INTRODUCTION TO VEESLAB SYSTEMS
The Vee Systems have been developed by Sydney Structural Engineer, Trevor Valaire.
THE NEED FOR AN IMPROVED PRECAST SYSTEM.
Trevor has designed prestressed concrete structures for over 50 years and has acquired a wealth of experience and design expertise. Most of this experience has been in the field of insitu structures. This was mainly because there was not a serious and useful precast alternative.
The existing precast elements available were just that. Elements which provided one function but not too well. There was no integrated system which produced structures comparable to insitu structures.
They contained simplifications of assembly which rendered them as bulky, awkward and inefficient.
To be viable, a precast system should have the following attributes:
1. PROPPING
In order to provide significant advantages over insitu construction, a precast sysytem should be capable of spanning long distances during construction, without the need for propping. Once a system requires propping , it suffers most of the shortfalls of insitu construction. If you have to frame up for propping then the O H and D requirements of catch floors need to be satisfied for high floor clearance and the propping cost would equate to most of the cost of the formwork the system replaces so why waste it on a system where you have to pay for precast as well.
Once a building system requires propping, then that propping needs to extend three or four floor below thus delaying finishing trades access for 4 floor cycles after pouring. So in order to offer the maximum construction speed, the ideal system must not utilise vertical propping of floor components
2. FLEXIBILITY IN APPLICATION-VARIOUS STRUCTURAL FORMATS
The system needs to be capable of replicating the existing structural type
If the structure lends itself to a beam and slab system then a precast system needs to have such a system available.
Similarly if it has been deemed desirable that a flat plate system should be used for a project , then the systems should have a flat plate form in their repertoire.
3. CONTINUITY.
Virtually all insitu concrete structures take advantage of continuity for structural efficiency , to achieve shallow structural depths, reduce deflections and to minimise maintenance.
Most existing precast systems can only achieve simply supported structures.
In order to compete with insitu structures , percast structures need to be able to achieve continuity between all components. Beams need to be continuous for structural efficiency as do slabs . The connection between column and beam/slab needs to be able to attain continuity for stability of the structure as a whole . A floor panel needs to form a rigid diaphragm to transmit lateral loads to core structures and columns.
4. COMPOSITE ACTION.
Just like insitu structures, the precast components should work compositely with one another . The topping slab needs to be composite with the precast slab members, The precast slab members need to be composite with the precast beam members to form Tee beams.
5. DEFLECTIONS
Deflections are inextricably rely on a combination of loading, sections properties, continuity , composite action and prestressing.
In electing to have a fully self supporting system not relying on propping during construction places an even more demanding situation than in insitu structures.
To counteract deflections we need to address each of the contributing factors. Loading needs to be minimised within the bounds of the project requirement so structural members neee to be light consistent with high stiffness. This is a function of member geometry thus section properties. Voiding of the structural elements to reduce weight consistent with high section modulus is highly desirable. High quality concrete also contributes. Maintaining the full cross section as being effective at all loads is another quality which minimises deflection and thus prestressing becomes an indispensible method of improving section properties. It follows that the higher the level of prestressing then the wider is the range of loads for which the section properties remain optimal. High levels of prestressing depend on the method of stressing and the quality of the concrete at the time of stressing. Pretensioning entails the lowest level of initial prestress for safety reasons and also the highest level of prestress lossed due to low concrete properties and high elastic shortening, shrinkage and creep losses. Posttensioning addresses all of these drawbacks to pretensioning along with the advantages of profiling the prestress cables to optimise the location of the cables and the load balancing benefits of profiled cables to counteracting deflection, reducing the shear load on the concrete.
Higher levels of prestress increase the shear loading capacity of a concrete member.
All of the above pointers make it imperative that for optimal concrete behaviour the structural members should be prestressed by the posttensioning procedure.
6. SPEED OF CONSTRUCTION.
A precast concrete system needs to be able to be built quickly.
Joints need to be made quickly and not have to wait for insitu components to reach strength before the following precast to be erected. Connections need to be drop-in or bolted. Slab components need to be light so that the area installed with each crane movement Is maximised and to reduce the cost of transport.
I believe I have achieved all of this with Vee Slab Systems.