By: Javier Encinas, PE | November 26, 2019
ASDIP STEEL includes the design of steel non-composite or composite beams. This structural engineering software is based on the latest AISC 360 provisions. This article provides an engineering background overview of steel and composite beam structural design.
Beams are structural elements that support loads applied transversely, which produce a combination of shear and bending stresses. Steel beams can be designed as non-composite or as composite beams. If non-composite, the loads are resisted by the steel beam alone.
Composite action is developed when two load-carrying structural elements, such as a concrete floor slab and its supporting steel beams, are integrally connected and deflect as a single unit, substantially increasing its strength and stiffness. A reduction in steel weight is often possible by taking full advantage of a composite system. Since the concrete slab exists anyway and the shear connectors are inexpensive and easy to install, it is structurally advisable to use composite construction whenever possible.
The construction sequence effect in composite beam design.
The construction sequence plays an important role in the design of composite beams. Since the steel beam and the concrete slab will work structurally together, it’s important to consider the loads that will be applied before and after the composite action takes place. If the beam is unshored, the steel section alone will resist the beam selfweight and the concrete slab weight, but the composite section will resist the live load. On the other hand, if the beam is shored, then all the loads will be resisted by the composite section.
Depending on the construction sequence, the steel beam alone must be designed for the pre-composite stage. In this stage, the beam will be subjected to construction dead and live loads (CD+CL), defined as the loads applied before the concrete has reached 0.75 f’c. These loads will include the weight of the beam and slab, and the weight of the construction workers. Once the concrete has reached the 0.75 f’c boundary, the full loads may be applied to the composite section. These loads will include the full dead and live loads for the structure in service (D+L). The figure below shows an example of the shear and moment diagrams for both the pre and post-composite stages.
How do you design a steel non-composite beam?
In the construction stage the steel beam is not composite, and the compression flange, which is attached to the web in the plane of the beam, may or may not be laterally braced, thus the buckling concepts of compression members apply to beams as well. Steel beams design is affected by the location of the lateral bracing. ASDIP STEEL calculates the Cb factor and finds the unbraced length of each segment along the beam accordingly. With this information and the local buckling properties of the section, the shear and bending capacities are calculated for the different limit states.
The flexural behavior of a steel beam is a function of the unbraced length Lb for the limit state of lateral-torsional buckling.
It should be noted in the left figure that when the lateral support of the compression flange is adequate the bending strength is governed by yielding. In this case the section can be fully plastified and reach the plastic moment Mp.
How is the composite action achieved?
To achieve a composite action between the concrete slab and the supporting steel beam, shear connectors are usually provided to transfer the horizontal shear force across the interface and prevent vertical separation. Full composite action is achieved when the shear connectors are able to take the full shear, assuming that either the steel beam is fully plastified or the effective concrete slab is stressed to its maximum capacity in compression, whichever is less. When full interaction is not present, the beam is said to be partially composite.
It’s common practice to pour the concrete slab on formed steel deck, which may be oriented either parallel or perpendicular to the steel beam. For the perpendicular orientation, the concrete below the top of the deck should be neglected in determining the composite section properties. The AISC has established some geometric requirements for the deck-stud-slab system, as shown in the figure below.
What are the properties of the composite section?
When a beam is loaded, the bending resistance is provided by a couple of internal forces, one in tension and one in compression, times the lever arm between them. For the regions of positive moments the tension is provided by the plastic stress of the steel beam, and depending of the location of the plastic neutral axis, the compression is provided by the concrete slab. For the negative moment regions, the tension is provided by the rebars in the slab, and the compression is provided by the steel beam. To satisfy the equilibrium, both forces balance each other, this is, C=T. The figure below shows schematically the plastic stress distribution for both cases.
Conservatively, the AISC allows to consider the steel section alone (non-composite) for the negative moment capacity. When the compact steel section is braced and the rebars are adequately developed in the negative moment regions, the nominal flexural strength may be determined from the plastic stress distribution shown.
Composite beam design may be cumbersome and error-prone. ASDIP STEEL includes the design of steel and composite beams, with multiple options to optimize the design easily within minutes.
Detailed information is available about this structural engineering software by visiting ASDIP STEEL. For information on the use of ASDIP STEEL to design steel and composite beams see our blog post How to Design Steel and Composite Beams Using ASDIP STEEL.
Javier Encinas, PE
ASDIP Structural Software