In the era of computerized structural analysis and design, advanced building regulations are developed to more precisely calculate the performance of structures in such extreme loading conditions as earthquakes. Structural engineers using the commercially available software are entitled to receive in-depth knowledge about each piece of software they use, along with all the underlying adjustment factors applied and the modifications done in the background. Sometimes the modification factors or adjustments are done without even giving warning to users.
I am dealing with numerous consultants, very reputable and prestigious firms. I can tell from personal experience how this serious, consequential issue is easily overlooked even by firms in structural engineering business for decades. This is partly due to a lack of transparency from the software companies, but mainly because we, the engineers, are not using the software correctly.
I have performed a case study on finite element analysis of steel moment frames. I made my own FE environment, using VB to validate and compare it to commercially available alternatives. In the case of steel structures’ analysis using SAP2000 or ETABS, if the design method for steel members is chosen to be “Direct Analysis,” whether or not the design is done, the software will apply a stiffness reduction factor on the structure! The same is true for RISA; a user has to manually uncheck the “adjust stiffness” box for it to not reduce the stiffness. The results, then, turn out almost 20% to 30% higher than analysis done without the adjustment factors. The applied adjustment factor is 0.8xϮb per AISC-360-10 section C2.3. According to the applicable chapter in AISC, the reduced stiffness only pertains to strength and stability limit states; it does not apply to drift, deflection, vibrations, and determination of period of structure. Since the deflection and period of structure are typically the governing limit states for the design of moment frame structures, especially for multistory frame, this can have significant impact on the design.
The stiffness reduction will yield higher deflections and about 20%-30% over-designed moment frames. The reduced stiffness, however, will not always produce conservative results. Lower stiffness is associated with higher natural period which could be used to reduce seismic demand for dynamic or even static analysis procedure.
I acknowledge the fact that engineers shall have complete knowledge about the design process and the features of the analysis software, however more transparency is needed from the software providers about this issue, maybe a simple warning about the deflection results to pop up after switching design method will do the job. Analysis results shall not be based on “adjusted stiffness” which is solely used for design purposes. Until this issue is addressed by software companies, I recommend setting the design method to “Effective Length” before checking for deflection and period of structure. Then if the desired design method is “Direct Analysis” switch the design method back to “Direct analysis” only to check member DCRs.
Edmond Hagopyan, P.E.
