Thursday, 25 September 2014

Post tensioned slab design - Its more interesting than it sounds (I hope!)

We have a lot to say Merci to the French for...the croissant, the baguette, the company that makes our lunch every day and the rather clever system of "post tension" construction we are using to build this building.

This system of post-tensioning was pioneered by a Mr Eugène Freyssinet  in 1933 for the foundation of a marine terminal in France and is now used extensively all over the world in bridges, elevated residential or commercial buildings, foundations, walls, and columns.

The site from above clearly showing the layout of the the metal strips...
Looking down at our construction site from above, it is possible that you may have noticed something a little unusual. The site is criss - crossed with a lines of shiny metal strips...

This strips of metal are actually hollow casings filled with wires and form part of the structural system which we use to hold up the building...This is called post tensioned construction, and is simply a method of producing prestressed concrete. 

The term prestressed is used to describe the process of introducing internal forces (or stresses) into the concrete elements during the construction process in order to counteract the external loads applied when the structure is put into actual use. This method of construction allows us to span large distances safely, whilst keeping the amount of concrete we use to a minimum.

For many of you, you can probably stop reading now as this is probably enough knowledge to anything but the most persistent of questions from the kindergarten... However if you would like to know more please continue...
So lets start with the basics. The materials we use.  We make most of the building from concrete, which has incredible compressive strength (it cannot be squashed easily but you are welcome to try) but unfortunately it has limited tensile strength so is not every good at resisting large forces pulling on it... This means concrete on its own can only safely really be used to span small distances. 

We would like clear open space in the building, not filled with columns so we require spans over 12m in length. Sort of like lots of little bridges. For this we use a post-tensioned contruction method, first pioneered all those years ago.

Adding post-tensioned reinforcement instead of just steel bars (rebar) alone combines the action of reinforcing the tension zones with the advantages of compressing the concrete slab

With our type of post-tensioned construction, the silver strips seen criss crossing the construction site are actually high strength steel cables which are placed inside the concrete slab from end to end. Once in place concrete is poured directly on top of them and once this concrete  has set hard, they will be pulled tight and stretched (tensioned) by a machine to impart internal forces on the concrete.

When tensioned and locked-off to the anchors, this post-tensioned system increases the load the slab can bear and reduces "sag" by lifting the slab and counteracting forces that could be pushing down on the slab and cause it to crack. This allows our concrete spans to increase to over 12 metres of clear open space and the actual slab thickness by around 30cm a floor (6 floors) to decrease overall building height.

High and low points visible on the slab
We get additional benefits by installing the  post-tensioned reinforcement in a draped profile to create high and low points instead of running in a straight line. This routes the post-tensioned reinforcement through a high point over the slab's supports, and through a low point in between those supports. Now optimum efficiency cab beobtained because the post-tensioned reinforcement is appling forces directly in the tension zones, the concrete is compressed, and the post-tensioned reinforcement is creating an uplift force in the middle of the spans where it is needed the most.

Construction process

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel wires of a diameter of 15.2mm, housed inside a galvanised metal tube, it is these covers that you see so visibly on the site 

A tendon has anchors on each end to transmit the forces into the structure. Long tendons may have intermediate anchors along their length to allow for stressing at construction joints. (see left and right)

To get an idea of the high strength of this type of steel, a typical steel strand used for post-tensioning will fail at about 243,000 psi. In contrast, a typical piece of reinforcing bar (rebar) will fail at around 60,000 psi. To be very sure we also send samples of this steel wiaway for independent testing before we start the work.

The handling and installation of the post-tensioning tendons does require special skill and knowledge. The prestreessing team will install the empty cases in the precise locations dictated by the engineer and shown on the post-tension field placement drawings.  These have to be very accurate

 When these empty tubes have been placed  the wires are inserted in the tubes. There is often a different number of wires in each tube as the specific forces applicable in each area are calculated.

In our elevated slab construction, the tendons typically are grouped in bundles in order to increase the spacing between tendons and improve the constructability of the slab.
After the concrete is placed, it must achieve proper strength before the tendons are tensioned. On our site this is 25 n/mm3 which is about 70% of the actual eventual strength of the concrete .

 The tensioning of the tendons, also known as the stressing operation, is achieved by using a hydraulic jack. At least one end of each tendon will have been installed with a length of prestressing steel cable protruding  from the edge of the slab; this is known as the stressing tail and unlike many other types of tail, this one is designed to be pulled....
 A plastic pocket former also will have been installed at this location to create a stressing pocket 
When the edge form and pocket former are removed, the strand tail and stressing pocket are exposed to allow the construction team to use the stressing jack to apply the force in the tendon.
The forces generated when the tendons are stressed are high enough to damage the structure or even cause injury to people working on the job if the installation and stressing are not done properly. 

Safety during stressing includes making sure that no one is working in the area where the tendon is being stressed. It is possible, but rare, for the cable to snap whilst being stressed.

The tensioning is done in 2 phases, firstly it is stressed  to a force equal to 25% of a strand's tensile strength.(on our site this is 15 MPa.) At this point spray paint is applied to provide a reference line, this is then held and checked for slippage and excessive elongation of the cable, which may indicate a problem with the integrity of the cable.

If everything seems stable then the jack is re-installed and stressing is carried out to the final stressing pressure - 59.6 MPa (a lot!)

As the tensioning is occurring, the steel is being elongated, and the concrete is being compressed. When the proper tensioning force is reached, the prestressing steel is anchored in place. The anchors are designed to provide a permanent mechanical connection, keeping the steel in tension, and the concrete in compression.
The steel elongation is measured and recorded for each tendon. This measurement is reviewed to determine and verify that the proper force exists in each tendon. Once the elongation measurements have been approved, the stressing tails can be cut off just inside the edge of the concrete slab, and the stressing pocket is filled with nonshrink grout (with a material strength of 30 MPa) to provide cover and protection over the end of the prestressing steel.
The act of stressing the tendons transfers force off of the formwork and into the tendons, which carry the force over to the columns or other supports. This means that the deck forms can be removed and cycled up to the next placement as soon as it is determined that all of the tendons in the current slab have been properly stressed.

The final product....

Stamped on the underside of the slab, its pretty important we dont accidentally damage those tendons.

The edge of the slab showing the tendons and the final thickness of the slab.