Seismic Design and Performance of Multitiered Ordinary Concentrically Braced Frames
Advisor: Professor Larry A. Fahnestock
Abstract
In tall, single-story buildings with steel concentrically braced frame (CBF) lateral force-resisting
systems, it is more efficient to replace a single brace or pair of braces between the base and the story
(roof) level with multiple bracing panels or tiers, leading to a multitiered braced frame (MT-BF).
MT-BFs do not have any intermediate floor diaphragms or out-of-plane bracing at the tier levels.
The versatility of MT-BFs makes them a popular choice for the lateral force-resisting system in a
variety of building types, including convention centers, sports facilities, and warehouses. However,
in a MT-BF, nearly all the mass is located at the top of the frame (typically the roof level), which
has important implications for seismic design, and their seismic behavior is more complex than
typical multistory CBFs.
Similar to multistory braced frames (MS-BFs), seismic energy in concentric MT-BFs is primarily
dissipated through brace inelastic response. Drift concentration has been known to occur in an
individual frame tier/story which can lead to excessive ductility demands on the braces and cause
low-cycle fatigue fracture in the braces. In MT-BFs, this phenomenon increases the propensity
for column instability since any unbalanced loads can only be resisted through column bending
which is concurrent with axial loads. Column vulnerability in MT-BFs is further heightened due
to the lack of out-of-plane supports at tier levels. These unique conditions have been the focus
of studies that support the first-generation of MT-BF design requirements introduced in the 2016
AISC Seismic Provisions (unchanged in the 2022 AISC Seismic Provisions).
Systems with modest levels of ductility, like ordinary concentrically braced frames (OCBFs)
are commonly used in the United States, and the requirements for multitiered ordinary concentrically
braced frames (MT-OCBFs), which is the focus of this dissertation, are based on a limited
initial evaluation. The primary feature of the requirements is an axial force amplification (150% of
the overstrength horizontal seismic load effect), intended to account for the induced in-plane flexural demands. A requirement for including out-of-plane moments arising from 0.6% of the vertical
component of the compression brace force at each tier, which was intended to account for effects
from buckling compression braces, was also included for column design. Prior to the introduction
of these design requirements, MT-OCBFs were designed as multistory frames without considering
flexural demands.
The stability response of columns in MT-OCBFs was studied in-depth in this dissertation.
Detailed nonlinear models were used to evaluate the seismic performance of an extensive set of
frame designs. This is the only body of work that employs a full collapse performance assessment
on MT-OCBFs to show that potential for column instability is reduced in designs considering the
new axial force amplification. The effectiveness of the amplified axial force demands was verified
by showing that the frame collapse probabilities are within acceptable limits. Thus, the work
presented in this dissertation substantiates the importance of considering the additional flexural
demands to produce seismically safe frame designs. MT-OCBF column instability is primarily
related to axial force and in-plane moment, and it was concluded that the out-of-plane moment
requirement can be eliminated since this does not influence column proportioning appreciably, and
performance is still acceptable. A column design simplification, which reduces design complexity
and material costs, is proposed for consideration in the next edition of the AISC Seismic Provisions.
Inspired by the fundamental knowledge gained on MT-OCBF column behavior, a method was
developed to strengthen the columns and improve the safety of existing frames with seismically
deficient performance. Finally an assessment of existing MT-OCBFs was conducted by simulating
realistic column base conditions that are intermediate between common pinned and fixed
base idealizations. It was shown that frame collapse performance improved since typical column
base plates provide greater rotational restraint than assumed in design. For older MT-OCBFs not
designed according to the current requirements, realistic base restraints can also be employed to
conduct a more accurate performance assessment before pursuing column strengthening, such as
proposed here. The findings of this study support safe MT-OCBF designs that are lighter and more
efficient, thus making the use of multitiered braced frame configurations even more attractive in
tall, single-story structures.