Applications
Hollow core is available from CPCI members all across Canada. Hollow core slabs can be used for most applications requiring a floor or roof system. Schools, office buildings, condominiums, hotels, senior citizen's apartments, commercial buildings, residential dwellings, houses of worship, nursing homes and educational facilities are all ideal applications. For hotels, motels and apartments the hollow-core slabs are oriented to either span between load bearing shear walls or to span from the central corridor to the exterior walls. Slabs can be cantilevered up to 1.5 m (5 ft).
Sizes
Hollow core is economical. Modular precast concrete floor and roof slabs are machine manufactured under controlled conditions in modern precast plants. Most slabs are 1220 mm (4’-0”) nominal width and 203 mm (8 in) deep – other sizes are available for special applications [152 mm (6”), 254 mm (10”), 305 mm (12”), 355 mm (14”)]. – consult your local CPCI member for available sizes.
Manufacturing
Hollow core slabs are made on long-line casting beds and cross-cut at 900 to the exact length for each project. Angle cuts are possible but cost more. It is desirable to have the plan dimensions fit the slab module, typically 1220 mm (4’-0”). Non-module plan dimensions can be accommodated using partial width slabs.
Prestressing
Hollow core slabs are prestressed. This means longer spans, shallow depth and the ability to carry heavy loads are easily accommodated. This also means better space planning and a lower floor/floor height. Lengthening the span is economical with fewer slabs to make and install. Maximum span/depth ratios of 45 are recommended for floors. A 203 mm (8”) slab can span up to 9 m (30 ft) depending on the loading. A 305 mm (12”) slab can span up to 14 m (45 ft) depending on the loading. CPCI member’s load tables will define the allowable live load that a given slab can safely support in addition to the slab self weight. The load capacity will be a function of the slab thickness, the amount of prestressing provided and the location of the prestressing strands.
Use of Slab Voids
Hollow core is cast with continuous voids to reduce weight and cost and, and as a side benefit, for use to conceal electrical wiring or mechanical ducts. When properly coordinated for alignment, the voids in hollow core slabs can be used for electrical or mechanical runs. For example, routing of a lighting circuit through the cores can allow fixtures in an exposed slab ceiling without unsightly surface mounted conduit. Slabs detailed to distribute heated air through the cores can be used as the thermal mass in a passive solar application.
Finished Soffit
Hollow core, cast on smooth steel forms, has a finished underside. The smooth ceiling provided by a hollow core slab requires only caulking of the longitudinal joints. The underside of slabs can be used as a finished ceiling as installed, by textured paint, or by applying an acoustical spray.
Bearing Supports for Hollow Core
Hollow core slabs can be supported on many types of supports designed to carry the dead and live loads required. Precast beams, precast walls, poured concrete beams and walls, masonry walls, insulated concrete forming system walls, wood and steel stud walls and structural steel beams are all suitable for use with hollow core as load bearing systems - consult your local CPCI member. Minimum bearing width is 75 mm (3 in). Hollow core in double bearing on a steel beam will require at least a 150 mm (6 in) top flange width.
Holes and Openings
Openings may be provided in hollow core systems by saw cutting after a deck is installed and grouted, by shoring and saw cutting, by forming or sawing the openings in the plant or by installing short slabs with steel headers. In laying out openings for a project, the least structural effect will be obtained by orienting the longest dimension of an opening parallel to a span, or by coring small holes to cut the fewest prestressing strands, or when several openings must be provided, aligning the openings parallel to the span to again cut the least number of prestressing strands.
Load Distribution
Hollow core slabs are designed as individual, one way, simple span slabs. When the slabs are installed and grouted together at the keyways, the individual slabs become a system that behaves similarly to a monolithic slab. A major benefit of the slabs acting together is the ability to transfer forces from one slab to another. In most hollow core slab deck applications, non-uniform loading occurs in the form of line loads, concentrated loads, or load concentrations at openings. The ability of individual slabs to interact allows these load concentrations to be shared by several slabs. The ability to distribute loads among several slabs has been demonstrated in published tests.
