By: Javier Encinas, PE | March 27, 2017
Beams are structural elements that support loads applied transversely, which produce a combination of shear and bending stresses. The structural design of steel beams includes the check of multiple limit states for both shear and bending. For composite beams, it’s important to consider the loads that will be applied before and after the composite action takes place.
In the pre-composite stage, the flexural behavior of a steel beam is a function of the unbraced length Lb for the limit state of lateral-torsional buckling. In a composite beam, shear connectors are usually provided to transfer the horizontal shear force across the interface and prevent vertical separation. The number and spacing of connectors will determine whether the section is fully or partially composite.
In addition to the strength checks, the design of a steel or composite beam includes the check of deflections under the action of different loads in both the pre and post-composite stages. The calculated deflections will be checked against allowable limits.
Evidently all the design checks described above are dependent on the structural properties of the steel and composite sections. Let’s consider two main cases:
- An existing beam will have new and heavier loads, so the adequacy of the beam needs to be checked. In this case, the structural properties of the section are known and we can do all the checks described above. No problem.
- A new beam needs to be designed and installed. In this case the section properties are unknown, and there are only some constraints in beam depth due to space limitations to accommodate HVAC, piping, etc, above the ceiling. The same design checks described above need to be done, but this time there are literally dozens of different possible sections in the AISC Manual that could be installed. How to proceed to find the right section?
Of course you could check all possible sections in the depth range, one by one, then set apart the winners and disregard the losers, and then pick your best winner. All this plan sounds very nice in theory, but it might take you hours of hard work. Fortunately ASDIP STEEL has a feature called Design Manager which will do exactly this, quick and easily, as shown below.
After setting up the beam type and depth limitations, click on the Find Sections button to populate the table with all the steel sections that pass the design (winners). Sort the table by weight or by ratio, and pick the desired selection. Finally click the Selectbutton to accept the selection and update the results with the new section. Clean and easy.