Member strength checks (ULS) (Composite beams: BS 5950)
Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam.
Shear capacity (Vertical)
The vertical shear resistance is determined in accordance with clause 4.2.3 of BS 5950-1:2000. Where the applied shear force exceeds 50% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below).
Shear capacity (Longitudinal)
The longitudinal shear resistance of a unit length of the beam is calculated in accordance with clause 5.6. You can set the position and attachment of the decking and details of the reinforcement that you want to provide. Tekla Structural Designer takes these into account during the calculations.
The following assumptions are made:
- the applied longitudinal shear force is calculated at the centre-line of the beam, and at the position of the lap (if known). If the position of the lap is not known, then the default value of 0mm should be used (that is the lap is at the centre-line of the beam) as this is the worst case scenario.
- the minimum concrete depth is assumed for calculating the area of concrete when the profile decking and beam spans are parallel,
- the total concrete area is used when the profile decking and beam spans are perpendicular,
- the overall depth of the slab is used for precast concrete slabs. that is the topping is assumed to be structural and any voids or cores are ignored.
In the calculations of the longitudinal shear resistance on the beam centre-line and at the lap, the areas used for the reinforcement are shown in the following table.
Decking angle | Reinforcement type | Area used |
---|---|---|
perpendicular | transverse | that of the single bars defined or for mesh the area of the main wires^{[1]} |
other | that of the single bars defined or for mesh the area of the main wires^{[1]} | |
parallel | transverse | that of the single bars defined or for mesh the area of the main wires^{[1]} |
other | single bars have no contribution, for mesh the area of the minor wires^{[2]} |
^{[1]}These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1
^{[2]}These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1
The contribution to longitudinal shear of the profiled decking, v_{p}, is dependent on:
- whether the ribs are perpendicular or parallel
- whether the beam is in an internal or edge condition
- the effectiveness of the attachment of the deck to the flange
In some cases, the contribution of the decking will be zero. Also, where the decking is discontinuous, a reduction in v_{p} can be necessary based on a minimum distance of the shear connectors from the edge of the deck.
The reduction factor for shear connectors in close proximity to the edge of the decking i.e. closer than 1.5 * d_{s} is given by
k_{ds} = min(1.0, (0.4 + 0.4 * e_{s}/d_{s}))
where e_{s} is the “Edge distance from centre of connector”; defaulted to 30 mm and d_{s} is the stud diameter.
If the decking spans at some intermediate angle (α) then the program calculates:
- the longitudinal shear resistance as if the sheeting were perpendicular, v_{1},
- the longitudinal shear resistance as if the sheeting were parallel, v_{2},
- then the modified longitudinal shear resistance is calculated from these using the relationship, v_{1}sin^{2}(α) + v^{2}cos^{2}(α).
Moment capacity
for the low shear condition the plastic moment capacity is determined in accordance with clause 4.4.2. For the high shear condition the approach given in clause 5.3.4 is adopted.
The overall depth of the slab is used for precast concrete slabs. That is the topping is assumed to be structural and any voids / cores are ignored.
In this calculation the steel section is idealized to one without a root radius so that the position of the plastic neutral axis of the composite section can be determined correctly as it moves from the flange into the web.