Civil Engineering Reference
In-Depth Information
6
Design of Axial Force
Steel Members
6.1 INTRODUCTION
Members designed to carry primarily axial forces are found in steel railway bridges as
truss members [chords, vertical members (hangers and posts), and diagonal members
(webmembersandendposts)],spanandtowerbracingmembers,steeltowercolumns,
spandrel columns in arches, knee braces, and struts. These members may be in axial
tension, compression, or both (due to stress reversal from moving train and wind
loads) and must be designed considering yield strength and serviceability criteria.
Members in axial tension must also be designed for the fatigue and fracture limit
states, and members in compression must also resist instability. Furthermore, some
axial tension and compression members are subjected to additional stresses due to
flexure
∗
and must be designed for these combined stresses (see Chapter 8).
6.2 AXIAL TENSION MEMBERS
Axial tension main members in steel railway superstructures are often fracture
critical and nonredundant. Therefore, the strength (yielding and ultimate) and fatigue
limit states require careful consideration during design. Brittle fracture is considered
by appropriate material selection, detailing, and fabrication quality assurance (see
Chapters 2, 3, and 5).
6.2.1 S
TRENGTH OF
A
XIAL
T
ENSION
M
EMBERS
The strength of a tension member is contingent upon yielding of the gross section,
A
g
, occurring prior to failure (defined at ultimate strength) of the effective net section,
A
e
,or
F
y
A
g
≤ φ
F
u
A
e
,
(6.1)
∗
Typically from bending forces due to end conditions (frame action, connection fixity, and eccentricity),
presence of transverse loads, and/or load eccentricities.
227
