When the principal operations on a main member, such as punching, drilling, and cutting, are completed, and when the detail pieces connecting to it are fabricated, all the components are brought together to be fitted up, i.e.,temporarily assembled with fit-up bolts, clamps, or tack welds. At this time, the member is inspected for dimensional accuracy, squareness, and, in general, conformance with shop detail drawings. Misalignment in holes in mating parts should be detected then and holes reamed, if necessary, for insertion of bolts. When fit-up is completed, the member is bolted or welded with final shop connections.
The foregoing type of shop preassembly or fit-up is an ordinary shop practice, routinely performed on virtually all work. There is another class of fit-up, however, mainly associated with highway and railroad bridges, that may be required by project specifications. These may specify that the holes in bolted field connections and splices be reamed while the members are assembled in the shop. Such requirements should be reviewed carefully before they are specified. The steps of subpunching (or subdrilling), shop assembly, and reaming for field connections add significant costs. Modern CNC drilling equipment can provide fullsize holes located with a high degree of accuracy. AASHTO specifications, for example, include provisions for reduced shop assembly procedures when CNC drilling operations are used.
Where assembly and reaming are required, the following guidelines apply:
Splices in bridge girders are commonly reamed assembled. Alternatively, the abutting ends and the splice material may be reamed to templates independently.
Ends of floorbeams and their mating holes in trusses or girders usually are reamed to templates separately.
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 compression member and decreasing the geometric length of each tension member by the amount of axial deformation it will experience under load (see Art. 2.7).
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 sections), 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 geometric 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 connecting 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 sections 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 configuration 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 the geometric length.
The angles that were set in geometric position remain geometric. Therefore, the S curves induced in the no-load assembly vanish. Secondary bending stresses, for practical purposes, have been eliminated. Further loading or a removal of load, however, will produce some secondary bending in the members.
Figure 2.1b illustrates the effects of method 3. Dotted lines represent the shape of a truss reamed by method 3 and then fully connected, but without load. As the members are fitted up (pinned and bolted), the truss takes its cambered position. In this position, as when they were reamed, members are straight and positioned to their cambered angles, hence have no induced bending. The solid lines in Fig. 2.1b represent the shape of 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 its geometric length. The angles that were set in the cambered (no-load) position are still in that position. As a result, S curves are induced in the members, as indicated in Fig. 2.1b by exaggerated S curves in solid lines. Secondary stresses due to bending, which do not occur under camber load in methods 1 and 2, are induced by this load in method 3. Further loading will increase this bending and further increase the secondary stresses.
Bridge engineers should be familiar with the reaming methods and see that design and fabrication are compatible.