Designers have a multitude of choices in building design, especially in the selection of a flooring framing system. Post-tensioned flooring systems offer a great deal of flexibility, and when properly designed, can significantly impact the cost of materials and speed of construction.

Typical systems

The structural floor framing systems for cast-in-place concrete are designated primarily as either one-way or two-way systems. The essential difference between the two framing systems is in their method of load distribution to supporting elements. Each of these systems are further divided into subcategories, depending on slab span, loading requirements, and framing configuration of beams or locations of thickened slab areas. Examples of one-way systems include one-way beam and slab, joist slab, and skip joist slab. Two-way slab systems include flat plates, flat slabs with drop caps, flat slabs with drop panels, and floor slabs with "slab bands." Slab bands are wide, shallow, continuous sections of thickened floor below a two-way slab, which increases the stiffness of the slab in the longer of the two span directions.

Flat plate slabs are commonly used in the design and construction of multi storied structures for construction cost efficiency and reduced building height. When a structure calls for additional flexural stiffness and strength, stiffening elements such as joists, beams, or drop panels may be used. These floor systems provide the required structural efficiency for a given weight per unit floor area and reduce the overall floor weight. Post-tensioned floor slab systems reduce the materials used, resulting in savings for vertical members and foundations.

Flexibility and openings

One argument, frequently deterring owners and contractors in using post-tensioning, is the lack of flexibility to accommodate floor penetrations, either planned or as part of future changes to meet specific tenant requirements. A post-tensioned flooring system reduces the spacing between reinforcement, leaving more flexibility for small penetrations. It is a relatively easy process to locate the tendons in a floor to ensure that small penetrations will not cut the post-tensioning. In most cases the post tension shop drawings will give a reasonable estimate where it is safe to drill. If the drawings show tendons close to the proposed penetration, then the exact tendon location can readily be determined on site with the aid of a metal detector or ground penetrating radar. Drilling small holes for fixing dowels is generally safe, provided the depth of penetration into the slab is less than the clear cover for the tendons.

Large penetrations for new stairs, elevators or air conditioning ducts will require an analysis of the floor system by an engineer, regardless of whether the floor is post-tensioned or not. If large penetrations coincide with prestressing tendons, some strengthening along the edges is required and must be installed prior to cutting, in the same way as for reinforced concrete floors.

If it becomes necessary to cut unbonded tendons, a qualified contractor should be engaged to ensure that proper procedures are followed. VSL, a designer, manufacturer and installer of post-tensioning and specialty reinforcement systems, has developed specialized equipment that grips the tendons from the two ends on either side of the cut so that the energy is released in a controlled way when the strands are cut. When the strand has been cut, the pressure on the jack is released, de-tensioning the tendon completely. After the penetration has been completed, new stressing anchorages are provided at the edges, the tendons cut to length, and re-stressed with a normal stressing jack.

Considerations for floor framing system selection

Selection of the floor framing system depends on a number of factors. Below is a list of some the primary factors to consider:

• Typical span-Suitability and economy of different floor framing systems depend on the span length.
• Span ratio-Consider the ratio of the span in the x-direction to the span in the y-direction. For nearly square column grids, two-way systems are more suitable than one-way systems.
• Overall structural height of the floor framing-This determines the total building height and thus the cost for cladding and vertical services, which is particularly important for high-rise buildings.
• Column support locations-Random column placement may increase the complexity of a beam and slab framing system. Two-way slab systems are more adept at handling complex load paths.
• Building services-Consider flexibility for the layout of under-ceiling mechanical/electrical services. Free routing underneath the soffit versus penetrations through beams will impact the project.
• Structural weight per unit area (average)-This determines the size of vertical supporting members, foundations, and, in seismic areas, size of the lateral load-resisting system. The use of ribbed slabs, waffle slabs, or voided slabs helps minimize weight.
• Serviceability and strength-Consider requirements for in-service behavior (such as deflections and cracking) and for strength. Depending on the project's needs, stiffness, moment capacity, or both are important criteria to think about in the following way: Is the floor framing part of the lateral load-resisting system? Floor systems with beams are preferable if frame action is required.
• Exposed soffits/suspended ceilings-Flat plates or waffle slabs offer a pleasing aesthetic where the soffit is exposed.

Conclusion

With purposeful floor framing system selection, the structural engineer can positively impact the final costs to the project. As a follow up to this article, "Efficient floor framing systems: Preliminary sizing of post-tensioned floors" will be included in the October issue of Structural Engineer. This feature will educate readers on some of the fundamental principles of post-tensioning and offer tips for starting your next design with more accuracy.

John Crigler, P.E. is senior vice president and technical manager of VStructural LLC (VSL). Crigler serves as the secretary/treasurer of the American Segmental Bridge Institute. He also serves on the Post-Tensioning Institute Technical Activities Board. In addition, he is a member of the American Concrete Institute and the American Society of Civil Engineers. Crigler earned a bachelor's degree in civil engineering from Virginia Tech. He can be reached at 410-850-7000.