Wall Bracing to Slab-on-Ground Floors
By: David L. Kelly, consultant — retired from Meadow Burke
This article originally appeared in the July 2007 issue of ‘Concrete International.’
In the Tilt-Up construction method, concrete wall panels are typically cast in a horizontal position on a ground-supported slab. Because the size (and weight) of a typical Tilt-Up panel can be quite high, the panels are normally cast near their final locations, and each panel is lifted and moved from the casting slab to its final position in one continuous operation. Before the crane rigging is disconnected and the crane boom is moved to the next panel, however, each panel must be laterally supported by temporary braces. There are issues and responsibilities that arise during this important phase in the life of a Tilt-Up building—the time after panels have been erected but before the lateral-force-resisting system for the building stabilizes the individual panels.
A QUESTION OF RESPONSIBILITY
Many parties have vested interests in a successful construction phase for a Tilt-Up concrete building. Those directly concerned with panel erection and stability include:
- The General Contractor (GC);
- The Tilt-Up Panel Subcontractor;
- The Tilt-Up Insert and Bracing Supplier; and
- The Delegated Engineer who provides specialty service for, in this case, design of the lifting and bracing devices for the Tilt-Up panels.
Clearly, the temporary bracing is within the domain of these parties. But, if the temporary bracing is tied to the slab-on-ground, does that domain also extend to the Slab-on-Ground Floor Subcontractor and the Engineer of Record (also termed the Engineer in Responsible Charge or the Prime Professional)?
MORE THAN A FLOOR
For most Tilt-Up buildings, the slab-on-ground floor will typically comprise a major part of the lateral-force-resisting system for the structure, so the Engineer of Record must ensure that a load path exists from the wall panels (normally the walls act as shearwalls) through the floor and to the subgrade. The slab-on-ground floor must also serve as a high-quality wearing surface or as a substrate for the final flooring materials. The slab-on-ground designer (often, the Engineer of Record) and the floor subcontractor must therefore give careful attention to subgrade stiffness, surface hardness and finish, service roads (fork lifts and rack loads), joint spacing, joint load transfer, joint filler, slab reinforcement, and slab thickness. These factors are detailed in ACI 360R-06, “Design of Slabs-on- Ground,”2 and ACI 302.1R-04, “Guide for Concrete Floor and Slab Construction.”
Before a Tilt-Up building is completed, however, its slab-on-ground floor is often quite literally the factory floor. The slab serves as a staging area for materials as well as the major forming surface for the panels. This is no small role, as the formed surface may be the future exterior surface of the panel, largely defining a building’s overall aesthetic quality. In a list of potential construction loads that should be considered by the designer, ACI 360R-06 includes concrete trucks, dump trucks, hoisting equipment, and cranes used for steel erection, tilt wall erection, and setting equipment. The guide further notes that “Some of these loads can exceed the design limits and, therefore, the construction load case should be anticipated, particularly relative to early-age concrete strength.” Unfortunately, the guide appears to provide no warning regarding the effects of bracing loads on a slab-on-ground floor.
Although ACI 551.1R-05 does state that temporary bracing is typically designed by the engineer that designs the lifting inserts for the Tilt-Up panels, it also provides little guidance to the Engineer of Record regarding the effect of bracing on the slab-on-ground floor. There is at least a statement, however, that hints at a possible overlap in design responsibility for the concrete floor: “If a strip of floor slab is left out until the panels are erected, deadmen for anchoring the braces may be required until permanent connections are made.”
So, who is responsible for ensuring that a slab will be capable of resisting bracing loads?
The Engineer of Record may say that panel bracing and associated hardware can be classified as means and methods that are in the domain of the GC. After all, when the building design is underway, how will the Engineer of Record know how the panels will be braced or what bracing and anchor types will be used (the engineer may not even know who the GC will be)?
When selected, the GC may say that only the contract documents were bid. Special slab provisions for bracing loads, such as thickening or adding reinforcing to the slab-on-ground, may not be the GC’s concern unless the GC is self-performing the casting and erection of the panels. The GC may also be reluctant to change the slab design shown on the drawings to accommodate the bracing forces—not only could the changes be uncompensated, the GC could also incur responsibility for the service-life performance of the slab. Finally, the GC can always argue that only the Engineer of Record is intimately aware of the design requirements and has the necessary expertise to properly design the slab.
The Tilt-Up subcontractor will have no contractual control over the slab-on-ground conditions and is often dependent on a bracing supplier for providing a highly critical and highly specialized component of the Tilt-Up system.
