Build with tilt-up, the most advanced construction method for:
- warehouse facilities
- distribution centers
- office complexes
- shopping centers
- industrial buildings
- low-rise commercial
- waste water tanks
- low to mid-rise structures
When you construct a new building, regardless of its purpose, you want a building that works, is attractive, reasonably priced, and efficient - a building in which you can take pride! You can have all of these benefits with tilt-up concrete construction.
Tilt-up concrete construction is not new; it has been in use since the turn of the century. Since the mid-1940s it has developed into the preferred method of construction for many types of buildings and structures. Nationwide, over 15% of all industrial buildings are tilt-up, ranging in size from 5,000 to over 1.5 million square feet. They are typified by their attractiveness, efficiency and longevity.
Design-build - the fast delivery system for your project where the building construction proceeds while the design is being developed, speeding completion. Tilt-up lends itself to this process because construction of the wall panels can proceed whle the rest of the building is being designed.
You'll get earlier occupancy - when you decide to build, you want to proceed quickly. With tilt-up, you get compression of disciplines - many phases proceed simultaneously. Therefore, you can move into your building sooner.
Assembly line prefabrication - load-bearing reinforced concrete wall panels are cast horizontally on the floor slab or a casting slab, and are tilted up into their final vertical position with high-capacity mobile cranes. The entire process is designed for efficiency and speed of erection.
Trades get in sooner - because the floor slab is poured first, the other trades can work sooner, and have a better work environment. That saves both time and money.
Materials and labor are more readily available - site-cast tilt-up uses ready mixed concrete usually available near the job site. Contractors use local labor.
In-place material costs are more competitive than precast, masonry and steel structures; fewer skilled laborers are needed to achieve high-quality construction. In addition, the speed of erection and the ease of other subcontractors' work, further reduces costs.
Costs are easier to control because there are fewer variables with tilt-up. Ready mixed concrete, made from readily available native materials, has price stability. Vertical forming and scaffolding are eliminated; less skilled labor is needed; and, the short construction cycle precludes cost run ups. Weather is less of a problem, since the floor slab provides a stable work surface for the trades.
Positive cash flow - materials are used as soon as purchased, allowing for early payment, unlike off-site fabricated building components requiring long lead times.
Less heat and air-conditioning will be required for your tilt-up building; and, smaller, less costly mechanical systems can often be used. The thermal mass inherent in concrete reduces the heating and cooling peaks and loads. Insulation systems are available that enable the construction of integral sandwich walls or lightweight interior insulation. The national Energy Policy Act of 1992 mandates improved energy performance in commercial buildings, and recognizes the effect thermal mass contributes to reducing heating and cooling loads.
You'll have lower insurance premiums because:
- concrete provides more fire resistance than comparable steel buildings or wood-frame structures;
- tilt-up concrete structures have been shown to withstand wind storms and earthquakes better than comparable buildings of other materials; and
- security is better with solid concrete walls.
Maintenance - Your new tilt-up concrete building will require less maintenance. The exterior can be left unpainted with no damage from the elements. If painting is desired, it need be repainted only every five to ten years. Concrete interiors are less subject to damage, and easier to wash down.
Expansion. Your tilt-up concrete building can be designed and built to allow easy expansion, simply by detaching and relocating the concrete panels or cutting new openings.
Sound Control. If you are in a noisy area, such as near an airport, you will enjoy the added benefit of the sound reduction properties of concrete. The mass absorbs the sound rather than letting it through as in metal and wood-frame buildings.
Safety. Most work is on the ground - there is no vertical formwork, no scaffolding, since walls are constructed horizontally. In addition, since the floor slab is poured first, workers have a safer working surface.
Labor crews are smaller - without vertical forming, or other costly erection processes, few workers are needed, especially during the lifting operation. The short project cycle presents less opportunity for accidents.
Any tilt-up structure can be an attractive building in which you can take pride. The large tilt-up panels lend themselves to a variety of decorative treatments:
- An unlimited array of colorings that can be added to the concrete mix or textured paints of any color may be used for beautiful effects.
- Textures produced with improvised techniques and form-liner cast surfaces in a variety of patterns, including striated, fractured fin, stone, brick and woodgrains.
- Exposed aggregate and mechanical tooling surface treatments.
