UFC 4-023-03
25 January 2005
APPENDIX C
WORKED REINFORCED CONCRETE FRAME EXAMPLE:
TIE FORCE AND NONLINEAR ALTERNATE PATH ANALYSIS
C-1
INTRODUCTION.
A typical reinforced concrete 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 has been 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. The automated
procedure described herein eliminates the step by step hinge placement described in
earlier sections of the UFC. In the models developed, 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 and example
illustrated in the paragraphs below 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).
C-2
PRELIMINARY DESIGN.
The structure is a five-story reinforced concrete moment frame building. It is
four bays by five bays in plan, with a 25 ft. x 25 ft. typical bay. The function of this
building is office use only, with occupancy under one hundred people. See Figures C-1
and C-2 for drawings of the building and the orientation of the members.
C-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 only.
C-2.2
Model Assumptions.
1) Members are represented by centerline elements (no end offsets)
2) All connections were assumed to be moment connections
C-1
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