The bracing supplier will normally employ the services of a delegated engineer who designs the braces and their connections for wind loads. The delegated engineer will rarely have a contract or the authority to design the slab-on-ground. In fact, the slab-on-ground could very well be in place at the time the bracing design is completed. Finally, all bidders can argue that they would risk losing the contract if they were to include services not indicated in the contract documents.
So, what are the possible consequences of these confused relationships?
- Anchors may pull out of the slab. There are no post-installed anchors that can resist a substantial load in concrete less than 5 inches (125 mm) thick, so a floor slab designed with no consideration of bracing loads may be too thin to develop the required anchorage forces for the braces;
- The floor slab may crack. Even if the slab is thicker than 5 inches (125 mm), it may not have enough strength to prevent flexural cracking when wind forces cause uplift at the anchor points. This can lead to anchor pullout at loads below the expected capacity of the anchor; and
- The floor slab may slide. Slab panel joints are deliberately designed to avoid transferring tension forces, and slabs are often constructed with polymer vapor retarders—friction forces may therefore be limited. Sliding can lead to panel misalignment or, in extreme cases, allow a panel to topple.
Who (or What) Dictates Bracing Forces?
Guidelines for determining bracing forces during construction are published by the Tilt-Up Concrete Association (TCA). The current version of TCA’s guidelines for temporary wind bracing uses the wind force recommendations provided in SEI/ASCE 37-02,6 the U.S. standard providing minimum design load requirements during building construction. Per SEI/ASCE 37-02, a basic wind speed with a 50-year mean recurrence interval can be adjusted for the reduced construction period. In this case, the construction period is assumed to be 6 weeks to 1 year, so the reduction factor is 0.8. Most of the U.S. is located in a 90 mph (40 m/sec) wind zone (50-year recurrence interval, 3 second gust speed), so TCA recommends a minimum wind speed of 72 mph (32 m/sec) for bracing loads, resulting in the equivalent uniform pressures on solid sign-type structures given in Table 1. In coastal regions, higher wind loads are required to be used for building design, but SEI/ASCE 37-02 allows the use of a 90 mph (40 m/sec) building design wind speed (72 mph [32 m/sec] bracing design wind speed) if special precautions are taken during hurricane season.
Per Section 8 of the TCA bracing guidelines, the floor slab, footing, or deadmen should be designed with sufficient strength and weight to resist the applied brace loads, using a minimum safety factor of 1.5. The section provides a method for estimating the required thickness of a floor slab to safely anchor wall braces and recommends the designer consider the type and location of joints, the thickness and reinforcement in the slab, the strength of the concrete in the slab, and the size and location of slab closure strips. For sliding resistance, a friction coefficient of 0.5 is recommended.
The required slab thickness is estimated using Eq. (1) through (3) that are based on the geometry illustrated in Fig. 3. These equations include consideration of sliding resistance only. Slab bending due to uplift must be checked separately.
S = (1.5WpH2)/[2(V + Y)] (1)
U = S (V/X ) (2)
T = (2S + U)/[wc(J – P)] (3)
Where S is the sliding force in lb/ft (kN/m) including the factor of safety; U is the uplift force in lb/ft (N/m) including the factor of safety; T is the required slab thickness, ft (m); Wp is the wind pressure in psf (kPa); H is the total panel height in ft (m); V is the height of brace connection above the floor slab in ft (m); Y is the distance from top of footing to top of slab in ft (m); X is the distance from face of panel to the floor connection for brace in feet (m); wc is the unit weight of the concrete in lb/ft3 (kN/m3); J is the distance from inside face of the panel to the first slab joint beyond the anchor point in feet (m); and P is the closure strip width in feet (m).
The guidelines note that the method for estimating the slab thickness may not be adequate for conditions at building corners or where bracing loads will be concentrated. They also recommend that the floor slab be designed “by a professional engineer to resist the applied brace forces furnished by the brace designer.” Unfortunately, unless the building is being constructed by a Design-Build firm or the Engineer of Record brings a bracing supplier into the design process, it’s unlikely the slab designer will have any contact with the bracing supplier. So, even the TCA bracing guidelines don’t provide a clear assignment of responsibility for bracing load effects on slabs.
