Composite Beams Design

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. The stiffness of a composite floor is substantially greater than that of a concrete floor and its supporting beams acting independently. In addition, a 20 to 30% savings 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 program performs the design of a simply supported, either interior or border W-section steel beam supporting a concrete slab in composite action subjected to uniformly distributed loads, taking into account the different stages of loading depending on the construction method used. In addition, the program computes the number and spacing of the shear connectors required to develop either full or partial composite action, as well as the maximum deflection under live load.

Since the program works with a database of W-section properties, it is possible to select a steel beam to be analyzed without the necessity of entering its geometric properties. In addition, the program calculates the properties for the composite section and the steel section alone, useful when a bottom cover plate is used.

Input Data

The required input data consists of the beam and slab dimensions, the material strengths, the bottom cover plate data, and the method of construction used. In addition, two types of service loads are required:

a) Construction load is the load (in excess of self-weight) that is applied BEFORE the concrete has reached 75% of its required strength, and is resisted by the beam itself, such as the weight of workers and formwork.

b) Superimposed additional load is the load applied AFTER the concrete has reached 75% of its required strength, such as live load, partitions and ceiling.

The program requires the stud diameter to be specified in order to design the shear studs. Figure to the right shows the required input data.

Example

Design an interior member of the fully-composite floor system shown below, assuming it is constructed without temporary shoring and subjected to a live load of 182 psf. The 5" concrete slab is on W14x26 beams with 6 " x 3/4" cover plate. Find out the required number of 5/8" shear studs. Use f'c = 3000 psi and Fy=50 ksi..

Output

To specify the W-section to be analyzed choose the option Select from the Design menu or double-click on the Designation field in the TEMPLATE page. The program automatically computes the properties and stresses in the composite section depending on the construction method specified, and compares them with the allowable values.

The program computes the maximum live-load deflection, which is also given as a function of the beam length, in order to be compared with the allowable maximum deflection of L/360.

In addition, the program calculates the required number of studs and spacing between pairs for the specified stud diameter to be used. For a single line of studs, use a half of the calculated spacing.