Page 65 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.27

FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.27

end to the other. While both arrangements are used, one may have advantages over the other in a
particular situation.
Horizontally curved girder bridges are similar to straight-girder bridges except for torsional
effects. If use of falsework is to be avoided, it is necessary to resist the torques by assembling two
adjacent girders with their diaphragms and temporary or permanent lateral bracing and erect the
assembly as a stable unit. Diaphragms and their connections must be capable of withstanding end
moments induced by girder torques.
Truss bridges require a vast amount of investigation to determine the practicability of a desired
erection scheme or the limitations of a necessary erection scheme. The design of truss bridges,
whether simple or continuous, generally assumes that the structure is complete and stable before it
is loaded. The erector, however, has to impose dead loads, and often live loads, on the steel while the
structure is partly erected. The structure must be erected safely and economically in a manner that
does not overstress any member or connection.
Erection stresses may be of opposite sign and of greater magnitude than the design stresses.
When designed as tension members but subjected to substantial compressive erection stresses, the
members may be braced temporarily to reduce their effective length. If bracing is impractical, they
may be made heavier. Members designed as compression members but subjected to tensile forces
during erection are investigated for adequacy of area of net section where holes are provided for con-
nections. If the net section is inadequate, the member must be made heavier.
Once an erection scheme has been developed, the erection engineer analyzes the structure under
erection loads in each erection stage and compares the erection stresses with the design stresses. At
this point, the engineer plans for reinforcing or bracing members, if required. The erection loads
include the weights of all members in the structure in the particular erection stage and loads from
whatever erection equipment may be on the structure. Wind loads are added to these loads.
In addition to determining member stresses, the erection engineer usually calculates reactions
for each erection stage, whether they be reactions on abutments or piers or on falsework. Reactions
on falsework are needed for design of the falsework. Reactions on abutments and piers may reveal
a temporary uplift that must be provided for, by counterweighting or use of tie-downs. Often, the
engineer also computes deflections, both vertical and horizontal, at critical locations for each erec-
tion stage to determine stroke and capacity of jacks that may be required on falsework or on the
structure.
When all erection stresses have been calculated, the engineer prepares detailed drawings show-
ing falsework, if needed, necessary erection bracing with its connections, alterations required for any
permanent member or joint, installation of jacks and temporary jacking brackets, and bearing devices
for temporary reactions on falsework. In addition, drawings are made showing the precise order in
which individual members are to be erected.
Figure 2.11 shows the erection sequence for a through-truss cantilever bridge over a navigable
river. For illustrative purpose, the scheme assumes that falsework is not permitted in the main chan-
nel between piers and that a barge-mounted crane will be used for steel erection. Because of the lim-
itation on use of falsework, the erector adopts the cantilever method of erection. The plan is to erect
the structure from both ends toward the center.
Note that top chord U13–U14, which is unstressed in the completed structure, is used as a prin-
cipal member during erection. Note also that in the suspended span all erection stresses are opposite
in sign to the design stresses.
As erection progresses toward the center, a negative reaction may develop at the abutments (panel
point LO). The uplift may be counteracted by tie-downs to the abutment.
Hydraulic jacks, which are removed after erection has been completed, are built into the chords
at panel points U13, L13, and U13′. The jacks provide the necessary adjustment to allow closing of
the span. The two jacks at U13 and L13 provide a means of both horizontal and vertical movement
at the closing panel point, and the jack at U13′ provides for vertical movement of the closing panel
point only.
Other bridge types are also encountered as variations of the bridge types shown above. There are
also distinct different types of bridges, such as suspension, cable-stayed, and movable bridges, each
requiring erection planning and equipment especially suited to configuration and location.

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.