STORM AND STRESS
I’ll use a simple example to illustrate a few of the potential issues. An elementary school gymnasium has been designed with 30 feet (9.1 m) tall walls. The top of the footings is 2 ft (0.6 m) below grade and the finish floor elevation. The unreinforced slab-on-ground floor is 4 in. (100 mm) thick with 16 x 16 feet (4.9 x 4.9 m) joint spacing. The floor slab has a 4-foot (1.2 m) wide closure strip adjacent to the panel, and the first slab joint is 20 feet (6.1 m) from the inside face of the panel. Can the panel be safely braced to the floor?
The 30-foot (9.1 m) tall panel should be checked using a 12.5 psf (0.6 kPa) construction wind load. The brace connections to the panel will be at 2/3 the height of the panel or 20 feet (6.1 m) above the foundation. To keep a bracing angle of 50 to 60 degrees and to keep the brace anchor near the centerline of the first floor slab panel, the brace connections will be 12 feet (3.65 m) from the face of the panel.
Assuming that the full bracing load is resisted by only the floor slab panel closest to the wall panel
S = [1.5(12.5 psf)(30 ft)2]/[2(18 ft + 2 ft)] = 422 lb/ft
U = 422 lb/ft(18/12) = 633 lb/ft
T = [2(422 lb/ft) + 633 lb/ft]/[150 lb/ft3(20 ft – 4 ft)]
= 0.62 ft or 7.4 in. (in.-lb units)
S = [1.5(0.6 kPa)(9.1 m)2]/[2(5.5 m + 0.6 m)] = 6.1 kN/m
U = 6.1 kN/m(5.5/3.65) = 9.2 kN/m
T = [2(6.1 kN/m) + 9.2 kN/m]/[23.6 kN/m3(6.1 m – 1.2 m)]
= 0.19 m or 190 mm (SI units)
So, a 4-inch (100 mm) slab is too thin to provide sufficient sliding resistance. If the slab is already installed, however, what can be done to ensure worker safety?
RETROFIT
To provide more sliding resistance, it’s possible to install temporary ties across the floor joints using, for example, expansion anchors installed through metal plates (Fig. 4). The bracing designer can then assign uplift forces to the first slab panel and sliding forces to both the first and second slab panels. The slab thickness required to resist sliding is
T = [2(422 lb/ft) + 633 lb/ft)]/[150 lb/ft3(32 ft)]
= 0.31 ft or 3.7 in. < 4 in. (in.-lb units)
T = [2(6.1 kN/m) + 9.2 kN/m]/[23.6 kN/m3(9.8 m)]
= 0.09 m or 90 mm < 100 mm (SI units)
The slab thickness required to resist uplift is
T = (633 lb/ft)/[150 lb/ft3(20 ft – 4 ft)]
= 0.26 ft or 3.2 in. < 4 in. (in.-lb units)
T = (9.2 kN/m)/[23.6 kN/m3(6.1 m – 1.2 m)]
= 0.08 m or 80 mm < 100 mm (SI units)
For a 4-inch (100 mm) slab, the length required to provide sufficient weight to resist uplift is
(633 lb/ft)/[0.33 ft(150 lb/ft3)] = 12.8 ft (in.-lb units)
(9.2 kN/m)/[0.1 m(23.6 kN/m3)] = 3.9 m (SI units)
But, can the 4-inch (100 mm) slab resist the uplift? The brace connection is centered in a 12.8-foot (3.9 m) bending zone, so the amount of slab cantilevered from the brace point can be assumed to be half of this length.
The bending moment M will therefore be:
M = (0.33 ft)(150 lb/ft3) (6.4 ft)2
2
= 1014 ft-lb
ft
(in.-lb units)
M = (0.1 m)(23.6 kN/m3) (1.95 m)2 2
= 4.49 kN•m
m
(SI units)
The section modulus for a unit width (1 ft or 1 m) of a 4-inch (100 mm) thick slab will be 32-inch3 (1.67 x 106 mm3), so the flexural stress will be 380 psi (2.7 MPa). Because this is a life-safety issue, a flexural strength for the slab concrete of 5 (0.4 ) will be used. With f’c = 4000 psi (28 MPa), the flexural strength will therefore be 316 psi (2.1 MPa). The slab should be reinforced for bending, but, in this case, it’s already in place. Now what?
