structural design ‐ SLS
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
Structural design ‐ SLS
Restraint factors
Concrete Basements
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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
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
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.
Structural design ‐ SLS
BS EN 1992-1-1 Exp (7.1)Tightness Classes
Crack widths and watertightness
Structural design ‐ SLS
BS EN 1992-3 Cl 7.3
Tightness Classes - notes
Crack widths and watertightness
Structural design ‐ SLS
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.
Structural design ‐ SLS
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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.
Structural design ‐ SLS
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
Structural design ‐ SLS
cm ≈ ctu /2wk = sr,max cr = sr,max (sm - cm)
Test for restraint cracking
A section will crack if:
r = Rax free = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] ctu
Structural design ‐ SLS
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
Structural design ‐ SLS
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
Structural design ‐ SLS
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
Structural design ‐ SLS
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• 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
(Thick sections > say 750 mm)
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).
(Thick sections > say 750 mm)
• 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!
(Thick sections > say 750 mm)
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
Basement example
Slab 300 mmWalls 250 mmGFS 250 mm
C30/37 Class R cement
wk max =0.2 mm
Structural design ‐ Example
Concrete Basements
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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
Structural design ‐ Example
Characteristic actions on basement wall and adjacent slabs: LC1 water at ground level
Combination 1 Combination 2
Structural design ‐ Example
Characteristic actions on basement wall and adjacent slabs: LC2 no water
Combination 1 Combination 2
Compaction pressure
Structural design ‐ Example
Basement wall moment envelope, ULS
Ground w
ater
No
groundw
ater
Structural design ‐ Example
Basement wall reinforcement
904(LT edge restraint)
1130(LT edge restraint)
466Inside face cnom = 30 mm
1608(LT edge restraint)
3140(LT edge restraint)
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
Structural design ‐ Example
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?
Structural design ‐ Example
Concrete Basements
ASELB 29 April 2016 18
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
Structural design ‐ Example
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)
Structural design ‐ Example
Mitigating measures – Concrete strength and Cement class (type)
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
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
Construction, inspection and testing
Specification –National Structural Concrete Specification(NSCS)
Construction, inspection and testing
Materials
Inspections
Waterstops
Ties
Concrete Basements
ASELB 29 April 2016 19
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.
Construction, inspection and testing
Workmanship is key
Supervision?
Forms of contract
Risk vs £
Discuss with client!
Construction
Specification and construction
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 Concrete Basements
Case study - institutional
Concrete Basements
ASELB 29 April 2016 20
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
ASELB 29 April 2016 21
Concrete Basements
Pictures!The Fusion Shoreditch
The Fusion ShoreditchConcrete Basements
Case study - residential
Concrete Basements
Domestic case study
Tha
nks
to
Chr
is
Bucz
kow
ski
Concrete Basements
ASELB 29 April 2016 22
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
ASELB 29 April 2016 23
Introduction/backgroundPlanning a basement• Types & Waterproofing strategy• Site Constraints
Ground movements & Methods of construction
MaterialsStructural design• Loads• ULS• SLS• Example
Specification and constructionCase studies
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