Construction ts 1b6s2o

Construction ts for multistory structures Where to locate them, how to form them BY BRUCE SUPRENANT DEPARTMENT OF CIVIL ENGINEERING & MECHANICS UNIVERSITY OF SOUTH FLORIDA TAMPA, FLORIDA

onstruction ts are stopping places for concrete work. They’re needed because it’s not feasible to place all the concrete for most jobs in one pour. Breaking the job into smaller portions allows the contractor to: Match pour size with concrete delivery capabilities

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■ Choose the best crew size ■ Make best use of equipment ■ Reduce formwork costs by increasing form reuse Vertical and horizontal construction ts must be carefully located to produce these b enefits without weakening the structure. And to function properly the ts must be built correctly.

Where to install construction ts The architect or engineer often specifies construction t location

Figure 1. The ACI Code requires construction ts in elevated slabs to be located within the middle third of spans of slabs, beams, and girders. ts must be located at least two beam widths away from beam-girder intersections.

in the contract documents. Sometimes, though, the contractor is asked to submit a construction t plan after the bid letting. In either case, the engineer or contractor should consider economy, appearance, strength, and durability when choosing t location. Economy. If the contractor has input on where to put construction ts, he can match the pour size to his crew’s capabilities. Also, placing a construction t in a practical spot, such as between footing and walls, avoids complex, costly formwork. Appearance. Construction ts can influence how a structure looks. Often they are installed in an inconspicuous position. Sometimes, though, they are used as a feature of the structure. Some designers use reglets and rustication grooves to hide the ts. Whatever scheme is used, the ts should fit the architectural design. Strength. A construction t introduces a weak vertical or hori zo ntal plane in an otherwise monolithic concrete member. This obvious slip plane may reduce the strength of beams, columns, and walls. Test results show that construction ts reduce a member’s shear strength but not the flexural or bending strength. Tests conducted on simply ed beams with a vertical construction t at the center showed the same load-deflection and ultimate moment characteristics as an unted beam (Ref. 1). Measurements of crack widths also indicated similar performance between ted and unted beams. These results held true for a t that was deliberately debonded between the two concrete pours. The construction t in these tests was located in a region of negligible shear. In a region where the shear is significant, a construction t with a smooth surface reduces the shear strength of the member by 40%. But a beam having a t with a roughened surface has a failure mode and ultimate load similar to an unted beam. A 1⁄16-inch texture over the

Figure 2. Bulkheads form vertical construction ts for elevated slabs (above) and beams (below). Bulkheads are stiffened and braced to resist lateral concrete pressures.

entire face qualified as a roughened surface in these tests (Ref. 1). Du ra b i l i t y. The location of construction ts also is affected by whether or not water and salts can easily enter the t. Although concrete has low permeability to water and salts, construction ts don’t. Water and salt leaking through ts may cause rusting of rebar, p re s t ressing tendons, and anchorage hardware in parking structures. Locate construction ts at high points in the floor away from drains. Where durability is critical, use fewer ts by allowing larger floor pours. Consider using waterstop at some or all t locations.

Code (Ref. 2, Section 6.4.4) requires that: “Construction ts in floors shall be located within the middle third of spans of slabs, beams, and girders. ts in girders shall be offset a minimum distance of two times the width of intersecting beams” (Figure 1). There may be high shear forces even at midspan, for instance, where a beam intersects a girder. That’s why the code requires ts to be located at least two beam widths away from a beam-girder intersection. It’s a good idea to keep construction ts away from point loads (such as heavy equipment) for the same reason. Locate construction ts in walls and columns on the underside of floor slabs, beams, or girders, and at the tops of footings or floor slabs.

Figure 3. Keys are sometimes formed in bulkheads to increase t shear resistance. This t design also includes a waterstop. Note that the plywood bulkheads aren’t notched. Plywood pieces are cut, positioned under rebars, and nailed to 2x4 vertical s.

ACI requirements for construction t design Because construction ts are most likely to reduce shear strength, they should be located where shear forces are low. Under uniformly distributed gravity loads, shear forces are low in the middle of a flexural member span. Thus, the American Concrete Institute (ACI) Building

Plan View

Figure 4. Detail shows how to form an inconspicuous horizontal construction t without a rustication strip (Ref. 4).

To avoid cracking from settlement, pour concrete in columns and walls at least 2 hours before placing concrete in slabs or beams framing into them (Ref. 3). Deg concrete for lateral forces may require special design treatment of construction ts. Shear keys, diagonal dowels, or the shear transfer method (ACI Code, Section 11.7) may be used.

