Among the most powerful innovations in recent years in the structural engineering profession is the widespread adaptation and use of nonlinear analysis methods for seismic issues. The concept of nonlinear behavior has been around for decades, but only recently – say, within the last 15 years – have practical analysis methods been introduced and embraced by many. Among these is the static pushover method, which enables a representation of nonlinear behavior without the need to develop and run sophisticated response history analyses. …

A Fundamental Discussion

Nearly every day in our careers as structural engineers, we consider the following equations:

M_n = A_sf_y(d-a/2), M_n = F_yZ_x

Readers will no doubt recognize these as the nominal flexural strengths utilized in beam designs for reinforced concrete and steel, also known as (among other things) the flexural limit state. …

It is a well-understood concept and an inevitable law of statics that loads must be transferred between beams and columns. This is an idea that is not foreign to engineers. Since our first classes in structural analysis, we have been developing our expertise at analyzing and designing beam-column intersections. The idea of balancing the sum of forces at such locations is one that cannot be disputed. However, there are some vagaries with this concept when considering flat plate and flat slab systems, which by definition have no beam-column joints. Hence, moment transfer becomes a more complex issue. …

There was a time, which many readers may well remember, when elastic behavior of structures governed our thoughts when it came to their design. For seismic design, we understood that the response reduction factor (currently designated ‘R’) was a reflection of system ductility and gave us the latitude of designing the system for much lower forces than standard elastic design might typically predict. …

Engineers largely appreciate the differences afforded between allowable stress design (ASD) and ultimate strength design (LRFD) methods, and we generally follow the prescribed protocol for each procedure without much trouble. However, even though the ASD approach has been largely supplanted by LRFD, certain occasions require that we revisit the old ASD theory for reinforced concrete (or masonry). As such, it is appropriate that we recognize the subtle and not-so-subtle differences between the two. …

Among categorizations of seismic behavior that have been adopted in modern codes is extreme torsional irregularity. Torsional irregularity is not an unfamiliar concept, having been expressed in codes in various forms for decades. It is an issue that engineers have learned to deal with, particularly in seismically active areas. Extreme torsional irregularity, however, is a somewhat newer concept and subset within the larger issue of torsional behavior. It is something that can greatly limit and restrict flexibility in choosing seismic force-resisting systems and configurations. …

The ‘X’ Brace vs. ‘V’ Brace Conundrum

As practitioners in structural design, we sometimes scratch our heads in bewilderment with new code provisions. Usually, with a little homework, we can understand the logic behind the changes and see how they lead to better performance of the end product. …

Do ACI Seismic Provisions Apply?

The title of this article may seem like a trivial question, but it deals with an issue that in large measure might be overlooked. At first glance, one might think, “Of course not; gravity columns are designed for gravity, so why would I need to address the seismic provisions in Chapter 21 of ACI 318-11?” The answer is a simple matter of deformation compatibility, which ASCE 7-10 addresses for Seismic Design Categories (SDC) D through F in Section 12.12.5. …

Among the many code provisions which may be characterized as ambiguous, few seem as pervasive as the issue of full-height blocking for either wood or cold-formed steel construction. Opinions regarding the need for full-height blocking widely varying, and it seems that even the purpose of full-height blocking is subject to debate. …