Simplified Frame Analysis - Simplified Design of Reinforced Concrete Buildings - page 22

Chapter 2
Simplified Frame Analysis

2.1 INTRODUCTION

The final design of the structural components in a building frame is based on maximum moment, shear, axial
load, torsion and/or other load effects, as generally determined by an elastic frame analysis (ACI 8.3).
For building frames of moderate size and height, preliminary and final designs will often be combined.
Preliminary sizing of members, prior to analysis, may be based on designer experience, design aids, or
simplified sizing expressions suggested in this topic.

Analysis of a structural frame or other continuous construction is usually the most time consuming part of the
total design. For gravity load analysis of continuous one-way systems (beams and slabs), the approximate
moments and shears given by ACI 8.3.3 are satisfactory within the span and loading limitations stated.
For cases when ACI 8.3.3 is not applicable, a two-cycle moment distribution method is accurate enough.
The speed and accuracy of the method can greatly simplify the gravity load analysis of building frames with
usual types of construction, spans, and story heights. The method isolates one floor at a time and assumes that
the far ends of the upper and lower columns are fixed. This simplifying assumption is permitted by ACI 8.10.3.

For lateral load analysis of a sway frame, the Portal Method may be used. It offers a direct solution for the moments
and shears in the beams (or slabs) and columns, without having to know the member sizes or stiffnesses.

The simplified methods presented in this chapter for gravity load analysis and lateral load analysis
are considered to provide sufficiently accurate results for buildings of moderate size and height.
However, determinations of load effects by computer analysis or other design aids are equally applicable for
use with the simplified design procedures presented in subsequent chapters of this topic.

2.2 LOADING

2.2.1 Service Loads

The first step in the frame analysis is the determination of design (service) loads and lateral forces (wind and
seismic) as called for in the general building code under which the project is to be designed and constructed.
For the purposes of this topic, design live loads (and permissible reductions in live loads) and wind loads are
based on Minimum Design Loads for Buildings and Other Structures , ASCE/SEI 7-05. 2.1 References to
specific ASCE Standard requirements are noted (ASCE 4.2 refers to ASCE/SEI 7-05, Section 4.2). For a
specific project, however, the governing general building code should be consulted for any variances from
ASCE/SEI 7-05.

Design dead loads include member self-weight, weight of fixed service equipment (plumbing, electrical, etc.) and,
where applicable, weight of built-in partitions. The latter may be accounted for by an equivalent uniform load of not
less than 20 psf, although this is not specifically defined in the ASCE Standard (see ASCE Commentary Section 3.1.2).