Page 51 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.13

FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.13

For reaming truss connections, three methods are in use in fabricating shops. The particular
method to be used on a job is dictated by the project specifications or the designer.
Associated with the reaming methods for trusses is the method of cambering trusses. Highway
and railroad bridge trusses are cambered by increasing the geometric (loaded) length of each com-
pression member and decreasing the geometric length of each tension member by the amount of
axial deformation it will experience under load (see Art. 2.13).

Method 1 (RT, or Reamed-Template, Method). All members are reamed to geometric angles (angles
between members under load) and cambered (no-load) lengths. Each chord is shop-assembled and
reamed. Web members are reamed to metal templates. The procedure is as follows:
With the bottom chord assembled in its loaded position (with a minimum length of three abutting
sections), the field connection holes are reamed. (Section, as used here and in methods 2 and 3, means
fabricated member. A chord section, or fabricated member, usually is two panels long.)
With the top chord assembled in its loaded position (with a minimum length of three abutting sec-
tions), the field connection holes are reamed.
The end posts of heavy trusses are normally assembled and the end connection holes reamed, first
for one chord and then for the other. The angles between the end post and the chords will be the geo-
metric angles. For light trusses, however, the end posts may be treated as web members and reamed
to metal templates.
The ends of all web members and their field holes in gusset plates are reamed separately to metal
templates. The templates are positioned on the gusset plates to geometric angles. Also, the templates
are located on the web members and gusset plates so that when the unloaded member is connected,
the length of the member will be its cambered length.
Method 2 (Gary or Chicago Method). All members are reamed to geometric angles and cambered
lengths. Each chord is assembled and reamed. Web members are shop-assembled and reamed to each
chord separately. The procedure is as follows:
With the bottom chord assembled in its geometric (loaded) alignment (with a minimum number
of three abutting sections), the field holes are reamed.
With the top chord assembled in its geometric position (with a minimum length of three abutting
sections), the holes in the field connections are reamed.
The end posts and all web members are assembled and reamed to each chord separately. All
members, when assembled for reaming, are aligned to geometric angles.

Method 3 (Fully Assembled Method). The truss is fully assembled, then reamed. In this method,
the bottom chord is assembled and blocked into its cambered (unloaded) alignment, and all the other
members are assembled to it. The truss, when fully assembled to its cambered shape, is then reamed.
Thus the members are positioned to cambered angles, not geometric angles.
When the extreme length of trusses prohibits laying out the entire truss, method 3 can be used
sectionally. For example, at least three abutting complete sections (top and bottom chords and con-
necting web members) are fully assembled in their cambered position and reamed. Then complete
sections are added to and removed from the assembled sections. The sections added are always in
their cambered position. There should always be at least two previously assembled and reamed sec-
tions in the layout. Although reaming is accomplished sectionally, the procedure fundamentally is
the same as for a full truss assembly.
In methods 1 and 2, field connections are reamed to cambered lengths and geometric angles,
whereas in method 3, field connections are reamed to cambered lengths and angles. To illustrate the
effects of these methods on an erected and loaded truss, Fig. 2.1a shows by dotted lines the shape of
a truss that has been reamed by either method 1 or 2 and then fully connected, but without load. As
the members are fitted up (pinned and bolted), the truss is forced into its cambered position. Bending
stresses are induced into the members because their ends are fixed at their geometric (not cambered)
angles. This bending is indicated by exaggerated S curves in the dotted configuration. The configu-
ration shown in solid lines in Fig. 2.1a represents the truss under the load for which the truss was
cambered. Each member now is strained; the fabricated length has been increased or decreased to

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.