Frequently Asked Questions
ARCHITECTURAL PRECAST
HOLLOWCORE
PARKING STRUCTURES
SPORTS FACILITIES
Which finishes provide the best consistency?
A wide variety of finishes are available in architectural precast concrete, ranging from a smooth form finish to deeply exposed aggregates. As a general rule, a textured surface provides more uniformity than a smooth surface because the natural variations in the aggregates will camouflage subtle differences in the texture and color of the concrete. A medium sandblast finish, for example, generally provides more uniformity and consistency than an acid wash finish. Dividing larger areas into smaller ones with reveals or rustications can also help lessen any variation in texture that might be visible.
What is the largest panel dimension I can design?
It is more economical to maximize panel size and minimize the number of precast units on a project. This results in fewer erected pieces, fewer connections and fewer crane picks. However, the maximum size of a precast panel depends on a variety of factors. For example, the size may be limited by site conditions or the reach of the crane that will be used to set the pieces. A site with limited access, or one where the maximum panel weights are set by the crane capacity could be the overriding factor in determining panel dimensions. Similarly, the size or weight of precast panels may be limited by shipping or fabrication considerations specific to a region or individual precast supplier. Usually panels should not exceed a width of approximately 12'-0", without consideration for a special permit or escort. Also, panels that exceed 40' in length may require the use of prestressing to reduce handling stresses and minimize cracking. The maximum size of panels is also a function of the design loads and locations of building supports. In general, it is best to work with a MAPA precast supplier to determine the most economical sizes and dimensions for your project.
What is the optimum joint size?
The recommended precast panel to panel joint width for architectural projects is 3/4". This is the minimum nominal joint width needed to adequately account for production and erection tolerances and still maintain an effective minimum joint width that can be caulked. A 30' long spandrel panel is allowed, per PCI tolerances up to a 1/4" variation in length. Keep in mind that of 3/8" is the minimum width that caulk suppliers will warrant. It is also important that the joint between precast panels and window frames also maintain the same nominal joint width.
Should sealers be used on precast? And if so, when should it be applied?
Sealers are often specified to improve the weathering characteristics of precast panels, especially in urban areas where the building may be subjected to airborne industrial chemicals. Sealers can also help facilitate the cleaning and maintenance of the panels if they should become dirty. When sealers are used, they should be applied in the field, after all of the joints are caulked, repairs made and cleaning complete. Otherwise, it is likely that the panels will have to be recoated in spots, which could, in turn, lead to inconsistencies in color and finish.
What is recommended as a preferred distance from the architectural precast panel to the edge of slab?
The slab edge location should be clearly defined on the contract documents. It is recommended that a 1-1/2 inch dimension be allowed between the edge of slab and the precast panel to account for tolerance both in the slab as well as the precast. Pay particular attention to slab edge conditions along skewed or curved building edges as these areas are often the areas that cause the most difficulty during layout.
What about interior dimensions?
It is important to consider tolerances when designing the interior wall finishes and locations. For example, if inadequate space is left between the back of the precast panel and the inside face of the interior finish, connections may become exposed to view. Allowing at least an extra 1/2-inch between the back of the drywall and the theoretical back edge of the connection hardware is strongly recommended. When the distance between the back of the precast and the interior finish does not accommodate this, connections may have to be recessed. It is also a good practice for the engineer to specify the allowable locations for slab recesses and to provide reinforcing details to account for this.
What kind of span lengths can be achieved with hollowcore plank systems?
The design recommendations for span lengths vary slightly from product to product, but here are a few general rules of thumb to keep in mind. If we assume a uniform superimposed load of 100 pounds per square foot and an un-topped system, these guidelines apply:
6-inch depth hollowcore plank: |
22-foot spans |
8-inch depth hollowcore plank: |
29-foot spans |
10-inch depth hollowcore plank: |
35-foot spans |
12-inch depth hollowcore plank: |
40-foot spans |
16-inch depth hollowcore plank: |
50-foot spans |
Factors such as concentrated loads and large openings can affect the span capabilities of your system. Once you have determined the given loadings, fire endurance ratings, span lengths and slab thickness for your project, consult a local MAPA producer for their published load tables.
What assistance is available to me during the design of a hollowcore system?
MAPA hollowcore manufacturers have engineers on staff who are very familiar with the requirements for submittals, design, materials, concrete mixes, product delivery, storage, erection, field welding, and attachments. It is customary for the precast producer to perform the final engineering for the product to be supplied to the job. This includes loads specified by the Engineer-of-Record, embedded items, handling, shipping, etc. It is very important that the Engineer of Record insure that the specified floor and roof system is achievable prior to the selection of a producer. MAPA precasters can provide input on span lengths and openings as well as the latest innovations in connection technology and techniques on how to use hollowcore to its best advantage.
How do we design for openings in plank?