How to form construction ts Deciding where to put construction ts is only half the job. The other half is forming them correctly and cleaning the t surface before the next concrete placement. Bulkheads for construction ts are normally formed with wood, although expanded metal mesh has also been used (See Concrete Construction, July 1983, page 552; November 1986, page 966; Fe b ru a ry 1988, page 214). Forming costs are high because rebar has to through the bulkhead. Wood are notched or a gap is left in the plane of the rebar. Leaving a gap allows each bulkhead board to be positioned and removed without putting pressure on the bar. Excessive pressure might cause green concrete to spall or crack. Because they are cut to fit and usually damaged when stripped, bulkheads a re n’t reusable.

Construction ts in floor slabs usually are made of 2-by materials or plywood (Figure 2). Wall or beam bulkheads are usually made of plywood. Shear keys, if required, can be made by beveling 2x4 or 2x6 lumber (Figure 3). Ho ri zontal construction ts in walls and columns are generally spaced one story apart. Use chalk lines or nails driven on the inside of wall forms to mark the t location. Or use a wood strip nailed to the forms. For architectural concrete, more care is needed to get straight and level horizontal ts (Ref. 4). Nail a 1-inch wood strip to the inside face of the forms near the top (Figure 4). Put tie rods about 4 inches below the t to forms for the next lift. Place concrete to a level slightly above the bottom of the s t ri p. Remove the strip after concrete has set enough to hold its position. When forms are set for the next lift, locate another row of ties just above the t. Ties above and below the t hold the form tightly against concrete in the lower lift. Don’t overlap sheathing more than an inch over the lower lift. This also helps prevent leakage. Grooves or rustications are a good way to hide horizontal ts (Figure 5). Rustication strips are best attached with double-headed nails d ri ven from the outside. After nails

are pulled, the strips will pull loose from forms during stripping and can be left in the concrete longer to protect edges from spalling. Make sure that rebars behind the rustication strip have adequate cover.

Construction t surface preparation After placing and curing the concrete and removing the bulkhead, the hardened concrete t surface must be prepared. ACI 318-83 requires that: “Surface of concrete construction ts shall be cleaned and laitance removed. Immediately before new concrete is placed, all construction ts shall be wetted and standing water removed.” Use stiff wire brushes, scabblers, waterblasters, or sandblasters to clean surfaces and remove laitance. Expanded metal mesh bulkheads left in place also provide a good rough bonding surface (Ref. 5). The 1977 ACI Code required application of a neat cement paste to vertical construction t surfaces. This procedure was not always practical and is no longer required.

Team effort Understanding the why, where, and how of construction ts for multistory structures involves a team approach. The architect wants ts that are consistent with the

SHEAR WALLS NEED CONSTRUCTION TS TOO

Figure 5. Rustication strips at construction ts hide t lines and add to the architectural effect (Ref. 4).

architectural design. The engineer wants ts that do not reduce the strength or durability of the structure. The contractor is looking for ts that enhance his productivity. A good construction t plan involves all three parties. References 1. Monks, W. L., and B. M. Sadgrove, “The Effect of Construction ts on the Performance of Reinforced Concrete Beams,” Technical Report, Cement and Concrete Association (London), 1973. 2. Building Code Requirements for Reinforced Concrete, ACI 318-83, (Revised 1986), American Concrete Insti-

Earthquakes have shown engineers and contractors that construction ts in shear walls can form the weakest link in the loadresisting mechanism of a structure. During the 1964 Alaska and 1971 San Fernando earthquakes, h o ri zontal construction ts in both medium and low-rise buildings were damaged, some beyond repair. Especially troublesome were lightweight concrete floor slabs connected to normalweight concrete shear walls through two construction ts (one at the bottom of the slab and one at the top of the slab). An adequate construction t for beams and columns may not be satisfactory for walls that need to resist large shears generated during earthquakes. Earthquakes generate many cycles of high-intensity shears that can deteriorate tute, Box 19150, Detroit, Michigan, 48219, 1987. 3. Specifications for Structural Concrete for Buildings, ACI 301-84, (Revised 1985), American Concrete Institute, 1986. 4. Formwork for Concrete, Publication SP-4, Fourth Edition, (Revised second printing), American Concrete Institute, 1981.

ts. This repeated loading reduces the t interface roughness, thus reducing the frictional resistance. Don’t rely on t roughness to resist seismic shears. Use shear keys or extra vertical re i n f o rc i n g bars placed at the t interface. Minimize the number of hori zo ntal construction ts by slipforming the shear wall tower or by making each concrete pour two or three stories high. Consider deg the construction t using the shear friction concepts from the ACI Building Code. The shear wall is an important load-resisting mechanism and the designer spends many hours analyzing the forces and stresses in the wall. The weakest link, the construction t, needs some design time too. 5. Building Movements and ts, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois, 60077, 1982.

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