AISC DESIGN GUIDE 14
Staggered Truss Framing Systems
|Publication Date:||1 December 2001|
A staggered-truss frame is designed with steel framing members and concrete floors. Most often, the floor system is precast concrete hollow-core plank. Other options, including concrete supported on metal deck with steel beams or joists, can be used.
With precast plank floors, economy is achieved by "stretching" the plank to the greatest possible span. 8-in.- thick plank generally can be used to span up to 30 ft, while 10-in.-thick plank generally can be used to span up to 36 ft. Specific span capabilities should be verified with the specific plank manufacturer. Therefore, the spacing of the trusses has a close relationship to the thickness of plank and its ability to span. 6-in.-thick precast plank is normally only used with concrete topping.
Hollow core plank is manufactured by the process of extrusion or slip forming. In both cases the plank is prestressed and cambered. The number of tendons and their diameter is selected for strength requirements by the plank manufacturer's engineer based upon the design instructions provided by the engineer of record.
The trusses are manufactured from various steels. Early buildings were designed with chords made of wide-flange sections and diagonal and vertical members made of chan-nels. The channels were placed toe-to-toe, welded with separator plates to form a tubular shape. Later projects used hollow structural sections (HSS) for vertical and diagonal members.
Today, the most common trusses are designed with W10 chords and HSS web members (verticals and diagonals) connected with gusset plates. The chords have a minimum width of 6 in., required to ensure adequate plank bearing during construction. The smallest chords are generally W10x33 and the smallest web members are generally HSS4×4×¼. The gusset plates are usually ½-in. thick.
The trusses are manufactured with camber to compensate for dead load. They are transported to the site, stored, and then erected, generally in one piece. Table 1.1 is a material guide for steel member selection. Other materials, such as A913, may be available (see AISC Manual, Part 2).
The plank is connected to the chords with weld plates to ensure temporary stability during erection. Then, shear stud connections are welded to the chords, reinforcing bars are placed in the joints, and grout is placed. When the grout cures, a permanent connection is achieved through the welded studs as illustrated in Fig. 1.2. Alternatively, guying or braces may also be used for temporary stability during construction.
The precast plank is commonly manufactured with 4,000 psi concrete. The grout commonly has 1,800 psi compressive strength and normally is a 3:1 mixture of sand and Portland cement. The amount of water used is a function of the method used to place the grout, but will generally result in a wet mix so joints can be easily filled. Rarely is grout strength required in excess of 2,000 psi. The grout material is normally supplied and placed by the precast erector.