- Trompe l'oeil painting and simple forming on the casting surface can produce the effect of three-dimensional surfaces.
The strength of tilt-up concrete buildings was proven in the Northridge, California earthquake, where tilt-up walls stood, even when roof connectors failed. What's more, buildings constructed in the 1940s show little sign of age after 50 years. In fact, buildings dating back to as early as 1906 are still in service today. Tilt-up structures withstand wind and hail storms and are impenetrable by the smallest rodent, insect, or even the most determined human.
Of minimal concern is localized damage to a concrete building from a truck or fork-lift. Metal or wood buildings will usually sustain substantial damage from similar incidents.
Fire resistance of concrete can extend the building's life, plus tilt-up panels may be used for the interior fire walls, and buildings may be spaced closer together under many building codes.
Tilt-Up Construction Guidelines...
Tilt-up construction is one of the fastest growing industries in the United States. At least 4,000 buildings enclosing 160 million square feet are constructed annually. This is due, in part, to the economics of tilt-up, which combine reasonable cost with low maintenance, durability, speed of construction, and minimal capital investment. These advantages are most evident in buildings greater than 10,000 square feet with 20-foot or higher side walls and repetition in panel size and appearance.
A successful tilt-up project begins long before concrete is poured. As with any project, the key is thorough planning. An experienced tilt-up contractor can be successful on a wide variety of projects. The beginner, however, should choose more carefully. This article explains the basic methods and skills required for successful tilt-up construction.
Site evaluation. Planning begins with an evaluation of the site. A large, flat, open site is ideal, but not common. Therefore, evaluate the site with regard to slab layout and the movement of materials and equipment around the slab. If the site is tight around the building, consider sequencing the pouring of slabs and panels from within the building. A plan or sketch showing the pouring and erection sequence relative to the site and floor plan of the building can identify potential problems.
Engineering. Engineering is a critical phase of tilt-up projects. Consult an engineer with experience or familiarity with tilt-up construction and current design methods. Current recommendations for many aspects of tilt-up, particularly engineering, are presented in ACI 551R (Ref. 1).
Engineering tilt-up panels to withstand service and lifting loads is critical. But using a design that is too conservative could produce panels that are thicker and heavier than required. The net result could be larger footings, bigger cranes, more panels, and more joints - all of which can increase the cost of the project to the point where it is not cost-competitive.
The thickness of a concrete panel usually is determined by a quantity called the slenderness ratio. This is the ratio of the unsupported panel height (usually the distance between the floor slab and the roof structure attachment) to the panel thickness. The generally accepted slenderness ratio on tilt-up walls is 50. However, a qualified engineer should make the final determination.
Floor slabs must be designed to support crane loads during erection. These loads may exceed building occupancy loads in some cases, so select the crane early in the project. Most contractors use at least a 5- or 6-inch-thick slab, unless structural requirements dictate a thicker one.
Panel connections to the footings, floor system, roof, and between panels also are details designed by an engineer, and must be determined before construction.
The loads applied during lifting generally have the greatest affect on the panels. Locations of openings, lifting inserts, and other parameters must be considered. Additional reinforcement usually is needed to accommodate these loads. Tilt-up accessory suppliers can be helpful in this aspect of design. Most will provide panel layouts indicating locations of lifting inserts and other information critical for erecting the panel.
A product of the design phase should be a drawing of each panel, preferably showing both the front and back, and insert and embed locations. The contractor or engineer should produce a building floor plan showing the layout of every panel on the slab and the erection sequence.
Several other items must be considered during the planning and engineering phases of the project. These include surface treatment, anticipated weather conditions, and material and equipment availability. For example, if the panels are to have deep reveals, a thicker panel might be required since the cross section of the panel could be reduced at a critical point by the reveal.
Careful consideration should be given to the size, location, and attachment of temporary wall braces. Here, again, tilt-up accessory suppliers can be helpful. Experienced tilt-up engineers also can offer economical bracing schedules.
Footings and floor slabs. Once planning is complete, construction can begin. Install footings as level as possible, with attachment plates (if used) placed properly. Spread footings are used most often for tilt-up buildings, but pier footings can be considered if conditions warrant their use. Footing locations, heights, and dimensions should be checked and verified for correctness. Crane time is expensive if modifications must be made while a panel is suspended over an incorrect plate or footing. Setting a panel down after it has been lifted interrupts the construction schedule. Also, panel lifting hardware may be designed for only one pick, so repeated handling can reduce the safety factor for that hardware.
