UFC 4-023-03
25 January 2005
APPENDIX D
WORKED STRUCTURAL STEEL FRAME EXAMPLE:
TIE FORCE AND NONLINEAR ALTERNATE PATH ANALYSIS
D-1
INTRODUCTION.
A typical steel frame commercial building design and analysis example has
been prepared to illustrate tie force and alternate path calculations. The structure is
assumed to require a Low Level of Protection; hence, horizontal and vertical tie forces
are required and the Alternate Path Method is applied to any element that cannot
provide the required vertical tie force capacity.
The example was prepared using tools and techniques commonly applied by
structural engineering firms in the US. Computer software that is typical of that used for
structural design was employed for preliminary design and for the alternate path
analysis. Per the option given in the UFC, static nonlinear analysis was performed
using material nonlinearity methods available in these codes. Nonlinear hinges are
placed in all members and at all connections in the model, and only "activate" when
capacity values are exceeded in the nonlinear hinge definitions. The goal of the
analysis was to confirm that flexural members would bridge over failed columns, since
loss of an external column or internal column would result in a collapsed area in excess
of the maximum areas permitted by the UFC (750-ft2 and 1500-ft2, respectively).
D-2
PRELIMINARY DESIGN.
The structure considered is a five-story steel moment frame building. It is four
bays by five bays in plan, with a 25 ft. x 25 ft. typical bay. The intended function of the
building is office use only, with occupancy under one hundred people. See Figures D-1
and D-2 for drawings of the building and the orientation of the members.
D-2.1
Scope of Model.
As is often the practice in a structural design office, only the lateral-resisting
system was modeled. The gravity beams and flooring were designed but not modeled
in the analysis program. These elements are typically not included as a part of the
lateral load resisting system. Progressive collapse evaluation through the alternate path
method thus considers only the primary lateral load resistance structure.
D-2.2
Model Assumptions.
1)
Members are represented by centerline elements (no end offsets)
2)
All connections assumed to be partially restrained (PR) moment connections
3)
Column to foundation connections are considered pinned
4)
Each floor was taken as a rigid diaphragm
5)
Beams were not analyzed or designed as composite sections
6)
All steel shapes had a yield strength (fy) of 50 ksi and an ultimate strength (fult)
of 65 ksi. The modulus of elasticity (E) was 29,000 ksi.
D-1
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