Page 305 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 239

For continuous attachment, use n equal to four times the attached length
along each faying surface.

Bolting Light-gage Members Bolting is employed as a common means of
making field connections in light-gage steel construction. The AISI Specification
for the Design of Light-gage Steel Structural Members requires that the distance
between bolt centers, in line of stress, and the distance from bolt center to edge
1
of sheet, in line of stress, shall not be less than 1 /2 times the bolt diameter nor
P
(9.103)
f b t
where P load on bolt, lb (N)
t thickness of thinnest connected sheet, in (mm)
f basic design stress, psi (MPa)
b
That specification also recommends a limit of 3.5f for the bearing stress, and a
b
maximum allowable tension stress on net section of
0.1 3 d f (9.104)
s b
where d bolt diameter, in (mm)
s spacing perpendicular to line of stress, in (mm)

CHOOSING THE MOST ECONOMIC STRUCTURAL STEEL*

Structural steel is available in different strengths and grades. So when choosing
steel members for a structure, the designer must compare their relative cost
based on cross-sectional areas and prices. For two tension members of the same
length but of different steel strengths, their material-cost ratio C /C is:
2 1
C 2 A 2 p 2
(9.105)
C 1 A 1 p 1
where A and A are the cross-sectional areas and p and p are the material
2
1
1
2
prices per unit weight. If the members are designed to carry the same load at a
stress that is a fixed percentage of the yield point, the cross-sectional areas are
inversely proportional to the yield stresses. Therefore, their relative material
cost can be expressed as
C 2 F y1 p 2
(9.106)
C 1 F y 2 p 1
where F and F are the yield stresses of the two steels. The ratio p /p is the
1
y2
y1
2
relative price factors. Values of this factor for several steels are given in engi-
neering handbooks.*
*Brockenbrough and Merritt, Structural Steel Designer’s Handbook, McGraw-Hill.