I Struct E Part 3 Exam Preparation

Concrete

Charles Johnson

When to use concrete In building superstructures

1. Minimum depth

Flat slab- passive steel or post-tensioned

Ribbed or waffle slab

2. Complex floor shapes

3. Acoustics

4. Vibration

5. Steel supply problem

6. Personal preference

7. Cost

Concrete Design The Big Issues

1. Shrinkage and thermal cracking

2. Durability

3. Weight

4. Workmanship / Finishes

1. Shrinkage and thermal cracking

In UK buildings:

Mostly shrinkage

Long term.

Total Long Term Strain K ~ 300 x 10-6

Joint spacing:

Internal: 50 - 75 m

Un-insulated roof slabs: 25m

Beware of effect of shrinkage on distant cores

1. Shrinkage and thermal cracking

In UK structures exposed to ground/water

only consider early shrinkage - little long term drying

Drying shrinkage K ~ 100 x 10-6

Thermal effects relatively more important, especially in massive structures with high hydration heat.

K ~ 12 x 10-6 /degree C

2. Durability

Not normally important issue in internal environments

[cover controlled by fire]

External:

Cement Content

Cover

Concrete Density (i.e. compaction & curing)

Special protection against aggressive conditions

Sulphates BS8110 Tab 6.1

Chlorides

3. Weight

Say 300mm ave. conc thickness per floor. = 7.5 kN/m2

Equiv. steel build: 120 conc + 1.5 kN/m2 beams = 4.5 kN/m2

Say three floors, and footings ~ 20% of footprint:3 kN/m2 x 5 = 15 kN/m2 extra bearing pressure per floor.

x3 floors = 45 kN/m2

Or, Say 6m x 5m grid and 150 kN/m2 safe bearing pressure:Concrete solution needs 1.8m2 larger pads than steel

Pre-cast components: Beware of transport limits:

~ 40T total? 10T axle load?

25T mobile crane: ~ 10T capacity @ 7m radius (i.e. 4m3)

Initial sizing-1-: Basic L/D

Beams: L/10 - L/20 Slabs: L/25 - L35

On beams: Use short span Flat slabs: Use long span

Cantilevers: L/5 - L/8 Corbels: L/1 - L/3

Initial sizing-2-: Bending

As = M .0.95fy 0.8d

[in finding d remember links/distribution steel ]

As < ~ 2% . Less on very large sections. Also check no compression steel needed: M . < 0.156* or M < 0.156* fcubd2

bd2fcu

* Varies if redistributing moments (down to 0.104) b = width of compression zone. Check bars fit comfortably / how many rows?

Initial sizing-3-: Deflection

BS8110 basic L/D Tension steel factor Factor for steel in compression zone (even if not

needed for compression in bending) Best to add compression steel, but check quantity no

too great uneconomic.

Initial sizing-3-: Shear

First check to do in retaining walls, foundations, pile caps, etc.

Avoid shear reinforcement in these if poss. So size section assuming:

vc = ~0.5 N/mm2 for 0.5% As= ~0.7 N/mm2 for 1.0% As

Also important in flat slabs. Good initial sizing charts in Scheme design guide if you want

to avoid shear links (vc only). If not... First perimeter length = col. perimeter + 12d Min links = 0.4 N/mm2. So v~ 1.0 N/mm2

Columns

For short columns Le/b < 15): From N = 0.35Acfcu = 0.7 Acp fy

p=1% N = 15Acp=2% N = 18Acp=3% N = 21 Ac

Min column size 220 x 220 Check min size and cover for fire resistance

Specially over 2h.

Reinforcement quantities

Volume of steel x 7800 = Wt of steel in kg For reinforcement estimates:

Calc. Vol of steel/ known m3 of conc. in sample areas Per (1m2 of slab/wall x h) Per (1m length of beam x A)

+ 10 for laps, + 10% contingency x 7800

Shear Walls and cores

Minimum thickness 200mm but may have problems with areas of steel congestion (e.g. beam junctions).

So unless quite simple say 250min Avoid using risers as cores: large holes above ceilings. Lifts OK but assume no wall on door side Stairs OK but risk of Arch. wanting it open later. To estimate As

Use Z~ 0.5d. Deduct min dead load from wind tension. Ideally no resultant

tension at working loads, then no cracking.

Stairs

May need greater crowd load capacity Decide form of support

Avoid support on masonry.- Difficult construction sequence On conc. walls: use bend-out bars or post-drill

Contractors will usually prefer to pre-cast Min depth: support flights on side of landing, and landing onto

side wall. (take span of flight to centre line of landing) Simplest: span flights + landings onto end walls For weight: use av. vertical thickness For bending strength use min (slope) thickness.

Water-retaining/excluding

BS8007. Concrete Grade C35A Generally design for 0.2mm crack width instead of the 0.3mm

implied in BS8110 Check for flexural crack width

Use design chart e.g. Batty and Westbrook Moment capacity limited by crack width, which is also dependent on

bar size and bar spacing. Check for thermal and shrinkage cracking

Affects distribution/minimum steel Full external restraint; and steel must control all shrinkage cracks,

or Enough joints to permit free shrinkage

Bending design chart for limiting

service crack width.(From Batty and

Westbrook)

When to use concrete In building superstructuresConcrete Design The Big Issues1.Shrinkage and thermal crackingIn UK buildings: 1.Shrinkage and thermal cracking In UK structures exposed to ground/water2. Durability3. WeightInitial sizing-1-: Basic L/DInitial sizing-2-: BendingInitial sizing-3-: DeflectionInitial sizing-3-: ShearColumnsReinforcement quantitiesShear Walls and coresStairsWater-retaining/excludingBending design chart for limiting service crack width.(From Batty and Westbrook)