Minor mistakes in floor slab construction often can be masked or corrected. A poor floor slab, however, can affect the appearance of the tilt-up panel. The slab should be smooth and hard and contraction joints should be positioned where they will have minimal impact on the panel. If the joint must fall beneath the panel, clean the joint and fill it with caulk. Floor sealing or hardening compounds must be compatible with any chemicals or paints used on the panels or there may be problems when the panels are stripped or painted.
Forming tilt-up panels. The standard practice in laying out panels is to snap a chalk line on the floor slab. These lines indicate panel perimeters and the forms should be placed against them. Wood 2xs are the most common material used for side forms. Often the panel depth is designed to fit the depth of standard dimension lumber, so 5 1/2- and 7 1/4-inch-thick structural panels are common. The form sides can be supported and secured to the slab by a wood or steel angle support. Any common concrete anchor can be used to attach forms to the slab. Remember that holes in the slab left by anchorages must be repaired.
There are several ways to form individual panels. One way is to form the perimeter of a series of panels, then use 1x or 2x strips to divide the area into panels. Advantages of this method include less forming and reduced forming lumber costs. Close joint tolerances from panel to panel can be maintained since adjacent panels share a form.
A variation of this method is to cast a large slab, then saw joints into the slab to form individual panels after the concrete has been troweled.
Use cant strips at the juncture of the side form and slab. They reduce spalling when stripping forms, help close off the bottom of the form to reduce concrete leakage, and give a neater appearance. A bead of caulk often is added as an additional seal between cant and reveal strips and the slab.
Form door and window openings after framing the panel perimeter. Brace the interior of the openings to prevent bowing or movement. Use cant strips between the opening forms and slab. If the opening is closer than 24 inches to a panel edge, a strongback may be needed for additional support during panel lifting and placement.
Apply form release agent and bond breaker to the slab and forms as recommended by the manufacturer. A wide variety of materials are available. Compatibility between bond breakers, form release agents, and paints or coatings used on the panels is critical. Check compatibility by consulting with the product suppliers.
Surface treatments. It is popular to impart a pattern or texture to the face of tilt-up panels using reveal strips. Typically, these strips are anchored to the base slab after side forms are erected, but before reinforcement is placed. Concrete nails or other anchors often are used to fasten the strips. This surface treatment must be carefully planned and executed, but the results are striking. A reveal strip that is 1 inch off is difficult to correct once the panel is erected. Other finishes include exposed aggregate, sandblasted, and brick or stone facings.
Reinforcing. The steel grid for reinforcing tilt-up panels is typically tied in-place after the side forms are erected. Standard Grade 40 or 60 bars are used. The use of plastic support chairs instead of steel chairs is recommended to avoid rust on the panel face. If steel chairs must be used, use a type that is plastic-tipped. Check to make sure the tips are in place just before the pour. If insulated panels are used or the pour is on a sand bed, use chairs with sand feet. The weight of the steel can force a standard chair foot into the sand or insulation. Use the correct chair height to maintain the proper depth of the reinforcing.
Embeds and inserts. The next step is to install embeds and inserts. Embeds are pre-fabricated steel plates with lugs that are cast into the panel to attach it to the footing, other panels, or the roof system, or for attachment of building accessories after the shell is completed. They can be attached to the side forms if they are on the panel edges, or they can be wired to the reinforcing.
Inserts provide the attachment points for lifting hardware and braces. They usually are sized by the supplier, who also should furnish engineering drawings showing insert locations. Install inserts according to the manufacturer's recommendations. If there is a field change in panel size, opening location, or other conditions, the insert supplier should be contacted to confirm the location and selection of hardware.
Concrete placement. Before placing concrete for tilt-up panels, clean the base slab. Use compressed air to blow away dirt, leaves, and other loose debris. Also, remove any standing water on the slab.
Concrete placement methods for tilt-up panels are the same as those for floor slabs. Since the panels are structural members, make sure the concrete mix meets specifications. Direct chute placement is the most economical method, but pumping and bucket placement also work. Consolidate the concrete to ensure good flow around the reinforcing steel. A trowel finish is suitable for most projects.
