Question

sort out the post-tensioning systems （recent systems）

no more than 500 words

Answer 1

Post-tensioning is a method of reinforcing (strengthening)

concrete or other materials with high-strength steel strands

or bars, typically referred to as tendons. Post-tensioning

applications include office and apartment buildings, parking

structures, slabs-on-ground, bridges, sports stadiums, rock

and soil anchors, and water-tanks. In many cases, post-

tensioning allows construction that would otherwise be

impossible due to either site constraints or architectural

requirements.

Although post-tensioning systems require specialized

knowledge and expertise to fabricate, assemble and install,

the concept is easy to explain. Imagine a series of wooden

blocks with holes drilled through them, into which a rubber

band is threaded. If one holds the ends of the rubber band,

the blocks will sag. Post-tensioning can be demonstrated by

placing wing nuts on either end of the rubber band and

winding the rubber band so that the blocks are pushed

tightly together. If one holds the wing nuts after winding,

the blocks will remain straight. The tightened rubber band is

comparable to a post-tensioning tendon that has been

stretched by hydraulic jacks and is held in place by

wedge-type anchoring devices.

BENEFITS

To fully appreciate the benefits of post-tensioning, it is

helpful to know a little bit about concrete. Concrete is

very strong in compression but weak in tension, i.e. it will

crack when forces act to pull it apart. In conventional

concrete construction, if a load such as the cars in a

parking garage is applied to a slab or beam, the beam will

tend to deflect or sag.

ADVANTAGES/APPLICATIONS

There are post-tensioning applications in almost all facets of

construction. In building construction, post-tensioning

allows longer clear spans, thinner slabs, fewer beams and

more slender, dramatic elements. Thinner slabs mean less

concrete is required. In addition, it means a lower overall

building height for the same floor-to-floor height. Post-

tensioning can thus allow a significant reduction in building

weight versus a conventional concrete building with the same

number of floors. This reduces the foundation load and can

be a major advantage in seismic areas. A lower building

height can also translate to considerable savings in

mechanical systems and façade costs. Another advantage of

post-tensioning is that beams and slabs can be continuous,

i.e. a single beam can run continuously from one end of the

building to the other. Structurally, this is much more efficient

than having a beam that just goes from one column to the

next.

Post-tensioning is the system of choice for parking structures

since it allows a high degree of flexibility in the column lay-

out, span lengths and ramp configurations. Post-tensioned

parking garages can be either stand-alone structures or one or

more floors in an office or residential building.

CONSTRUCTION

In building and slab-on-ground construction, unbonded

tendons are typically prefabricated at a plant and delivered to

the construction site, ready to install. The tendons are laid

out in the forms in accordance with installation drawings that

indicate how they are to be spaced, what their profile (height

above the form) should be, and where they are to be stressed.

After the concrete is placed and has reached its required

strength, usually between 3000 and 3500 psi (“pounds per

square inch”), the tendons are stressed and anchored. The

tendons, like rubber bands, want to return to their original

length but are prevented from doing so by the anchorages.

The fact the tendons are kept in a permanently stressed

(elongated) state causes a compressive force to act on the

concrete. The compression that results from the post-

tensioning counteracts the tensile forces created by

subsequent applied loading (cars, people, the weight of the

beam itself when the shoring is removed). This significantly

increases the load-carrying capacity of the concrete.

Since post-tensioned concrete is cast in place at the job site,

there is almost no limit to the shapes that can be formed.

Curved facades, arches and complicated slab edge layouts are

often a trademark of post-tensioned concrete structures.

Post-tensioning has been used to advantage in a number of

very aesthetically designed bridges.

ENSURING QUALITY CONSTRUCTION

The amount of post-tensioning strand sold has almost

doubled in the last ten years and the post-tensioning industry

is continuing to grow rapidly. To ensure quality construction,

the Post-Tensioning Institute (PTI) has implemented both a

Plant Certification Program and a Field Personnel

Certification Training Course.