Openings may be provided in hollowcore systems by forming openings in the plant, by installing steel headers, by shoring and saw cutting, or by cutting after a deck is installed and grouted. In laying out openings for a project, the least structural effect will be obtained by orienting an opening parallel to a span, or by coring small holes to cut the fewest prestressing strands.
An opening greater than 2' x 2' should be cast into 8'-0" wide hollowcore plank. This would usually occur if the plank were a wet-cast product. The opening could be plant or field cut in an extruded or dry-cast product. If the size of the opening exceeds the safe limitations of cast in the use of a structural header is used to create the required opening.
Openings less than 2-feet can be cast in or field cut in the plank providing that all of the pertinent information (location and size) is provided to the precaster prior to manufacturing. The general contractor must coordinate with all trades involved to get information to the hollowcore manufactured during the shop drawing phase so that additional reinforcement can be designed into the plank to carry the required design loads. An opening smaller than 10" square can not be cast into a plank due to production limitations.
Small openings are typically core drilled in the field by the trade (plumbing, electrical, etc.) involved after the plank has been installed and grouted. Again, the location and size of these openings should be provided to the precaster during the shop drawing phase so that the plank can be designed to accommodate these cores. Please do NOT cut prestressing strand in the plank without the approval of the hollowcore manufacturer.
What should be shown on the drawings for a precast hollowcore plank project?
Feel free to contact MAPA or a local precast manufacturer when considering a precast design. They are most helpful in the early stages of a project and can guide you through the process. Typically the following information would be provided to the precaster on the drawings:
Span Directions |
Loading Requirements |
Connection Information |
Fire Resistance Requirements |
Topping Requirements (for example 3/4-inch leveling coat; or 2-inch composite or non-composite topping) |
Openings: sizes and locations |
Should hollowcore plank be used in an exterior application for balcony sections in the high-rise building I am designing?
Residential construction is an ideal application for hollowcore plank. However, we are all familiar with the hot, humid summers and the cold, wet winters of the mid-Atlantic region. There is the potential for problems due to water infiltration and subsequent freeze/thaw damage in the hollow cores when used in an external application. All MAPA precasters of hollowcore plank recommend that you specify that the cores be filled for the balcony area or plan for that piece(s) to be solid slab section. This is easily and commonly manufactured for high-rise residential projects.
Is it common practice to weld alternate ends of the hollowcore plank? When is this practical?
Connections are required in hollowcore slab systems for a variety of reasons. Most are for localized forces. Connections are an expense to a project and, if used improperly, may have detrimental effects by not accommodating volume changes or other movements that occur in a precast structure. Connections may develop forces as they restrain these movements. In specifying connections, the actual forces must be addressed. If no force can be shown to exist, the connection should not be used.
The precast engineer often makes recommendations to the engineer-of-record regarding industry common practices. Typically, for buildings over 100-feet in length, with hollowcore set on structural steel, the precast floor system would require only one hard connection. This is accomplished by specifying a welded connection at opposite or alternating ends of the plank. The entire slab is then grouted in the normal manner. This allows the thermal movement in the floor slab to occur without damage to the embedded weld plates. This practice makes sense for a structural steel frame, but not for hollowcore bearing on masonry, nor total precast concrete systems. The practice is to avoid over-restrained systems for large footprints.
It is always a good idea to discuss your concerns with the precast manufacturer before production and construction begins.
How do I determine the fire rating or resistance of hollowcore plank? Should I specify a UL Rating?
Like all precast, prestressed concrete products, hollowcore slabs have excellent fire resistance. Depending on thickness and strand cover, ratings up to a 4-hour endurance can be achieved. A fire rating is dependent on equivalent thickness for heat transmission, cover over the prestressing strand for strength in a high temperature condition and end restraint. Underwriters Laboratory (UL) publishes fire ratings for various assemblies. The fire ratings should be considered in determining the slab thickness to be used in preliminary design.
A typical 8-inch thick hollowcore plank has a 2-hour fire rating. For a higher rating (3- or 4- hour), typically a concrete (or gypsum based) topping would be applied or a spray-on fire resistant material can be added to the underside of the plank. This is also shown in the UL directory.
Model codes like the IBC have prescriptive fire ratings. This is the best way to indicate ways to accomplish specified fire ratings and should be the first choice. UL listings are another way to indicate how to achieve the specified fire ratings. The code does not require that UL listings be provided. These listings are the result of proprietary tests on specific precast units produced and tested by specific companies. If UL labels or numbers are required then the specific details in the UL Directory are exactly and only what will meet this requirement.
The PCI manual "Design for Fire Resistance of Precast Prestressed Concrete" illustrates the code accepted practice of rational fire design for precast and prestressed concrete products. In the case of hollowcore, an equivalent thickness is calculated based on the cross-sectional properties of each brand of hollowcore. Rational fire design is used for situations not covered by the code.
The Engineer-of-Record will decide if the floor slabs can be considered restrained or un-restrained. Tables in ASTM E119 discuss the restraint conditions and are also shown in the PCI Hollowcore Design Manual. Required fire ratings should be clearly specified in the contract documents.