As with other types of concrete construction, plan for the unexpected. If the weather will be cold, have insulation blankets ready. If it looks like rain, delay the pour or have a suitable covering material available. Provisions should be made to block off a pour if a concrete truck breaks down or gets stuck in traffic. On hot or windy days, be prepared to cure the panels by water misting or by applying a suitable curing compound.
Panel erection. The erection sequence should be determined well in advance, but it's a good idea to review it immediately before panel erection. Also, thoroughly review safety procedures with all tilt-up crew members to help prevent accidents. Discuss crane operation, bracing and anchorage details, cable releases, and job communication.
Locate and clean inserts and embeds and attach braces before lifting the panels. It is much quicker and safer to do this work while the panel is flat rather than doing it on a ladder after the panel is upright.
Don't remove braces until after the roof and decking are installed. Once braces are removed, workers can patch holes in the floor and complete other finish work.
Panel finishing. The finish of a panel is limited only by the creativity of the architect and the abilities of the contractor. Common sandblasted or exposed aggregate finishes can be done immediately after panel erection. Painting, however, must wait until partial curing has taken place and residue from the bond breaker has been removed.
Most tilt-up concrete panels have an uneven or splotchy appearance when first stripped. These splotches usually fade after time. Uneven bond breaker application, standing water, slab porosity, and other factors can produce this effect. Sandblasting eliminates most of these inconsistencies. Washing also can improve appearance, but most tilt-up panels are eventually painted.
Before cleaning and painting panels, caulk joints and correct significant imperfections. The most frequently used paints are acrylic-based. Textured paints can be used for special effects. Banding or striping is a popular technique to produce variety and interest in tilt-up buildings. Reveals or recesses cast into the panel often are painted a contrasting or darker color for accent. The trompe l'oeil, or "fool-the-eye," effect is quite striking and popular.
Insulated panels. Insulated tilt-up panels are a rapidly growing market providing new opportunities to experienced and new tilt-up contractors. Several proprietary systems enable contractors to insulate panels during their construction or after the building is erected. The basic forming and pouring process must be modified slightly to accommodate the sandwich wall systems. Higher side forms are needed to accommodate the insulation, and some systems require concrete placement on separate days. Tilt-up sandwich panels with as much as 6 inches of insulation and R values of 30 can be built. Usual applications include coolers and freezers.
|What is the typical thickness of panel?|
5-1/2" thick with .9 to 2.0 PSF of reinforcing. Four-hour fire walls are 6-1/2" thick with approximately the same amount of reinforcing.
|What is the minimum size building that is economical?|
Some as small as 5,000 square feet can be economical, if they are relatively tall. Several small buildings clustered together may also prove economical. Special finishes on the walls, such as exposed aggregate or form liners may make tilt-up an economical choice.
|What is the minimum height buildings that is economical?|
Tilt-up is more economical than concrete block above 14 feet in height, and the block is generally more economical under 12 feet, unless there is a considerable quantity of wall.
|What is the maximum height panel that is economical?|
Tilt-up panels can be used up to 60 or 70 feet in height, but approximately 50 feet is the economical limit. This is due to bracing and crane limitations, more than the panel itself.
|What is the maximum length panel that is economical?|
Panels up to 96 feet in length have been used in some areas, but special cranes and spreader bars are required for panels over 30 feet in length. It is generally better to have the panels all nearly the same length and less than 30 feet.
|Are there any limits to the number or location of openings?|
There is no real limit to the number or location of openings, but their location can be critical. Openings closer than the minimum from the end of the panel supporting a concentrated roof load can add considerable reinforcing, thickened concrete or steel columns. The minimum distance is one-eighth the eave height, or two feet, whichever is less.
|What size crane is needed to lift the panel?|
It is best to let the crane company determine this, based on the size and weight of panels involved. A rule of thumb for the size, however, is two to three times the weight of the panel.
|Are there any site conditions that limit tilt-up?|
Yes, the following should be considered:
a. Access by the crane to the job site.
b. Relatively flat terrain to allow the crane operation.
c. Any power lines, ditches, railroad tracks, or other obstructions which limit crane operation.
