Page 104 - Handbook of Civil Engineering Calculations, Second Edition

PART 2
STRUCTURAL STEEL DESIGN

Structural Steel Beams and Plate Girders

In the following calculation procedures, the design of steel members is executed in accor-
dance with the Specification for the Design, Fabrication and Erection of Structural Steel
for Buildings of the American Institute of Steel Construction. This specification is pre-
sented in the AISC Manual of Steel Construction.
Most allowable stresses are functions of the yield-point stress, denoted as F y in the Man-
ual. The appendix of the Specification presents the allowable stresses associated with each
grade of structural steel together with tables intended to expedite the design. The Commen-
tary in the Specification explains the structural theory underlying the Specification.
Unless otherwise noted, the structural members considered here are understood to be
made of ASTM A36 steel, having a yield-point stress of 36,000 lb/sq.in. (248,220.0 kPa).
The notational system used conforms with that adopted earlier, but it is augmented to
2
2
include the following: A f = area of flange, sq.in. (cm ); A w area of web, sq.in. (cm );
b f width of flange, in. (mm); d depth of section, in. (mm); d w depth of web, in.
(mm); t f thickness of flange, in. (mm); t w thickness of web, in. (mm); L
unbraced
length of compression flange, in. (mm); f y yield-point stress, lb/sq.in. (kPa).

MOST ECONOMIC SECTION FOR A BEAM
WITH A CONTINUOUS LATERAL SUPPORT
UNDER A UNIFORM LOAD

A beam on a simple span of 30 ft (9.2 m) carries a uniform superimposed load of 1650
lb/lin ft (24,079.9 N/m). The compression flange is laterally supported along its entire
length. Select the most economic section.

Calculation Procedure:

1. Compute the maximum bending moment and the required
section modulus
Assume that the beam weighs 50 lb/lin ft (729.7 N/m) and satisfies the requirements of a
compact section as set forth in the Specification.
2
2
The maximum bending moment is M (1/8)wL (1/8)(1700)(30) (12) 2,295,000
in·lb (259,289.1 N·m).
Referring to the Specification shows that the allowable bending stress is 24,000 lb/sq.in.
3
3
(165,480.0 kPa). Then S M/f 2,295,000/24,000 95.6 in (1566.88 cm ).

1.87