Diaphragms
A series of hollow core slabs will provide a basic diaphragm capable of resisting lateral loads in the form of lateral earth pressures, wind loads or seismic loads by a grouted slab assembly provided proper connections and details are installed. The function of a diaphragm is to receive these loads from the building elements to which they have been applied and transmit the loads to the lateral-resisting elements which carry the lateral loads to the foundation.
Camber
Hollow core slabs have camber - an upward deflection produced by the effects of prestressing. Consideration should be given to the effects of differential camber in the planning stages. Joints between slabs of unequal spans or joints at which a change in the span direction occurs will have differential camber. This must be recognized and dealt with in the design layout. Wall locations may be used to hide some of these joints. Camber must also be accommodated when a topping is provided. The quantity of topping required must consider the amount of camber and the function of the floor. In occupancies where flat floors are not a requirement, a constant topping thickness may be used to follow the curvature of the slab. At the other extreme, if a "flat floor" is required in a structure consisting of multiple bays of varying length and changes in slab direction, the highest point will determine the top elevation of the topping. A greater amount of topping will then be required in "low areas". Camber, camber growth, and deflections must also be taken into account in roofing details. Drainage slopes should be large enough to overcome expected camber. Where changes in relative slab position can occur, counter flashings are suggested to accommodate these changes.
Top Surface Preparation
Untopped hollow core is the most economical system where applicable. The top surface of untopped hollow core slabs can be prepared for the direct installation of floor coverings by applying a cement based skim coat to the top surface of the slabs. Non-structural levelling concretes can be installed ranging from 12 – 50 mm (1/2 to 2 in) thick depending on the material used, or by casting a composite structural concrete topping. Structural toppings are often used in high seismic zones and where heavy loadings are to be supported.
Fire Resistance
Hollow core slabs provide excellent fire resistance. Depending on slab thickness and strand cover, fire ratings of 1 to 2 hour endurance can be readily achieved. A fire rating is dependent on equivalent thickness for heat transmission, concrete cover over the prestressing strands for strength in a high temperature condition and end restraint. Fire ratings of up to 4 hours can be achieved where required using deeper slabs, increased cross sections and increased strand cover – consult your local CPCI member. Follow the NBCC and Provincial Building Codes for slab fire ratings. Underwriters Laboratories Canada publishes fire ratings for various building assemblies.
Sound Transmission
Hollow core slab concrete floor-ceiling assemblies have excellent sound transmission characteristics. Sound Transmission Class (STC) ratings range from about 47 to 57 without topping. The Impact Insulation Class (IIC) ratings start at about 23 for a plain slab and can be increased to over 70 with the addition of under padding and carpeting.
Design Responsibilities
It is customary for CPCI members to perform the final engineering for the hollow core product to be supplied to the job. This would include design for vertical loads and lateral loads specified by the Engineer of Record, embedded items for specified connection forces and handling, shipping and erection. However, the Engineer of Record plays a very important role in the design process. Enough preliminary planning should be done to insure that the specified floor and roof system is achievable. The project should be engineered to avoid requiring changes from the contract documents.
The contract documents must clearly indicate design criteria to which hollow core slabs will have to conform. This is especially important when the hollow core slabs must interface with other construction materials. The forces to be transmitted through connections must be specified in the contract documents. CPCI members are best able to determine the most efficient connection element to be embedded in the slab. However, the balance of a connection which interfaces with other materials should be detailed in the contract documents.
The Engineer of Record also has a responsibility in the review and approval of erection drawings prepared by the precast producer. Review of these drawings is the last opportunity to assure that the producer's understanding of the project coincides with the intent of design. Erection drawings should be checked for proper design loads, proper details and bearing conditions, conformance with specified fire ratings, and the location of openings.
Speed of Construction
Fast construction means earlier completion and resulting cost savings. Hollow core slabs can speed up construction schedules. Manufacturing of the slabs can proceed in a CPCI member's high efficiency plant while site construction is underway. Slabs can be delivered to the jobsite and installed to meet the client's schedule regardless of weather conditions. The installed hollow core deck provides a working platform for other trades. The combination of hollow core with a structural precast concrete frame and/or precast concrete bearing walls will speed construction even more as the entire building shell is supplied and installed under a single contract.
