Precast Prestressed Planks Design & Detailing
The use of precast planks as a design option for roofing over our semi-basement garage seems to most as being the one and only option. If the ‘perit’ proposes a reinforced concrete slab design, then he is only doing so to make him pay more money on the total construction costs. But how much of this is true? Is there really a cost factor between the use of precast planks and cast in-situ concrete?
It is usually the case that clients opt for one option or the other according to which is the least expensive. It is in the company’s belief that, by providing the client with all the information possible, we can better explain the design solution and therefore arrive to a mutual agreement on the way forward. It is has become good practice to provide the client with a comparative statement of the estimated expenditures prepared prior to commencement of works. When properly detailed, it is a general rule that the price variation between one option and the other is negligible. What usually makes a difference is the way the precast planks are detailed.
It is quite often that due to lack of proper detailing, problems start arising soon after the precast slabs are loaded with the floors above. The significance of the planks tying requirements is not always given the proper importance. It should be taken as a general rule that the following design elements are envisaged when using precast planks in any type of construction:
- Reinforced spreader beams along the entire length of the precast plank supports;
- Concrete topping with mesh reinforcement above the precast planks. This is mostly important to eliminate differences in camber and elevation between adjacent planks at the joints and to evenly distribute loadings over adjacent planks. Reinforcement should be lapped with the spreader beams reinforcement in order to achieve adequate tying action;
- Alternatively, loose reinforcement should be provided between the concrete grouting of the precast planks as per tying reinforcement requirements
