METAL BUILDING SYSTEMS：SPECIFYING CRANE BUILDINGS.pdf
CHAPTER 15 SPECIFYING CRANE BUILDINGS 15.1INTRODUCTION Two out of every five metal building systems are constructed for manufacturing facilities where cranes are frequently needed for material handling. A building crane is a complex structural system that consists of the actual crane with trolley and hoist, crane rails with their fastenings, crane run- way beams, structural supports, stops, and bumpers. A motorized crane would also include electri- cal and mechanical components that are not discussed here. Our discussion is further limited to interior building cranes. The main focus of this chapter is on proper integration of the crane and the metal building into one coordinated and interconnected system. Any attempts to add a heavy crane to the already designed and constructed pre-engineered building are likely to be fraught with frustration, high costs, and inefficiencies. If the required planning is done beforehand, however, a cost-effective solution is much more likely. We are also interested in a relationship among the three main parties with design responsibilitiesarchitect-engineer, metal building manufacturer, and crane supplier. Occasionally, disputes arise when the contract documents do not clearly delineate their respective roles in the project. 15.2BUILDING CRANES: TYPES AND SERVICE CLASSIFICATIONS Several types of cranes are suitable for industrial metal building systems, the most common being bridge cranes (either top-running or underhung), monorail, and jib cranes. Occasionally, stacker and gantry cranes may be required for unique warehousing and manufacturing needs. Jib, mono- rail, and bridge cranes are examined here in this sequencein order of increasing structural demands imposed on a pre-engineered structure. Constraints of space prevent us from discussing gantry and stacker cranes, as well as conveyors and similar material handling systems. Within each type, the cranes are classified by the frequency and severity of use. Each crane must conform to one of six service classifications established by the Crane Manufacturers Association of America (CMAA). The six classes are:A (standby or infrequent service), B (light), C (moderate), D (heavy), E (severe), and F (continuous severe). Guidance for assigning a service classification is contained in CMAA standards 701and 742and in the MBMA Manual.3The Manuals Design Practices apply only to the cranes with service clas- sifications A to D. Information on cranes with service classification E or F, including design loads and impact factors, is given in AISE Technical Report 13.4 Another way to classify the cranes is by kind of movementhand-geared or electric. Hand- geared cranes are physically pulled along the rail by the operator and are less expensive, but slower, 423 than electric cranes. Hand-geared cranes act with less impact on the structure than their faster-running electric cousins. The operator controlling an electrically powered crane can be either standing on the floor using a suspended pendant pushbutton station or sitting in a cab located on the moving bridge. 15.3JIB CRANES Jib cranes require relatively little planning from the pre-engineered building designer. Floor-mounted jib cranes (also known as pillar cranes) do not depend on the building superstructure for support and bear on their own foundations (Fig. 15.1). Column-mounted jib cranes, on the other hand, are either supported from or braced back to the metal building columns and thus impose certain requirements on strength and stiffness of the structure. For example, Ref. 5 recommends that building columns sup- porting jib cranes be rigid enough so that the relative vertical deflection at the end of the boom is lim- ited to the boom length divided by 225. Floor-mounted jib cranes can rotate a full 360°, while column-mounted cranes are usually limited to a 200° boom rotation. A jib crane picks up the load by a trolley that travels on the bottom flange of the boom and carries a chain hoist. The hoist can be either electric or manually operated. Upon lifting the load, the boom rotates around the cranes stationary column and lowers the object to the desired location. These two operationstravel of the trolley and rotation of the jibare frequently performed manually. The length of the jib cranes boom varies from 8 to 20 ft. The lifting capacity ranges from 1?4to 5 tons,3with 1?2- and 1-ton jib cranes being the most popular. 