Jitendra Pathak, Business Unit Manager – Buildings & Structural Fittings, Freyssinet Menard India, outlines the major advantages of the use of post-tensioning in building structures. Economics of the post-tensioning slab system are discussed including relative material contents, speed of construction, and factors affecting the cost of post-tensioning. Finally, a discussion on the flexibility of post-tensioned building structures in terms of future uses, new floor penetrations and demolition is presented.
When Eugene Freyssinet developed and patented the technique of prestressing concrete in 1928 he little realized the applications to which his invention would be put in future years. Spectacular growth in the use of prestressed concrete took place after the Second World War with the material used to repair and reconstruct bridges in Europe. It is now an accepted Civil Engineering construction material.
In post-tensioning we obtain several distinct advantages: –
a) Designers have the opportunity to impart forces internally to the concrete structure to counteract and balance loads sustained by the structure thereby enabling design optimization.
b) Designers can utilize the advantage of the compressive strength of concrete while circumventing its inherent weakness in tension.
c) Post-tensioned concrete combines and optimizes today’s very high strength concretes and steel to result in a practical and efficient structural system.
Since the introduction of post-tensioning to buildings, a great deal of experience has been gained as to which type of building has floors most suited to this method of construction. Many Engineers and Builders can identify at a glance whether the advantages of post-tensioning can be utilized in any particular situation.
When is Post-tensioning Cost Effective?
In any basic comparison between post-tensioned and reinforced concrete one must consider the relative quantities of materials including formwork, concrete, reinforcement and post-tensioning. Other factors such as speed of construction, foundation costs, etc., must also be given consideration.
As can be seen from figure 3 below, post-tensioning should be considered as a possible economic alternative for most structures when spans exceed 7.0 metres.
In general, for spans in excess of 8.0 metres, savings in excess of $10 per square metre should be regularly attained in a direct cost comparison with reinforced concrete slabs. A graph below illustrates how the economies of the post-tensioning should be arrived at rather than comparison just the cost of PT submitted by an agency. It is always the design efficiency which contributes to the optimum combination of all factors – concrete, steel and PT; leading to most cost effectiveness.
Of course, the economics of post-tensioned buildings is heavily dictated by the design of the structure. The designer has a role to play in the minimization of material quantities, the selection of the most economical structural system, and the simplification of the detailing allowing for ease and speed of installation. A few design considerations are briefly mentioned below.
1. Partial Pre-stressing
Tensile cracking is allowed to occur, with crack control being provided by the bonded tendons and/or supplementary reinforcement. A cracked section analysis needs to be carried out to determine the cracked moment of inertia for use in deflection calculations as well as the steel stresses to confirm adequate crack control. The availability of computer software to carry out these calculations has meant that more often than not the amount of post tensioning is selected to satisfy deflection criteria.
2. Selection of Column Grid
A column grid spacing of between 8 and 10 metres for car parks, shopping centres and offices usually results in the most economical structure while maintaining architectural requirements.
3. Formwork Layout
Formwork layout should be selected to enable quick fabrication with a minimum of formply cutting. Widths of beams should be standardized in consultation with the main.
4. Construction Joint Treatment
Post-tensioning couplers should be avoided due to their cost and slow installation. Construction joints should be stitched with conventional reinforcement as far as possible.
Note that the amount of reinforcement required to keep a construction joint closed (saya crack width of 0.2 mm as for reinforced concrete) depends highly on the restraint of he overall frame. If the frame is very flexible, or alternatively if the construction joint is adjacent to very stiff elements such as core walls, then the amount of reinforcement required is quite low. On the other hand, if the frame is very stiff, large quantities of reinforcement will be required at which point an expansion joint should be positioned rather than a construction joint.
5. Simplicity In Detailing
As with all methods of construction the speed of installation is highly dependent upon the quality of the structural detailing. The designer needs to understand the installation process and be conscious of how their decisions on detailing affects all parties concerned on site. Detailing should be standardized and as simple as possible to understand. Congested areas should be carefully assessed and, as appropriate, large scale drawings and details produced.
6. L/D Ratios
Choosing the right L/D ratio for the structural system and applied loading is important. Choosing a high L/D ratio may minimize the amount of concrete, but will increase the amount of post-tensioning and/or reinforcement required, and perhaps cause increased vibration. Choosing a low L/D ratio in order to minimize post-tensioning may not secure the expected result due to minimum reinforcement rules and adequate residual compression levels to ensure shrinkage cracking is controlled.
7. Load Balancing
One of the major advantages of post-tensioning is to reduce the long-term deflection of the structure; however selection of too high a load to balance may incur prestressing costs reducing the economy of the prestressed solution. A combination of a lower level of ‘balanced load’ and the addition of normal reinforcement at peak moment regions will prove to be a more economical solution in most applications.
8. Terminate Tendons Wherever Possible
Often the amount of post-tensioning required within a member varies across its length. For example, end bays usually require a greater level of prestress to control deflections than internal bays. Terminating the post-tensioning once it is not required can be achieved by either terminating whole tendons or terminating individual strands using a ‘short dead end’.
9. The Use of Finite Element Analysis for Selected Projects
We find that the use of FEA methods for these types of structures allow for a better determination of structural load paths and enables the designer to detail and drape the post-tensioning tendons to better reflect the slab bending moments. This is what leads to economy.
