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Adjustable shoring systems

Types available and how they work

BY ANNE SMITH

S

horing has come a long way from the job-built lumber systems originally used. Many systems now available are modular and factorybuilt of steel or aluminum. Metal systems eliminate much of the cutting and measuring necessary with older methods. They also have predictable load capacities, minimizing the uncertainty involved when using job-built systems. Today's shoring systems are versatile too. Built-in or attachable height adjustment devices, interchangeable shore heads, and other accessories enable both wood and metal systems to accommodate different shoring conditions. To help speed construction, especially on multistory projects, many shoring systems are designed for fast erection, disassembly, and relocation. Fast-attach clamps, snap locks, integral couplings, and other connection devices reduce the need for nails, screws, and pins. After shores are assembled, workers can sometimes roll, crane-lift, or skid them from one location to another without disassembly, depending on the system used. Because adequate shoring is critical to the safety of a construction project, a primary consideration in choosing a system is its load capacity--shoring must be able to support concrete and formwork loads until the concrete becomes structurally self-sufficient. Of equal im-

portance is adequate bracing for any shoring system selected. Some of the systems described here have their own bracing; others require added braces or struts. With the many shoring systems available, you probably will find several systems that meet a project's load requirements. To choose from these systems, select the one that best meets project needs for adaptability and speed of erection or relocation. The following overview can help you match a shoring system to project re q u i re m e n t s. It describes six types of adjustable shoring methods and some of the features and capabilities of systems available in each category.

Single post shores

A post shore is a vertical support generally made of steel, aluminum, or wood. It can be used alone or in combination with other shoring systems and is often used for reshoring. Metal post shores typically have two parts: a base tube with a threaded collar and a staff tube of smaller diameter that fits into the base (Figure 1). Tubes can be round or square. Round-tube systems are available with maximum load-carrying capacities of 5,000 to 10,000 pounds. Square-tube systems have much higher load-carrying capacities (about 20,000 pounds). The safety factor (ratio of ultimate load

Figure 1. The components of a metal post shore. A lock pin, inserted into the desired hole, connects the staff to the base and determines the shore height. A handle on the base tube's threaded collar allows fine height adjustment.

Figure 2. Clamps overlapping 4x4 wood posts allow height adjustments of more than 5 feet and shoring heights of more than 20 feet, depending on post length. Using a special jack that grips the lower post, workers can raise the upper post about 1 inch per stroke for fine height adjustments after the shores are in position.

to allowable load) for both squareand round-tube systems ranges from 2.5:1 to 3:1 as recommended by the Scaffolding, Shoring and Forming Institute Inc. (SSFI). Many metal post shores use a pinand-hole system for adjustable heights of up to about 16 feet. The staff has a series of holes spaced at equal increments, usually 4 or 5 inches. A pin, inserted into the desired hole, connects the staff to the base and determines the shore height. After the shore is in position, an adjusting handle on the base tube's threaded collar allows fine height adjustment. On some systems, the pin is attached to the shore with a chain so there are no loose parts. Instead of an inner-outer tube arrangement, one square-tube post

shore system has a threaded central adjustment unit to which two extension tubes of equal cross section are attached. The extension tubes come in lengths from 1 to 10 feet in 1-foot increments and the central adjustment unit length is extendible from 2 feet to 3 feet, 1 inch. This allows shore heights of up to 25 feet. Workers turn a hammer nut on the central adjustment unit to make minor height adjustments after the shore is in position. Wood post shores often cost less than steel or aluminum shores, but they generally have lower load-carrying capacities so more shores are needed. Attachments are available to make a wood shore height adjustable. One system, which uses a pair of clamps to overlap two 4x4 wood posts, allows height adjust-

ments of more than 5 feet (Figure 2). Nail the clamps to the lower shore member, spacing them 12 inches apart, then slide the upper member through the clamps and adjust it to the desired height. A special jack allows fine height adjustments after the shore is in position. This system can shore to heights of more than 20 feet, depending on the length of 4x4s used. Maximum allowable load is 6,000 pounds with a 3:1 safety factor at an unbraced height of 10 feet. To make a one-piece wood post shore height adjustable, use a metal screw jack (Figure 3). This attachment nails to the bottom or top of a 4x4 or 6x6 post. Using its adjustment handle, a worker can screw the jack up or down 3 inches for a 6inch height adjustment range. Many jobs require post shores to be braced with wood to increase their stability. To allow attachment of wood braces to metal shores, nailer brace plates are available that accommodate most lumber widths. There's also a brace plate attachment for wood post shores that allows attachment of standard steel cross braces for forming shoring towers (Figure 4). To install the plates, simply nail them to each shore at 4-foot spacings. Steel studs on the plates hold cross brace ends, and double-headed nails, inserted through holes in the studs, secure the braces. Plates stay on the post shores permanently to ensure the tower has the same dimensions e ve ry time it's assembled.

