📄 Construction-Stage Torsion, Propping and Deflections in Composite Beam Design

Construction-Stage Torsion, Propping and Deflections in Composite Beam Design

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Overview

Composite beams behave differently during construction and at final stage. Before the concrete has cured, the beam generally acts as a non-composite steel beam. After the slab has cured and shear connection has developed, the beam acts compositely.

This is important for:

  • ASB beams with precast units

  • unequal slab spans each side of a beam

  • unpropped construction

  • construction-stage torsion

  • deflection comparisons between MasterFrame and Composite Beam Design

1. Construction-stage torsion

Where a beam supports slab or precast units on both sides, MasterSeries checks possible out-of-balance loading during construction.

For example, if one side of the beam supports a longer span than the other, the temporary loading may cause:

  • bending

  • torsion

  • shear

  • combined torsion and bending

The software checks the possible one-sided loading condition to identify the worst construction-stage torsional case.

2. Full construction-stage loading

The software may also check a case where both sides of the beam are loaded during construction.

This can produce a larger bending moment, even though torsion is reduced or removed because the loading is more balanced.

Therefore, it is possible for:

  • the one-sided torsion check to pass, but

  • the full construction-stage bending check to fail.

These are different temporary load cases.

3. Construction sequence limitations

The standard construction-stage checks are based on typical SCI composite beam design assumptions.

If the actual site sequence is more specific, for example:

  1. steel beam erected

  2. precast units placed

  3. beam/slab propped

  4. topping poured later

then not every intermediate temporary stage may be explicitly checked by the software.

If a particular temporary stage is critical, the engineer may need to assess that case separately.

4. Beam propping and slab propping

MasterSeries includes two different propping assumptions.

Beam propping

This assumes the beam itself is supported during construction. It removes construction-stage deflection and bending in the steel beam.

MasterSeries does not design the props or calculate the number, spacing, capacity or layout required.

Precast Slab propping

Construction stage props assume that precast slab units are temporarily supported with 0-4 props evenly distributed along the slab span, hence reducing the construction load transferred to the beam. If the beam has been propped then propping the slab will provide no benefit, however if the beam is unpropped, propping the slab can significantly reduce the construction loading on the beam.

Both options can be used together if they reflect the intended construction method, but the propping arrangement should be confirmed with the contractor.

5. Beam restraint from precast slabs

A precast slab bearing on a beam will provide the beam with a certain amount of restraint against lateral torsional buckling. 

Beam lateral restraint

The amount of lateral restraint provided to the beam at construction stage can be modeled in terms of 0-4 lateral restraints evenly distributed along the beam.

6. Why deflections change with propping

Composite beam deflection depends heavily on when the load is applied.

Unpropped construction

Dead load from wet concrete is applied while the beam is acting as steel-only, so dead-load deflection can be relatively large.

Live load is applied later, after composite action has developed, so the beam is stiffer and live-load deflection may be smaller.

Propped construction

If the beam or slab is propped, some dead load is applied only after the concrete has hardened. When props are removed, the composite beam attracts load and the shear studs may slip. This can affect superimposed dead and live-load deflections.

This is why deflections may appear to change unexpectedly when slab props are introduced.

7. MasterFrame vs Composite Beam Design deflections

MasterFrame analysis deflections and Composite Beam Design deflections may not match exactly.

The Composite Beam Design module accounts for:

  • construction-stage behaviour

  • final composite behaviour

  • partial shear connection

  • load sequence

  • creep and long-term modular ratios

MasterFrame graphical results use the analysis model stiffness assumptions and do not reproduce the full staged composite design calculation.

Key takeaway

  • Construction-stage checks are separate from final composite checks.

  • Unpropped beams act as steel beams during construction.

  • One-sided loading may govern torsion.

  • Both-sided construction loading may govern bending.

  • Propping changes the load history and therefore deflection results.

  • MasterSeries allows propping assumptions but does not design the props.

  • Differences between MasterFrame and Composite Beam Design deflections are expected.