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How do I determine optimum bay sizes when laying out a parking structure?
Efficiency and economy in precast concrete construction is driven by repetition, standardization and minimum number of pieces. Optimum bay sizes are most commonly based upon the width of the standard double tee module being used. A garage manufactured by a precaster utilizing 12' wide double tees would optimally use 36' or 48' bays while a garage constructed with 15' wide double tees would implement 30' or 45' bays. Precasters in the project region should be consulted to determine available modules.
What guidelines should be followed when determining drainage slopes within a parking structure?
PCI's Parking Structures: Recommended Practice for Design and Construction recommends a minimum 1% slope be maintained throughout a garage. Camber is generally not used as a drainage mechanism. Careful consideration should be given to product tolerances and camber of double tees when establishing drainage slopes.
What durability measures can be taken to assure a long service life for my garage?
The best first line of defense against concrete deterioration and steel corrosion is use of a high quality concrete mix and attention to proper construction practice. A high quality mix normally includes a low water / cement ratio (< 0.40), hard-rock aggregates, and a minimum compressive strength of 5000 psi. Proper construction practice includes attention to correct finishing and curing methods, adequate concrete cover over reinforcing steel and sufficient drainage. Beyond these fundamental measures, other items may be considered for garages exposed to especially harsh environments (coastal areas rich in airborne salts, geographical areas prone to heavy use of deicing salts and geographical areas subject to frequent and severe freeze-thaw conditions). These supplemental measures include corrosion-inhibiting admixtures, concrete densifying admixtures, surface applied sealers and traffic-bearing membranes. Cost and benefit of each of these measures should be discussed with local precasters and design professionals.
I just found a double tee on my garage with a cracked flange…now what?
Cracking in double tee flanges is occasionally encountered on newly erected parking garages. This is commonly the result of torsional stresses developed during loading, transporting or erection of the member. Similarly, this may occur when a double tee is "warped" in its final position to allow for proper drainage. This type of cracking does not jeopardize the structural integrity of the double tee. To avoid long-term durability problems, these cracks should be filled with an approved sealant or epoxy. Epoxy repairs should be covered with a cementitious material to limit UV exposure.
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Can I design a radius into the top edge of a seating unit?
It is very difficult to produce a radius on the top edges of a precast seating unit. A radius can only be tooled into the edge or created by rubbing the edge with an abrasive stone. Consider a chamfer instead.
What challenges are presented by cantilevering?
The sight lines and footprint constraints of many stadiums and arenas demand that the leading edge of upper seating tiers extend over the tiers below. Such cantilevering raker and bleacher framing presents designers and builders with special challenges. This problem exists regardless of the choice of support framing material, whether it is precast concrete, cast-in-place concrete or structural steel. In order to maximize construction economy every effort should be made to build a mechanism, prior to the precast's arrival on site, designed for the stable gravity support of the precast bleacher unit.
A detail of a full end support (available in detail section of web site) where the precast is provided with a full end support at the raker beam. From an erector's perspective this represents the quickest, most efficient framing method. From the precast concrete manufacturer's standpoint, this is the lightest, most economical "tub" section available framed in single, double or triple riser units.
How are the precast seating units supported?
Designers should verify that the stadia seating units have support on both ends, typically to a precast raker beam. Support must also be provided when large holes are cut into the seating units at mid-span. These openings are commonly required when a recessed camera platform or ADA designated area is needed. Tub seating units will need proper bearing so that they can be set in place, released from the crane line and then connected to the support structure.
How do we make the connection from stadia seat to support raker?
A pin that is field "shot" onto a steel support bearing or a pin that is placed in a field drilled hole in a precast support raker works best. This type of connection allows for field tolerances. The hole in the horizontal seat surface is typically grouted to lock the connection in place; a cover sealant can be used to protect the connection. A free or slide type variation of the pin connection can simply be made by placing a rectangular tin cap over the pin prior to the grouting of the hole in the horizontal seat surface.
How is the connection made from stadia seat to stadia seat?
These are typically spaced across the front nose of the stadia unit above and pinned down into the upturn of the stadia unit below. A hidden connection can be achieved by casting a threaded insert into the bottom surface of the lower stadia unit. When erecting the stadia units, threaded rods are installed into the cast-in inserts in the upper stadia units and grout is poured into the sleeves in the lower stadia units. When the stadia units are set on each other, the connections are automatic and the connections lock as the grout in the void hardens.
How will the structural support steel match the precast units?
It is important to recognize that precise dimensional coordination must occur in sports facility design. The forming methods used and repetition of precast components create a consistent quality in a controlled environment. Designing, detailing, and producing one internally coordinated material streamlines erection of the structure.
Will designing with two or three different materials impact our tight schedule?
A total precast concrete system provides a single source supplier. There is only one schedule and one field construction management which minimizes coordination conflicts. The precast concrete components are cast off-site regardless of weather constraints. Talk to a MAPA precaster regarding schedules and estimates.
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