Post-Tensioned BuildingsIzhak Z. Stern
November 18, 2008 — 1,452 views
Buildings with monostrand unbonded tendons post-tensioned slabs are becoming a widely used application of prestressed concrete. This method achieves performance and construction improvements over other construction methods. However, in order to reap the benefits of this method, proficiency is required in both structural design and construction.
Post-tensioned slabs is a preferred method for industrial, commercial and residential floor slab construction. The increasingly extensive use of this method is due to its advantages and its nature of easy application to a wide variety of structure geometry and design solutions.
Prestressed floor systems using monostrand tendons may be designed as either one or two way slab systems, and may be flat plate, flat slab waffle slab, or other slab sections. The prestressing is achieved by individually tensioning tendons, placed within internally greased protected plastic sleeves, arranged in the slab prior to casting. Compressive stresses are applied to the concrete via tendon anchors. Prestressing is performed within three to seven days of casting.
Unlike the multi-strand system (which is primarily suited for beams and girders) the monostrand method allows prestressing of slabs as thin as 15cm. while maintaining vertical optimal curvatures for the structure. The monostrand system is also simpler, requires less in site organization, and is more forgiving to construction variances.
Advantages afforded by unbonded slab prestressing as compared with alternative designs include:
- Increased speed of construction as prestressing allows for faster stripping and reuse of formwork.
- Thinner slabs resulting from post-tensioning by virtue of improved deflection behavior and improved section utilization.
Improved economy due to reduced slab thickness and associated concrete costs, reducing building weight with the corresponding foundation reductions, reduced building height with the corresponding decrease in building skin area, and a reduced amount of mild reinforcing rebar.
- Large area slabs can be maintained with no control joints. Simpler coordination between consultants due to a flat slab underside, the design and installation of utilities systems is simpler (heating, air conditioning, sprinklers, etc.)
- Increased design flexibility allows simple solutions even for structures with irregular geometry, without the need for transverse or longitudinal beams.
- Longer spans can be achieved improving the architectural structure flexibility.
- Long-term deformations due to creep, which are usually significant in concrete slabs, are almost nonexistent in unbonded tendons prestressed slabs.
- Longer building life cycle due to the uncracked nature of the prestressed concrete. This advantage also creates slabs more resistant to water penetration, and the structure behaves monolithically.
- Post-tensioned slab on grade - by post-tensioning, large area of slab on grade for any propose, can be designed without any joint, improving performance of the slab, increasing life span and reducing maintenance cost.
Typical projects are listed at: http://www.yde.co.il/Post-Tensioned-Buildings.aspx
About the Author
Izhak Z. Stern M.Sc., P.E.
M.Sc. Civil Engineering 1988 - University of Maryland, U.S.A.
B.Sc. Civil Engineering 1977 - Technion - Haifa, Israel.
Izhak Z. Stern has published and presented many papers on bridges and post-tensioned structures at proffessional conferences around the world.
I. Stern - Y.D.E. Engineers, LTD