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.
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.
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.
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
5. 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.
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.
7. 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.