The uplift can be reduced by adding ballast near the braces—a 55 gal. (200 L) drum filled with water will reduce the net uplift by about 450 lb (2 kN) (Fig. 5). Most floor anchors have an upward working load limit of 3200 lb (14.2 kN) in 4-inch (100 mm) thick concrete, so the maximum spacing between braces will be
3200 lb/633 lb/ft = 5 ft (in.-lb units)
14.2 kN/9.2 kN/m = 1.5 m (SI units)
The ballast will not eliminate the need to tie the slab panels, but it will reduce the flexural stress. The net uplift will be resisted by a slab length given by
[633 lb/ft – (450 lb/5 ft)]/[0.33 ft(150 lb/ft3)]
= 11 ft (in.-lb units)
[9.2 kN/m – (2 kN/1.5 m)]/[0.1 m(23.6 kN/m3)]
= 3.3 m (SI units)
The flexural stress from the resulting 5.5-foot (1.65 m) cantilever will be 281 psi (1.9 MPa). Although the 4-inch (100 mm) thick slab can be used to anchor the panel bracing, the thin slab will require that two slab panels are tied and numerous ballasting drums are used. What if the slab designer had considered bracing effects?
DESIGN IT
If the slab designer had considered bracing effects, a 5-inch (125 mm) thick slab could have been required in the first bay. The brace spacing would then be a function of the brace buckling and shoe capacities, not the brace anchor capacity. The uplift force from the braces would be resisted by a slab length of
(633 lb/ft)/[0.42 ft(150 lb/ft3)] = 10 ft (in.-lb units)
(9.2 kN/m)/[0.125 m(23.6 kN/m3)] = 3.1 m (SI units)
The resulting flexural stress would be only 188 psi (1.4 MPa). The slab would therefore require no flexural reinforcing and no ballast. Although the sliding force would still be too great to be resisted by the first floor slab panel alone, it would be necessary to install only light reinforcing across the first joint. Many projects have been successfully completed using an 8-foot (2.4 m) width of 6×6-W2.9xW2.9 (152×152-MW19xMW19) welded-wire reinforcement at the critical contraction joint. Alternatively, the Tilt-Up contractor could install temporary ties across the first slab joint or shoring to the panel or the footing (Fig. 6).
TYPICALLY TALLER
As the Tilt-Up industry continues to grow, wall heights grow as well. Multi-story Tilt-Up buildings are now becoming a popular application, leading to wall heights exceeding 75 feet (23 m).
Clearly, such tall walls are going to create numerous challenges that are outside the scope of this article. Knowing that the elevation of the upper brace connections will be about 60 percent of the panel height and the distance from the face of the panel to the lower brace connections will be 33 to 40 percent of the panel height, the building designer should consider numerous questions before the construction documents are issued, including:
- Where will the crane be located?
- Will the panels be braced to the inside or the outside of the building?
- If panels are braced to the inside, will the braces interfere with installation of intermediate floors?
- If panels are braced to the outside, will the braces interfere with the crane operations and traffic?
- Will the panels be strong enough to span between the bracing points?
Especially for tall structures, I recommend that the Engineer of Record design a bracing method into the structure. Available options include the use of:
- Thickened slab panels at the areas affected by bracing;
- Shores to transfer sliding forces to the panel foundation (Fig. 6);
- Concealed deadmen to transfer sliding forces to the subgrade (Fig. 7);
- Integral deadmen comprising continuous grade beams or drilled piers (Fig. 8);
- Discrete deadmen, consisting of helical or toggle anchors that are drilled or pressed into the earth at the bracing angle and tied directly to the brace, keeping in mind that the panels will probably need to be cast on a separate casting slab or braced to the outside; and
- Precast concrete blocks (often termed ecology blocks) that can be moved onto the site. These are typically used to anchor the lowest level of braces and must be designed using an appropriate coefficient of friction, including a safety factor of 1.5, as well as considering overturning.
PLAN FOR THE FUTURE
While the Engineer of Record must determine the service load conditions for a building, the Engineer of Record may have insufficient information to provide the final design of the floor slab for bracing effects. Perhaps, however, the following methods can be used to provide better value and reduced risk for owners, engineers, contractors, and subcontractors:
- In the design phase, use TCA guidelines to estimate the wind load, brace length, and bracing forces to determine the slab thickness, reinforcing, and affected slab area required to carry the bracing forces; and
- On the construction documents, indicate:
- The typical slab thickness and reinforcement required for service loads;
- Zones with increased thickness, reinforcement, or both required to resist the estimated bracing forces;
- Assumptions and forces used to arrive at the estimated values;
- A statement that the bracing forces have been estimated; and
- A statement that a Delegated Engineer must provide final bracing design and calculations indicating how bracing forces will be resisted.
This will allow the contractor more flexibility to discuss and consider alternate methods with his suppliers before the bid is rendered, when there is still time for optimization of the Tilt-Up system.