Scaffold-type frame shoring

Tubular-steel frame scaffolding was originally developed to support workers and relatively light loads of construction materials. Contractors later discovered that scaffold frames made excellent shoring supports because of their modular assembly and system of jacks that allow for easy height adjustments. Because the loads of concrete and formwork are much heavier than the loads scaffolding was originally designed to carry, manufacturers devised heavy-duty scaffold frames specifically for shoring.

Comparison of Adjustable Shoring Systems

Type Composition Maximum shoring height*

20 feet or more

Maximum load rating(lb)*

6,000

System advantages

Excellent height adjustment range when used with clamps (wood systems) or extension tubes (aluminum or steel systems). Can be used to augment other shoring systems.

Post shores

Wood

Aluminum or steel

25 feet

10,000 (for round-tube systems) 20,000 (for square-tube systems) 25,000 per leg (for round-tube systems) 100,000 per leg (for square-tube systems) Excellent height adjustment range when used with extension tubes or frames. Assembled towers can be rolled or cranelifted for fast relocation. Can support spans of 20 feet or more without intermediate vertical shoring Excellent height adjustment range when used with extension legs. Truss assemblies, including decking, can be rolled out from the bay then crane-lifted for fast relocation. No need for vertical shoring. Some brackets have rollers for easy movement of flying forms. No need for vertical shoring

A basic shoring tower is made of two frames connected by steel cross b ra c e s, forming the equivalent of four post shores braced for stability (Figure 5). Frames are made of steel or aluminum and come in seve ra l dimensions, ranging from 2 to 4 feet wide and 3 to 6 feet tall. Aluminum shore frames typically weigh 40% to 50% less than steel frames for the same load capacity, reducing labor requirements for assembly and relocation. By stacking frames and connecting them with coupling pins, towers of almost any height can be formed. Some systems have special extension frames or tubes that telescope from the legs of the base frame. Pins inserted into holes in the base frame legs set the desired extension height. Coupling pins for most frame sys-

Scaffold-type frame shoring

Aluminum or steel

Can be stacked to almost any height

Horizontal shoring

Aluminum or steel

Limited only by height of ledger or beam supports used 20 feet

Generally high; varies with span length 3,300 per lineal foot, per truss (depending on jack spacing)

Flying truss systems

Aluminum

Columnmounted brackets

Steel

Limited only by height of column or wall support used Limited only by height of column support used

100,000

Friction collars

Steel

50,000

*Maximum shoring heights and load ratings depend on the particular system used, bracing, and job conditions. Not all systems are capable of the maximums given here.

Figure 3. This metal screw jack turns a wood 4x4 or 6x6 into a heightadjustable shore. It nails to the bottom or top of the post and allows a height adjustment range of 6 inches.

Figure 4. A metal brace plate, nailed to a wood post shore, has two studs that allow attachment of steel cross braces. Double-headed nails, inserted through holes in the studs, secure the braces. tems are loose and inserted in the frame legs after stacking. Some systems, howe ve r, have built-in coupling devices to eliminate loose pieces. One system, for example, has a welded-on, horseshoe-shaped coupler that slides over the lower frame leg and locks into place with an attached key. Cross braces for frame systems come in a variety of lengths to accommodate different frame spacings. One system, for example, has nine lengths of cross braces for frame spacings from 3 to 12 feet. Cross braces for some systems have two holes at each end, allowing attachment in two positions. This reduces the number of cross brace sizes needed. One manufacturer paints each of its cross brace sizes a different color. This color coding system speeds tower erection by making it easier for workers to find the right size cross brace. Locks that quickly snap or slide into place attach cross braces to frame legs without the need for wing nuts, enabling fast tower erection and dismantling. Because multi-tiered towers usually require lum-

