These are members made up by a fabricator from two or more standard sections. Examples of common built-up sections are shown in Fig. 2.2. Built-up members are specified by the designer when the desired properties or configuration cannot be obtained in a single hotrolled section. Built-up sections can be bolted or welded. Welded members, in general, are less expensive because much less handling is required in the shop and because of more efficient utilization of material. The clean lines of welded members also produce a better appearance.
Cover-plated rolled beams are used when the required bending capacity is not available in a rolled standard beam or when depth limitations preclude use of a deeper rolled beam or plate girder. Cover-plated beams are also used in composite construction to obtain the efficiency of a nonsymmetrical section.
Cover-plate material is ordered to multiple widths for flame cutting or stripping to the required width in the shop. For this reason, when several different design conditions exist in a project, it is good practice, as well as good economy, for the designer to specify as few different cover-plate thicknesses as possible and to vary the width of plate for the different members.
For bolted sections, cover plates and rolled-beam flanges are punched separately and are then brought together for fit-up. Sufficient temporary fitting bolts are installed to hold the cover plates in alignment, and minor mismatches of holes in mating parts are cleaned up by reaming. For welded sections, cover plates are held in position with small intermittent tack welds until final welding is done.
Plate girders are specified when the moment capacity, stiffness, or on occasion, web shear capacity cannot be obtained in a rolled beam. They usually are fabricated by welding.
Welded plate girders consist of a web plate, a top flange plate, a bottom flange plate, and stiffener plates. Web material is ordered from the mill to the width between flange plates plus an allowance for trim and camber, if required. Flange material is ordered to multiple widths for stripping to the desired widths in the shop.
When an order consists of several identical girders having shop flange splices, fabricators usually first lay the flange material end to end in the ordered widths and splice the abutting ends with the required groove welds. The long, wide plates thus produced are then stripped to the required widths. For this procedure, the flanges should be designed to a constant width over the length of the girder. This method is advantageous for several reasons: Flange widths permit groove welds sufficiently long to justify use of automatic welding equipment. Runout tabs for starting and stopping the welds are required only at the edges of the wide, unstripped plate. All plates can be stripped from one setup. And much less finishing is required on the welds.
After web and flange plates are cut to proper widths, they are brought together for fit-up and final welding. The web-to-flange welds, usually fillet welds, are positioned for welding with maximum efficiency. For relatively small welds, such as 1â„4- or 5â„16-in fillets, a girder may be positioned with web horizontal to allow welding of both flanges simultaneously. The girder is then turned over, and the corresponding welds are made on the other side. When relatively large fillet welds are required, the girder is held in a fixture with the web at an angle of about 45 to allow one weld at a time to be deposited in the flat position. In either method, the web-to-flange welds are made with automatic welding machines that produce welds of good quality at a high rate of deposition. For this reason, fabricators would prefer to use continuous fillet welds rather than intermittent welds, though an intermittent weld may otherwise satisfy design requirements.
After web-to-flange welds are made, the girder is trimmed to its detailed length. This is not done earlier because of the difficulty of predicting the exact amount of girder shortening due to shrinkage caused by the web-to-flange welds.
If holes are required in web or flange, the girder is drilled next. This step requires moving the whole girder to the drills. Hence, for economy, holes in main material should be avoided because of the additional amount of heavy-load handling required. Instead, holes should be located in detail material, such as stiffeners, which can be punched or drilled before they are welded to the girder.
The next operation applies the stiffeners to the web. Stiffener-to-web welds often are fillet welds. They are made with the web horizontal. The welds on each side of a stiffener may be deposited simultaneously with automatic welding equipment. For this equipment, many fabricators prefer continuous welds to intermittent welds. When welds are large, however, the girder may be positioned for flat, or downhand, welding of the stiffeners.
Variation in stress along the length of a girder permits reductions in flange material. For minimum weight, flange width and thickness might be decreased in numerous steps. But a design that optimizes material seldom produces an economical girder. Each change in width or thickness requires a splice. The cost of preparing a splice and making a weld may be greater than the cost of material saved to avoid the splice. Therefore, designers should hold to a minimum flange splices made solely to save material. Sometimes, however, the length of piece that can be handled may make splices necessary.
Welded crane girders differ from ordinary welded plate girders principally in that the upper surface of the top flange must be held at constant elevation over the span. A step at flange splices is undesirable. Since lengths of crane girders usually are such that flange splices are not made necessary by available lengths of material, the top flange should be continuous. In unusual cases where crane girders are long and splices are required, the flange should be held to a constant thickness. (It is not desirable to compensate for a thinner flange by deepening the web at the splice.) Depending on other elements that connect to the top flange of a crane girder, such as a lateral-support system or horizontal girder, holding the flange to a constant width also may be desirable.
The performance of crane girders is quite sensitive to the connection details used. Care must be taken in design to consider the effects of wheel loads, out-of-plane bending of the web, and permitting the ends of the girders to rotate as the crane travels along the length of the girder. The American Iron and Steel Engineers and the AISC both provide information concerning appropriate details.
Horizontally curved plate girders for bridges constitute a special case. Two general methods are used in fabricating them. In one method, the flanges are cut from a wide plate to the prescribed curve. Then the web is bent to this curve and welded to the flanges. In the second method, the girder is fabricated straight and then curved by application of heat to the flanges. This method which is recognized by the AASHTO specifications, is preferred by many fabricators because less scrap is generated in cutting flange plates, savings may accrue from multiple welding and stripping of flange plates, and the need for special jigs and fittings for assembling a girder to a curve is avoided.
(Fabrication Aids for Continuously Heat-Curved Girders and Fabrication Aids for Girders Curved with V-Heats, American Institute of Steel Construction, Chicago, Ill.) Procedures used in fabricating other built-up sections, such as box girders and box columns, are similar to those for welded girders.
Columns generally require the additional operation of end finishing for bearing. For
welded columns, all the welds connecting main material are made first, to eliminate uncertainties
in length due to shrinkage caused by welding. After the ends are finished, detail material, such as connection plates for beams, is added.
The selection of connection details on built-up sections has an important effect on fabrication economy. If the pieces making up the section are relatively thick, welded details can provide bolt holes for connections and thereby eliminate punching the thick material. On the other hand, fabricators that trim sections at the saw after assembly may choose to drill holes using a combination drill-saw line, thus avoiding manual layout for welded detail material.