Steel Frame Design
Author: Joshua B. Kardon, S.E.
University of California, Berkeley
This case is an excerpt from "The Structural Engineer's Standard of Care."
A two-story, mixed-use, wood-framed building on a corner lot incorporated two full-height moment frames, one on each of two adjacent sides facing the streets, in order to accommodate storefronts and office windows. The location was in seismic zone 4, close to a known, active fault. The engineer of record produced a set of calculations for the frames. One frame was designed based on five lines of calculations; the other frame was designed based on one line of calculations, which read, "Similar." The calculations did not include any treatment of the vertical loads which the frame had to support, or any evaluation of earthquake-induced drift. A thorough and detailed computer-aided analysis performed during an investigation of the building showed that the frame as originally designed was adequate in terms of stiffness and strength for Code-required loads. In fact, the analysis showed the frames were a very efficient and economical design.
The calculations were not adequate to describe the design intent of the structural engineer. They did not include the evaluation of the performance of the frames under Code-required dead and live loads, or any required combinations of loads. The stresses and deflections induced by required or anticipated loads were not compared with allowable values.
Was the structural engineer negligent in his design of the steel frame? Calculations are not in themselves engineering. However, they do convey the thought process and the design intent of the engineer; they substantiate the judgment the engineer used in arriving at the engineering solution. The engineering effort is in knowing what to calculate, and how to model the real world, so that the solution, the real structure, actually performs the way the engineer wants it to. The quality of the calculations, their clarity, thoroughness and accuracy, can be considered an indication of the level of care and diligence exercised by the structural engineer. However, even the best calculations only substantiate, but do not substitute for, the judgment of the structural engineer. The structural engineer of this example designed his steel frames without exhaustive calculations, but as a detailed analysis indicated, not without apparently a clear understanding of good structural engineering design.
The in-plane lateral load-resisting design of the steel frames of this example was not, in this author's opinion, beneath the standard of care. There certainly was an absence of complete documentation substantiating the in-plane lateral design of the frame. However, the frame design, the actual size, configuration and details of the beams and columns, was not in error. The engineer may have proportioned the frame members correctly by intuition, but it was not an erroneous design.
Cite this page:
"Steel Frame Design"
Online Ethics Center for Engineering
National Academy of Engineering
Accessed: Monday, December 09, 2013