ber bracing to increase stability, nailer brace plates, like those used for metal post shore systems, are available with most frame systems. Load ratings for frame systems are based on a 2.5:1 safety factor. Most frame systems are made of round tubing and have load capacities from 5,000 to 25,000 pounds per leg, depending on the system used and height and bracing of the shoring tower. Some manufacturers make frames of heavy-wall square tubing to support load capacities of 25,000 to 100,000 pounds per leg, or 100,000 to 400,000 pounds per assembled tower (Figure 6). By using accessories, frame systems can be adapted to meet almost any shoring condition. One of the most useful accessories is the screw jack. Attached to the top or bottom of the frame legs, screw jacks allow

workers to level frames on uneven terrain, make minor height adjustments, and lower the frames for stripping clearance. Most screw jacks have maximum extensions from 12 to 27 inches. One tower system, howe ve r, has jacks that extend up to 36 inches. These jacks pin to holes spaced at 4-inch intervals in the frame legs and have adjusting screws for fine adjustments within a 5-inch range. Because the jacks allow 6 feet of extension when attached to both the top and bottom of a frame, they can be used in place of a second 6-foottall frame. Interchangeable U-heads, Jheads, and other support plates also increase the adaptability of frame systems, allowing them to support a variety of stringers. They quickly attach to the tops of frame legs or top

Figure 5. Two scaffold-type frames connected by cross braces form a basic shoring tower. Bottom and top screw jacks allow height adjustment.

screw jacks, depending on system setup. Caster attachments available with some frame systems enable workers to roll assembled towers from one bay to another for reuse. Consisting of a steel wheel mounted on a height-adjustable support bracket, the attachment clamps to the bottom of each frame leg, just above the screw jack. After rolling the tower into position, workers retract the wheel by turning an adjusting screw. Assembled frame towers also can be crane-lifted for reuse at another location.

Horizontal shoring

Made of aluminum or steel, horizontal shores have an inside member, usually an I-beam or plate section, which telescopes from an outside lattice member or boxbeam (Figure 7). Resting on beams or ledgers, these systems can support deck spans from about 41/2 to more than 20 feet without intermediate vertical shoring. They also can be used as joists directly under plywood, replacing timber joists. When a horizontal shoring is loaded to the capacity recommended by its manufacturer, built-in camber enables the member to produce a level slab as cast. To speed erection and disassembly, these systems usually have hammer-in-place wedge locks, which quickly set shore length, and tapered bearing prongs for easy stripping. Some systems have mated bearing prongs that make it easy to accurately align adjacent shores. Also available are hanging devices to suspend the shore from ledgers.

Figure 6. Squaretube frames generally support greater loads than round-tube frames. This square-tube steel system supports up to 400,000 pounds per tower.

Figure 7. An underneath view of decking supported by horizontal shoring. The shore ends have bearing prongs (not visible here) that support the shores on beams; no vertical shoring is needed.

Flying truss systems

Flying truss tables are ideal for high-rise projects requiring rapid cycling using minimum labor. Supported on a previously cast slab, the aluminum trusses are topped with aluminum or wood joists to support plywood decking or pan forms for a complete bay (Figure 8). Two or three trusses are used, depending on the table width. Adjustable bracing holds the truss units

in a fixed position. Using jacks, workers lower the truss assembly during stripping onto rollers or gliders positioned on the deck directly under the bottom chord. This allows them to easily push the entire assembly, including the plywood decking, out of the bay for crane lifting to the next floor. On ave ra g e, a crew of eight can fly and reset a truss assembly every 15 mintes.

Modular in design, flying trusses are available in many lengths and can be joined by splice connections to meet various length requirements. Different cross brace sizes allow truss spacings of about 6 to 20 feet to accommodate different table widths. Standard truss heights are 4, 5, or 6 feet, but most systems have adjustable extension legs to accommodate ceiling heights greater

