In the context of composite beam design using the Allowable Stress Design (ASD) method, shored and unshored construction methods lead to different design considerations and stress limits due to the varying load-sharing mechanisms between the steel beam and the concrete slab.
Shored Construction:
In shored construction, temporary supports are provided during construction to carry the weight of the wet concrete and any additional loads until the concrete slab reaches its design strength. During this period, the composite section (steel beam and concrete slab) acts together to resist all loads, including the self-weight of the beam, slab, and any additional loads.
The design of a shored composite beam generally involves:
Calculating the composite moment of inertia (Ic) for the transformed composite section.
Determining the moment capacity of the composite section (Mc) using the composite moment of inertia.
Checking the steel beam for its ability to resist the loads until the concrete slab reaches its design strength.
The allowable bending stress for the composite section is based on the normal ASD limits, typically 0.66Fy for bending stress and 0.9Fy for shear stress in the web of the steel beam.
Unshored Construction:
In unshored construction, the steel beam alone resists the weight of the wet concrete and any additional loads until the concrete slab gains sufficient strength. After the concrete reaches its design strength, the composite section resists the additional live loads.
The design of an unshored composite beam generally involves:
Calculating the transformed moment of inertia (It) for the steel beam alone.
Determining the moment capacity of the steel beam (Mt) using the transformed moment of inertia.
Checking the composite section for its ability to resist the additional live loads.
In unshored composite beam construction, the allowable bending stress for the steel beam can be increased by a factor of 1.15 (or the allowable stress can be raised to 0.9Fy) during the construction period when the composite section is not fully effective. This increased stress limit is applicable only during the construction phase when the steel beam is subjected to temporary loading conditions. Once the composite action is fully effective, the allowable stress should be based on the normal ASD limits.
In summary, the primary difference between shored and unshored composite beam design in ASD lies in the distribution of loads between the steel beam and the concrete slab, which impacts the stress limits used during the design process.