d. Other buildings very close to where panels must be placed.
|What really holds the tilt-up panels in place?|
The roof structure acts as a diaphragm to horizontally support the wall at the top, and the curb on the footing supports it laterally at the bottom. The panels are generally not connected together to allow for expansion and contraction without cracking. The panels are only positively connected to the roof at their centers near the top.
|What information is needed to bid a building, and how long does it take? |
With the preliminary floor plan, desired height, wall finishes, mansards, type of roof and preliminary soil report, a bid on the Varco Pruden Tilt-Plus Deign can be completed in a week.
|What information is needed to design and detail a building, and how long does it take?|
The finalized floor plan, the soil report and decisions on all the bid options are needed, along with any planning commission requirements. With this information, the design and detailing for plans to submit to the building department will take approximately four weeks.
|How do you insulate a tilt-up wall?|
It can be furred on the inside for some applications, or made into a sandwich panel. Burke has a system to use two concrete layers with rigid insulation between. The inside wall is structural and load-bearing, while the outer concrete layer is suspended from it to allow for temperature changes without cracking.
|What is the recommended floor thickness?|
A minimum of 5" to allow the crane to operate on it.
|What sub-base is recommended under the floor?|
This depends somewhat on the soils report, but normally about 6" of crushed rock over 95 percent compacted soil.
|Is a moisture barrier required under the floor?|
Not under warehouse-type uses, unless specified by the soils report. However, it might be considered in floor areas where floor coverings are used and the water table is high.
|What strength concrete, slump and type of mix is recommended in the floors?|
A minimum strength of 3,000 PSI at 28 days with a low water cement ratio. The slump should be as low as possible and still facilitate placing. A super plasticizer might be considered to allow for as low a water content as possible.
|What type of joints and how frequently should they occur in the floors?|
The typical floor will have joints both ways at no greater than 20 feet on center. They will be steel keyed cold joints in the long direction, and plastic zip strips perpendicular. Both are furnished in the Burke package and will be detailed on the foundation plan.
|What equipment is needed to cast the concrete floors?|
The floor needs to be carefully built to act as the casting bed, the crane platform and the working surface of the building. It must be strong at an early age for the ready-mix trucks and crane, and smoothly finished since it will mirror the panel finish. For these reasons, it needs to be vibrated well and struck off in large widths. The Texas Vibratory Screed does this very well and also serves as a straight-edge for inserting the zip strip joint material. However, any long Creed apparatus can be used in conjunction with good vibration and finishing techniques.
|How should the concrete floor be cured?|
With a chemical liquid curing compound, a combination bond-breaker/curing compound, or a cure-seal-hardener product. The Burke package includes a membrane bond-breaker for this purpose to ensure compatibility with the bon-breaking features.
|Will the crane damage the floor?|
Not if it has been properly constructed with the correct sub-grade and compaction. The crane should extend outriggers when lifting heavy loads, and use rolling outriggers when driving with heavy loads. The crane should not be allowed to get near the edge of the slab, especially when deep fills are used.
|How should electrical, mechanical and structural floor penetrations be blocked outs?|
Wherever possible, electrical, mechanical, and structural objects should be stopped below the floor surface and the opening filled with sand. Place a thin layer of plaster over the sand and trowel smooth. Coat with bond-breaker to prevent adhesion. Joints in the floor that will have panels cast over them should also be filled with plaster and coated with bond-breaker.
|Are isolated or continuous foundations better for tilt-up?|
This varies with soil conditions and the slope of the terrain. Both can work well, but if there is much panel extension below the floor, the continuous footing will be needed to permit the back-filled soil pressure. If there are a number of openings in the panel, the continuous footing allows better support for the panel. The continuous foundation also permits getting closer to property lines. The Burke design usually utilizes continuous foundations under the panels.
|How are inserts held in place during casting of the panels?|
They should be firmly wired to the reinforcing steel so that they won't be displaced during the placing and vibration operations. You should not attempt to wet-set these inserts for any reason.
|Are there any precautions necessary with inserts?|
a. They should be accurately located vertically, horizontally and for depth.
b. They should not be placed closer than one foot from any edge or opening.
c. They should not be placed in line vertically with the brace inserts that might interfere during the panel setting.