Friction collars

Similar to shore brackets, friction collars mount onto columns to support forms without the need for vertical shores. Instead of bolting into the column, howe ve r, the two sides of the collar bolt tightly together around the column like a clamp (Figure 10). Available for round or rectangular columns, the collars have a 2:1 safety factor and come in load capacities up to 50,000 pounds. Each collar has two adjustable swivel jacks to support forms. Unlike shore brackets, friction collars do not have rollers for easy movement of flying forms. Figure 8. Three flying truss tables, each supporting decking for an entire bay, are shown. The table on the left is being crane-lifted to the next floor. The one in the middle has been rolled out of the bay and is ready for crane attachment as soon as the crane is free. than 9 feet and grade changes such as ramps or sloping floors. On some systems the legs telescope from leg sockets integral to the truss. The legs retract into the sockets during flying. One truss system also has retractable top extension staffs to shore variable soffit heights. Screw jacks, fitted to the base of each extension leg, allow minor height adjustment and leveling of the truss table. A 5-foot truss shore can support loads of about 1,400 to 3,300 pounds per lineal foot, depending on jack spacing. lows the bracket to be positioned up to 10 3/4 inches from the column. A bearing plate on top of the b ra c k e t's adjusting screw supports form I-beams under concrete loading. To strip the forms after the deck is poured, workers lower the adjusting screws. Some brackets have rollers to make it easy to move flying forms (Figure 9). The I-beam rests on the roller after it's lowered, allowing easy horizontal movement of forms by winch or form puller so they can be hooked to a crane and flown to the next floor. One manufacturer has specially designed flying deck forms for use with its shore brackets. Partially assembled (steel form joists and Ibeams joined), the forms collapse like accordions for delivery to the jobsite. This allows up to 12,000 square feet of forms to be hauled on one flatbed truck. Once onsite, the forms are easily opened, squared, and bolted into position. Load capacities of columnmounted brackets accommodate a wide range of shoring requirements. One manufacturer, for example, makes brackets to support shoring loads from 10,000 to more than 100,000 pounds with a 4:1 safety factor.

Before you buy

Although it's most important to select a shoring system based on its performance capabilities, consider s e ve ral other factors before buying. Often it's more economical to lease rather than buy a shoring system if it can be used on only one or a few p ro j e c t s. Check with the shoring manufacturer to see if leasing the system is an option, then compare the costs of leasing versus buying.

Column-mounted shore brackets

Brackets that bolt to columns or walls can support flying deck forms without vertical shores. Because loads are transmitted to columns and walls and not new slabs, most reshoring can be eliminated too. This leaves clear spans under the deck so other trades can work unhindered. Each shore bracket has an adjusting screw that workers can turn manually to make minor changes in form elevation before placing the concrete deck. One bracket system adjusts horizontally as well as ve rt ically; a special extension piece al-

Figure 9. A column-mounted shore bracket eliminates the need for vertical shoring or reshoring, leaving clear spans under the deck. This bracket has a roller to allow easy horizontal movement of flying forms.

Figure 10. Friction collars that bolt onto round (left) or rectangular (right) columns support decking on adjustable swivel jacks.

Shoring Safety

Even shoring systems with loadcarrying capacities adequate for job conditions can fail if not properly erected. The following publications from the Scaffolding, Shoring, and Forming Institute Inc. (SSFI) serve as valuable jobsite references for proper shoring erection and safety p ro c e d u re s. When in doubt about any erection procedure, howe ve r, always consult your shoring equipment supplier.

Safety rules (50 cents each)

Single Post Shore Safety Rules Steel Frame Shoring Safety Rules Flying Deck Form Safety Rules Rolling Shore Bracket Safety Rules Ho ri zontal Shoring Beam Safety Rules

Make sure the shoring system has all the components necessary to meet your project needs. Shoring failure could result if you try to augment a shoring system made by one manufacturer with components from another manuf a c t u re r's system. The systems may have different safety factors and dimension tolerances. Find out if the shoring manufacturer offers jobsite assistance and training. Some manufacturers will help lay out, assemble, and inspect their systems. Some will even train crews in the correct handling and usage of their systems to help ensure proper assembly and maximum labor efficiency.

Erection procedures

Recommended Steel Frame Shoring Erection Proce dures ($1 each) Recommended Safety Requir ments for Shoring Concrete Formwork ($2 each) A safety slide presentation, Shoring Safety Dos and Don'ts, also is available from SSFI for $95. To order any of the above publications or the slide presentation, contact SSFI, 1230 Keith Building, Cleveland, OH 44115 (216-2417333).

PUBLICATION #C900921

Copyright © 1990, The Aberdeen Group All rights reserved

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