Canadian Success Story
The Spiroll hollow core manufacturing process was invented in 1962 in Winnipeg. A vibrating machine which used zero slump concrete was developed to extrude a hollow core slab over the prestressing tendons. Today this same process (with a host of subsequent refinements) is used all around the world to manufacture hollow core floor and roof building slabs in a wide variety of widths, depths and cross sections. Approximately 18 million square feet of hollow core slabs are produced annually across Canada - enough hollow core every year to cover Toronto’s SkyDome 200 times!
Bromley Place, Calgary, AB
This 31-storey apartment building is the tallest totally precast concrete building in Canada. The building was constructed using precast concrete interior and exterior shear walls and hollow core floors.
Owner & Contractor: MBS Construction (1977) Ltd.
Architect: IKOY Architects
Consulting Engineers: J. R. Spronken & Associates / W. H. Milley & Associates
For more information about precast, prestressed concrete:
Click on: www.precastsearch.com
• To find a CPCI member near you
• Province > Structural Precast > Company > Hollow core slabs
Click on: www.cpci.ca
• Products and Systems > Hollow Core > information & CAD details
• Specifications > Guide Specification for Hollow Core Slabs
• Brochures/Publications > Structural Technical Brochure
• Designer’s Knowledge Bank > searchable information database
Technical Publications
• CPCI Design Manual > order from www.cpci.ca
• PCI Manual for the Design of Hollow Core Slabs > order from www.pci.org
Design Software
Click on: www.blackmint.com > CONCIDE Beam software
Call CPCI toll free: 1-877-937-2724 for a plant tour and/or an in-house hollow core presentation
Questions
1. Hollow core can be supported on: (check all that apply)
Steel beams
Precast walls
Masonry walls
Wood or metal stud walls
Insulated Concrete Formwork (ICF) walls
2. Hollow core can have a: (check all that apply) consult your local CPCI member
1 hour fire rating
2 hour fire rating
3 hour fire rating
4 hour fire rating
3. (a) The maximum recommended span for 203 mm (8”) hollow core floors is:
6 m (20 ft)
9 m (30 ft)
11 m (36 ft)
3. (b) The maximum recommended span for 305 mm (12”) hollow core floors is:
14 m (45 ft)
17 m (56 ft)
11 m (36 ft)
4. Camber due to prestressing concrete is:
A downward deflection
An upward deflection
Can be an upward or downward deflection
5. For maximum economy: (check all that apply)
Lay out buildings around a 1220 mm (4 ft) slab width module
Run the slabs in different directions
Keep slabs the same length wherever possible
Minimize slab spans
6. Which of these statements is true? (check all that apply)
Hollow core can not be used in high seismic zones
Hollow core is not as strong as the same thickness of poured-in-place concrete
Hollow core slabs can be joined together to form a diaphragm to resist horizontal loads
Impact Insulation Class (IIC) ratings vary with different floor coverings
7. The overall design of the building structure is the responsibility of the:
Architect
General contractor
Hollow core contractor
Engineer of Record
8. Construction using hollow core slabs is generally:
Faster than other systems
Slower than other systems
About the same time to complete
9. Hollow core can be used on: (check all that apply)
Only 1 and 2 story buildings
Low rise buildings – up to 5 stories
Medium rise buildings – 6 to 12 stories
High rise buildings – 30 stories or more
10. Bonded structural concrete toppings are used on hollow core floors: (check all that apply)
to distribute heavy loads
are always necessary regardless of use
to tie the structural diaphragm together in high seismic areas
to prevent differential slab deflections
As part of this self-directed Continuing Education learning activity, you are required to read additional online material on designing with precast concrete. To access this free material, go online to www.cpci.ca, click on the Designer’s Knowledge Bank logo and register. Click on Advanced Search, check all market category boxes and enter the words Hollow Core Slabs in the Keywords Search block. You’ll be offered several articles. You can download the entire series of articles free of charge.
Other Resources:
CPCI Structural Precast Concrete Technical Brochure – free from CPCI
CPCI Infrastructure Technical Brochure – free from CPCI
