Worked example 4 Differential shrinkage
36 Expanding material
to plug gap Friction can cause cracking Movement Rotation If no plug, hard material can prevent rotation
Rotation
Rotation can cause spalling Figure 4.1
Examples of potential failures at movement joints.
It is possible to deal with movement at bearings using movement joints, and care should be given to the design and construction, as for bridge decks, to minimise the risk of failures. In general it is recommended to seek solutions that do not require movement joints. Figure 4.1 describes potential failure mechanisms that can occur even with a structural topping.
4.3 Movement joints
If the bearing material creates large friction forces (use neoprene or similar to avoid this), this can lead to large tension stresses in both the support and the precast slab or beam.
If the space between the precast slab or beam and the face of the supporting member is not adequate for the required movement or if in time it it fi lls up with hard material, then cracking can occur.
If the effects of movement and/rotation cause the line of action to move too close to the edge of the support, local spalling can occur.
4.4 Actions and restraints
4.4.1 Action effects
In addition to the effects of direct loading (imposed variable and permanent actions) the following action effects on the elements supported by the bearing must be considered:shrinkage (both long term and early thermal) temperature changes (both seasonal and short term) creep.
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In addition to the above action effects the following restraints must be considered: internal, e.g. from reinforcement, differential shrinkage
edge restraints end restraints.
For detailed consideration of these effects and restraints refer to Movement, Restraint and
Cracking in Concrete Structures26.
When designing bearings the following details should be checked: calculation of the bearing area
bearing layout
the detail of the reinforcement in the end of the supported member the detail of the reinforcement in the supporting member
tolerances
construction issues – especially any additional forces imposed on the bearing through ‘barring’ the units into final position, see Section 6.8.
The design and detailing of the reinforcement at supports is critical. The supported member has to be designed to bear safely onto the support without spalling of the end cover and also to sustain any forces that may come from shrinkage of the fl oor, through shortening of the fl oor, if prestressed, and from thermal, live and further dead load movements, see also Section 4.1.
Prestressed members used for fl ooring are commonly pre-tensioned and the main prestressed steel continues to the end of the member. Reinforcement in supporting and supported members should be detailed to ensure effective anchorage, allowing for deviations, see Figure 4.2.
di = ci + Δai with horizontal loop bars
di = ci + Δai + ri with vertically bent bars
ci = nominal concrete cover
Δai = a deviation (see Section 4.8)
ri = radius of bend (see Table 4.1)
4.4.2 Restraints
4.5 Design considerations
4.6 Allowance for
anchorage of reinforcement
at supports
Design of Hybrid Concrete Buildi37 37
38 c2 >a1+Da2 d3 r2 r3 d2 >a1+Da3 c3 Figure 4.2
Effect of reinforcement on bearing dimensions.
Table 4.1
Minimum bend radii for reinforcement to avoid damage to reinforcement.
Bar diameter Minimum radius of bend
φ ≤ 16 mm 2 φ
φ > 16 mm 3.5 φ
Bearings that allow limited movement, e.g. neoprene pads, not only distribute the bearing forces over uneven supports but also allow limited rotational and longitudinal movement of the supported member to take place. The bearing pad also defi nes the area of load transfer and thus has a direct effect on the detailed design of the ends of the supporting and supported members.
In the absence of other specifi cations, the bearing strength, fRd = fbed ≤ 0.85 fcd where fbed is the design strength of the bearing material may be used.
The layout of a bearing is critical to its successful execution. The concrete surfaces must be separated in areas where load transfer is not intended and must be bedded appropriately where load transfer is required. To ensure that spalling does not take place in the contact area at the end of the supported and supporting concrete, the provision of suffi cient bearing length must be provided. This should allow for constructional tolerances and ensure the overlap of reinforcement between the supporting and supported concrete. The required allowances are shown in the Figure 4.3 and are described in Eurocode 2, Cl. 10.9.5.2. These will lead to the design of minimum bearing shelf and nib sizes.
4.7.1 Design of the bearing
area
4.7.2 Bearing layout
4.7 Bearings that allow
limited movement
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The nominal length, a, of a simple bearing may be calculated as:
a = a1 + a2 + a3 + √(Δa22 + Δa 32)
where
a1 = net bearing length with regard to bearing stress = FEd /(b1fRd) but not less than the values in Table 4.2
FEd = design value of the support reaction
b1 = net bearing width
fRd = design value of the bearing strength
= 0.85fcd
a2 = distance assumed ineffective beyond outer end of supporting member
(see Table 4.3)
a3 = distance assumed ineffective beyond outer end of supporting member
(see Table 4.4)
Δa2 = allowance for distance between supporting members (see Table 4.5)
Δa3 = allowance for deviation of the length of the supported member
= ln /2500 ln = length of member in mm b1 a1 >Da2+Da3 a3+Da3 a1 a a2+Da2 Figure 4.3
Critical dimensions for bearings.
Relative bearing stress, σEda/f
cd ≤ 0.15 0.15 to 0.4 > 0.4
Line supports (fl oors and roofs) 25 30 40
Ribbed fl oors and purlins 55 70 80
Concentrated supports (beams) 90 110 140
Key:
a σEd is the design bearing stress
Table 4.2
Minimum value of a1 (mm).
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Relative bearing stress, σEda/f
cd ≤ 0.15 0.15 to 0.4 > 0.4 Reinforced concrete ≥ C30/37 Line Concentrated 5 10 10 15 15 25 Reinforced concrete < C30/37 Line Concentrated 10 20 15 25 25 35 Key
a σEd is the design bearing stress
Detailing of reinforcement Type of support
Line Concentrated
Continuous bars over support (restrained or not)
0 0
Straight bars, horizontal loops, close to end of member
5 15, but not less than end cover Tendons or straight bars
exposed at end of member
5 15
Vertical loop reinforcement 15 End cover + inner radius of bend
Support material Δa2
Precast concrete 10 ≤ l /1200 ≤ 30 mm Cast in-situ concrete 15 ≤ l /1200 + 5 ≤ 40 mm
Note:
l is clear distance between supports in mm
An example calculation is shown in worked example 5.
Table 4.4
Distance a3 (mm) assumed ineffective from outer end of supported member.
Table 4.5
Allowance for deviations for the clear distance between the face of the supports.
Table 4.3
Distance a2 (mm) assumed ineffective from outer end of supporting member.
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