d. The concrete should be vibrated well around the inserts.
e. There should be a positive method of preventing concrete from entering the attachment portion of the insert.
f. If there is need to move a lifting insert, consult the erection engineer for proper relocation.
|What are the safety factors for lifting and bracing inserts?|
The Federal OSHA and American National Standards Institute use a two-to-one safety factor inclusive of all loads for tilt-up, and a four-to-one safety factor for precast lifting inserts. Bracing inserts do not have a standard, but manufacturers and designers normally Up one-and-a-half-to-one or two-to-one.
|Are there any safety checks required with erection hardware?|
a. Make sure all hardware is in good working order, unbent and free of cuts and defects.
b. If coil bolts are used, be certain all bolts are unbent, have washers, and penetrate clear through the coil.
c. If hardware is used that bears on the panel surface, be certain it bears well, using grout pads if necessary with exposed aggregate or textures.
d. Any strongbacks or added reinforcing specified for lifting must be correctly in place at the time of the erection.
e. All cables, hooks, swivels, and bails must be free to rotate in the line of action or the load.
|What are the tolerances of placing the inserts?|
The inserts should be placed within one inch horizontally and vertically, and within1/4" for depth.
|Where can I obtain information on necessary crane rigging, sling lengths, boom length and spreader bars?|
The rigging configuration, spreader bar length and sling lengths are specified in theerection booklet prepared by Burke. The crane size and boom lengths are determined by the crane company.
|At what phase of the job should I plan the erection sequenced?|
Before the panels are cast, have the crane company representative assist in the layout to ensure that the panels can be lifted in the sequence cast.
|What safety precautions are necessary before and during lifting?|
a. Provide the crane company with a set of panel lifting details well before the day of lifting so that they can plan necessary rigging.
b. Double-check that the rigging configuration and that the spreader bars and slings are the correct length.
c. Watch for ditches, holes and overhead obstructions that might prevent the crane from moving.
d. Have a safety meeting at the beginning of the erection to ensure that everyone knows what is expected and how to operate safely.
e. Make certain everyone on the rigging crew knows how the lifting hardware is supposed to work.
|What wind velocity should panels be braced for?|
Due to the variation in moisture content and gusts, wind bracing is normally designed for a particular force. Most panels are designed for a 15 pounds per square foot wind force with a safety factor of 1-1/2. This amounts to approximately a 65-mile-per-hour wind with gusts.
|What safety factor is necessary for brace design?|
Most designers use a minimum safety factor of 1-1/2.
|How many braces are needed per panel?|
A minimum of two are needed for aligning the panel, but large panels require more. Check with the design charts provided with the erection design for type and quantity of braces.
|When should the braces be attached to the panel?|
When they are still on the casting bed if at all possible. It speeds and simplifies the erection when the braces are raised with the panels.
|When and how should braces be removed from panels?|
The panel should be removed after the roof connections and/or diaphragm are completed. Braces should be disconnected from the floor first, and then carefully disconnected at the top and lowered to the ground. If they are dropped or allowed to slide down the wall, they may be damaged.
|Is knee bracing and cross lacing of braces a good idea?|
It complicates and prolongs the lifting because it must be in place before the crane is released. While it is expensive to do, it is sometimes necessary on tall panels in windy areas.
|Is cable bracing adequate for panels?|
If correctly placed and connected, it can be effective. It is more difficult to adjust or plumb panels with cables than with braces, but exceptionally tall panels may require it.
|Are there any precautions necessary when bracing to the floor?|
Be certain that the floor is of adequate thickness and strength. The inserts should not be near edges or joints, and must be placed in large enough slab sections to withstand uplift forces.
|What do I brace to if the floor isn't available?|
Have a deadman designed for job site conditions, including the soil type and grade elevations.
|Can drill-in expansion type anchors be used?|
I don't recommend them. The majority requires some skill and certainty of setting that is not always obtained. Even when properly set, the working load with a 4-to-1 safety factor on 3/4" expansion anchors is much lower than the design load of most tilt-up braces.
|How do I brace and shore lintel panels?|
The panel bracing and shoring should be designed by a professional engineer, and the shores should be braced against lateral movement separately from the panel. All members should have firm bearing on adequate foundations, and the connections should be fixed before releasing the panel from the crane.