424CHAPTER FIFTEEN FIGURE 15.1Floor-mounted jib crane. (American Crane and Equipment Corp.) Common applications of jib cranes include machinery servicing operations, assembly lines, steam hammers, and loading docks. Sometimes, a pair of jib cranes and a monorail in combination with forklifts is sufficient to transport cargo from a loading dock to the area serviced by overhead or stacker cranes. Inexpensive jib cranes can relieve main overhead cranes of much minor work that would tie them up for a long time. Manufacturers of floor-mounted jib cranes normally supply the suggested foundation sizes for their equipment, but the foundation design is still the specifying engineers responsibility. Whenever floor-mounted jib crane foundations are added to an existing metal building, care should be taken not to interrupt any floor ties or hairpins which could be located in the slab. Otherwise the lateral-load-resisting system of the building could be damaged. Obviously, an addi- tion of the column-mounted crane needs to be approached even more carefully, because the existing building columns will probably need strengthening to resist the newly imposed loads. The basic design concepts for jib cranes are discussed in Ref. 6. 15.4MONORAILS 15.4.1The Monorail System The monorail crane is a familiar sight in many industrial plants, maintenance shops, and storage facilities. Monorails are cost-effective for applications requiring material transfer over predeter- mined routes without any side-to-side detours; their range of travel can be expanded with the help of switches and turntables. The monorail crane is essentially a hoist carried by trolley, the wheels of which ride on the bottom flange of a single runway beam. Monorails can be used to move the loads from 1 to 10 tons and can be either hand-geared or electric. Monorail runway beams have been traditionally made of standard wide-flange sections that could accept straight-tread wheels or from I beams supporting tapered-tread wheels. (The straight- tread wheel is essentially a short cylinder; the tapered-tread one is a short truncated cone.) Today, these standard beam sections are being increasingly displaced by proprietary built-up runway beam products with unequal flange configuration. Figure 15.2 illustrates one of these hard-alloy-steel inverted T products offered by crane manufacturers; it also shows the loads exerted on the runway by the hoist. Some advantages of the proprietary tracks over rolled beams include better wear resistance, easier rolling, longer service life, and weight savings. The tracks are specially engineered to over- come such common problems of the standard shapes as excessive local flange bending due to wheel loading. The advantages of the proprietary products make their use worthwhile for most monorail applications. 15.4.2Loads Acting on Monorail Runway Beams The vertical load V indicated in Fig. 15.2 includes the lifted weight and the weight of the hoist and trolley. It also includes impact that the AISC specifi- cation7prescribes to be taken as 10 percent of the maximum wheel load for pendant-operated cranes and 25 percent of the wheel load for cab-operated cranes. The side thrust S is specified as 20 percent of the lifted load includ- ing the crane trolley; this lateral force is equally divided among all the crane wheels. According to AISC, the longitudinal force L caused by trolley decel- eration is to be taken as 10 percent of the maximum wheel loads. Some building codes contain design provisions different from these, and the load percentages of the AISC specification apply only if not otherwise specified by other referenced standards. Also, some codes contain more detailed provisions for monorail cranes. The International Building Code8 SPECIFYING CRANE BUILDINGS425 FIGURE 15.2Loads acting on monorail runway beam (proprietary track shown). does not require any load increase due to impact for bridge cranes and monorail cranes with hand- geared bridge, trolley, and hoist, but for powered monorail cranes, the IBC specifies an impact load increase of 25 percent. For overhead cranes, various design authorities impose higher minimum requirements for impact and lateral loads acting on runway beams. For example,ANSI MH 27.19specifies the impact allowance for electric-powered hoists as 1?2percent of the rated load for each foot per minute of hoisting speed, with a minimum allowance of 15 percent and a maximum of 50 percent. For bucket and magnet appli- cations, it requires the impact allowance to be 50 percent of the rated load. Each of these forces must be resisted by suspension supports and lateral bracing. 15.4.3Suspension and Bracing Systems Monorails running perpendicular to the primary frames are ordinarily designed to span between the frames without any intermediate supports. When the monorails run parallel to the frames, additional support beams are needed. There are two basic suspension systems for attaching monorail beams to the frames or support beams: rigid and flexible. In a rigid, or fixed, system, the beam is connected to the frame by relatively stiff steel support members. Depending on the available vertical clearance to the frame, the support member can con- sist of a simple bracket welded to the underside of the frame rafter or supporting beam (Fig. 15.3a), or a longer steel section (Fig. 15.3b). The short bracket is usually capable of resisting both vertical and side-thrust reactions, but longer sections may need to be supplemented by diagonal angles. The suspended