FUTURE FLEXIBILITY, PENETRATIONS & DEMOLITION OF POST-TENSIONED
A question often asked of post-tensioned slab systems is what happens if we wish to make a penetration in the slab after construction.
From time to time it has been brought to our attention that certain members of the building profession see this question as a major obstacle and are reluctant to accept the use of prestressing in some types of buildings. This is often due to a perceived lack of flexibility in the structure when it comes to the formation of openings through the slabs sometime after construction.
This section will outline the options available to enable the designer to produce a building which is both economic to construct and easy to modify in the future.
Cutting Of Tendons
1. Bonded Tendons
Bonded tendons are located within oval shaped galvanized ducts which are injected with cement grout following the post-tensioning procedure. Consequently when such a tendon issevered, the free end will become de-tensioned but after a short transmission length the full tendon force will be effective. This distance is in the order of 800 to 1000 mm.
If a penetration is required that will need the termination of a bonded tendon, then the procedure follows that for a fully reinforced structure.
Cutting a bonded post-tensioned tendon is, structurally, the same as cutting through conventional reinforcement. The tendon, however, needs to be `terminated’ in order to give full corrosion protection (as does conventional reinforcement).
Tendons are easy to cut using a disc cutter. In fact, cutting tendons requires less effort than for a fully reinforced slab due to the relative amount of reinforcing material to be cut.
2. Unbonded Tendons
These tendons come individually greased and plastic coated and are therefore permanently de-bonded from the slab.
When unbonded tendons are severed, the prestressing force will be lost for the full length of the tendon.
When contemplating the cutting of an unbonded tendon it is therefore necessary to consider the aspects as noted below.
a) Cutting the tendons.
The strand is packed with grease which prevents an explosive release of energy when the tendon is severed. Even so a gradual release of force is recommended. This can be achieved by using two open throat jacks back to back. After cutting the strand, the force can be gently released by closing the jacks.
b) Propping the slab.
Adjacent spans may require temporary propping depending upon the number of tendons severed at one time. It is rare for a slab to carry its full design load. A design check based on actual loading at the time of the modification may show props to be unnecessary.
c) Forming the hole.
When the edge of the slab is re-concreted new anchors are cast in to enable the remaining lengths of tendon to be re-stressed, thus restoring full structural integrity. The above operations are not difficult but will require the expertise of a post-tensioning sub-contractor.
CALL FOR QUOTATION /TENDER FOR POST TENSIONING JOB
There are lot many companies of post tensioning operating and everyone has its own comfortness or method of submitting the quote. However, from owner’s point of view and benefits, it is very essential to know all the risks and costs associated from day one. It is always advisable to have the quote in terms of square metres of post tensioned slab area and cubic metres of post tensioned beams. The most common practice followed in India (and not observed anywhere else in world) is to quote on basis of metric tonnage consumption of strands. Since the design of slabs and beams for post tensioning is done by PT agency being hired, it is always possible to manage the consumption of strands and hence the total invoice value or cost to client can be considerably increased. Though, prima facie, such a method of per MT quote appears to be attractive due to low unit cost; but with high consumption the total value of project (as highlighted previously) exceeds.
With the concept of binding PT agency with area, the agencies are bound to do an efficient analysis and design of structure so as to maintain the overall consumption well within the range, as per codal provisions. Since there is very little chance of change in the PT slab area, the owner knows from day one of the project; what shall be the cost incurred by him for post tensioning work with no hidden or unknown cost factors.
While making assessment and evaluation of a post tensioning quote / bid / proposal, following points is to be insisted upon by owner to have a correct comparison of all –
- Unit for pricing : m2 for PT Slab Area and m3 for PT Beam Area
- Basis of Evaluation : Concrete, Rebar, PT System
- Cost structure of a PT system : Execution Time Impacts Cost
- Escalation of strand prices : The base price should be matching prevailing to market rates. Bidders can be asked to provide the copy of quote from strand supplier as a support at bidding stage itself.
- Material meeting specifications.
- Insurances and Performance Indemnity for the structure.
In conclusion it is worthy to reinforce a few key points.
There is a definite trend towards large spans in buildings due to the fact that there is now more emphasis on providing large uninterrupted floor space which can result in higher rental returns. Post-tensioning is an economical way of achieving these larger spans. For spans 7.5 metres and over, post-tensioning will certainly be economic and, as the spans increase, so do the savings.
The most significant factor affecting the cost of slab system post-tensioning is the tendon length. Other factors create a scatter of results leading to an upper and lower bound. Notwithstanding this, it is always advisable to obtain budget prices from a post-tensioning supplier.
The main structural schemes available are the flat plate, flat slab and banded slab, with the latter generally leading to the most cost-efficient structure. However, other factors such as floor to floor heights, services, etc., must be taken into account in the selection of the floor structure. For high rise construction and highly repetitive floor plates, the use of more specialized structural schemes is appropriate with emphasis on systems formwork.
It is not uncommon for post-tensioning to be rejected in certain types of building project due to a perceived lack of flexibility. However, tendons are usually spaced sufficiently far apart to allow penetrations of reasonable size to be made later, without cutting through the tendons. Should it be necessary to cut tendons this can easily be achieved using well established methods.