load applies a concentrated force on the supporting frame, and additional web stiff- eners and welding are generally needed to reinforce the rafters at those locations. Some manufactur- ers provide a single stiffener (Fig. 15.3), others provide double stiffeners (Fig. 15.4). Regardless of the actual detail, the contract documents should indicate who is responsible for the various components of the suspension system. Figure 15.4 illustrates one manufacturers approach, in which the frame stiffeners and a short shop-welded bracket are provided by the manufacturer, while the other compo- nents and bracing are provided by the crane supplier. A flexible suspension uses hanger rods instead of brackets (Fig. 15.5). The rods are typically attached to hangers placed above the top flange of the frame, and the rod length can be easily adjusted. According to published data,3,10flexible suspension tends to result in lower crane loads 426CHAPTER FIFTEEN FIGURE 15.3Monorail supports with rigid suspension: (a) minimum-length bracket; (b) long bracket. (Metallic Building Systems.) SPECIFYING CRANE BUILDINGS427 FIGURE 15.4Details of fixed suspension and stiffener welding. The manufacturer typically excludes the dashed items from its scope of work. (Nucor Building Systems.) FIGURE 15.5Web stiffener welding detail for flexible suspension. The manufacturer typically excludes the dashed items from its scope of work. (Nucor Building Systems.) and reduced wear. Monorail beams supported by hanger rods always require antisway lateral brac- ing for stability. Any suspension system should be vertically adjustable to bring the monorail beams to a true horizontal position prior to installation of the lateral bracing. Obviously, flexible suspen- sion makes adjustments easier. The side thrust S has been traditionally resisted by a shop-welded channel laid flat on top of an I beam (Fig. 15.3). The proprietary track, with its wide top flange, makes the channel unnecessary. The side thrust can also be resisted by intermittent lateral bracing of the girders top flange; such bracing must be designed not to interfere with vertical deflection of the monorail beam. Lateral brac- ing of this sort might be impractical in pre-engineered buildings with cold-formed purlins: the purlins have little lateral stability of their own and cannot accept bracing loads. Some additional structural members must then be introduced to resist the lateral bracing forces, or adequate lateral bracing provided between the purlins to distribute these forces into the roof diaphragm. The longitudinal runway force L may be resisted by a diagonal angle brace located in the plane of the monorail beam at approximately 100-ft intervals and at all runway turns.5Again, some added structural members (or at least boxed headers placed between the purlins) are needed to resist the bracing forces. The locations and conceptual details of all vertical and lateral supports should be indicated in the contract documents. The vertical supports should ideally occur at each main frame or at 20- to 25-ft intervals if the monorail runs alongside the frame. A special case arises when two parallel monorail beams must transition into a single perpendicular beam. This problem can be solved with monorail switches. 15.4.4Design Considerations for Runway Beams ANSI MH 27.1 requires that the allowable stress in the lower (tension) flange of monorail run- way beams be limited to 20 percent of the ultimate steel strength. It also specifies the deflection criterion for monorail runway beams with spans of 46 ft or less as the length between vertical supports divided by 450. The L/450 deflection criterion is also found in other sources, including the MBMA Manual. Monorail beams must be carefully spliced to allow for smooth wheel movement between the individual beams. The best splice detail involves full-penetration welding of the bottom flange in combination with bolted shear plates in the web. Some pre-engineered manufacturers prefer to use field-welded tie plates and locate the splices under each hanger (Fig. 15.6). To ensure that the splice does not interfere with the trolley travel, it is wise to require a test run of the trolley through the whole length of the monorail before accepting the work. Who supplies the runway beam and its supports? The metal building manufacturer already pro- vides the suspension supports and could also provide the runway beam if specifically required to do so by contract (normally, crane work is excluded from the manu- facturers scope of work). The runway beam supplied by a building manufacturer is likely to be of standard structural shape. Whenever proprietary runway beams or switches are specified, they should be furnished and installed by the monorail supplier. 15.4.5Special Requirements for Supporting Frame Rafters Whichever suspension system is selected, the weight of the loaded monorail needs to be transferred into the supporting frame rafter. A suspended load that attempts to te
