Practical Design Guidelines
Precast panel design involves considerations which are unique to precast. Some of these requirements vary greatly from insitu reinforced concrete design. This is due to the fact that precast panels are manufactured off site in a factory and are then transported to a site where they are then erected. Panel dimensions and details must take account all of the aspects of the manufacturing, transporting and erection of the panel and not only the ‘final-in-place’ design function of the panels. The following guidelines will help you to establish precast design parameters for your project:
1. Precast panels are generally manufactured in accordance with NZS3109. Designers of precast panels should be familiar with this document. In particular, it is important that the design makes allowance for the manufacturing tolerances given in Table 5.1. Generally, weld plates and fixings that are drilled and placed on site provide larger tolerance than cast-in bolted fixings.
2. The surface finish of precast panels is generally in accordance with NZS 3114 and has a F5, U3 finish.
3. There is an Approved Code of Practice for The Safe Handling, Transporting and Erection of Precast Concrete, first published in May 2002 by the Occupational Safety and Health Service of the Department of Labour. All precast concrete designers should be familiar with this document.
4. When determining the size of the panels, the following points need to be considered:
To make the most cost-effective panel, it should be as large as the other constraints permit. (The smaller the panel, the more expensive the rate per square metre).
- The maximum size of a precast panel is determined by the size of panel that can be transported (height restriction) and the maximum load the factory and/or erecting cranes can safely lift. Generally, a 3.5m panel that weighs not more than 13 to 14 tonne is acceptable. Tower cranes may limit the panel weight to six or eight tonne.
- The maximum load per truck is approximately 20 tonne. If each panel is greater than 10 tonne than the panels will need to be transported in single loads. This may be slow and uneconomical. Transport companies generally charge for a minimum 16 tonne load.
- Long narrow panel (e.g. low level factory panels of 2m high) need to be not much more than 8m to 10m in length. If panels are long and narrow, they can flex and develop cracks during tilting, transporting and erection.
- With multi-storey buildings, the floor-to-floor height is generally 3m or more. If the panel-to-panel connection is achieved using drossbach ducts, the starter bars on top of the panels may make the overall height of the panel too high to transport. Using grout sleeves will often reduce the overall height to an acceptable level.
5. Starter bars protruding from the face of panels need to be considered. In order to be able to store and transport panels, precast manufacturers bend the bars against the face of the panels. With Grade 300 bars this is acceptable, and the bars can be re-bent to the correct position on site. However, with high tensile bars this is not permitted. The designer should refer to NZS 3109:1997 amendment no. 2 (March 2004) new clause 3.3.8. Also refer to the Cement and Concrete Association of New Zealand Information Bulletin: IB 79.
6. Where panels require stitch joints, it is important that designers take into account the fact that panels need to be tilted onto at least one edge in order to facilitate storage and transport.
To avoid expensive construction problems, the face of the panel that the panel will rest on during tilting will require a stable edge without any reinforcing protruding from it.
Useful Links
National Precast Concrete Association
An international trade association representing plant-produced concrete product manufacturers.
New Zealand Concrete Society
Precast/Prestressed Concrete Institute
Dedicated to fostering greater understanding and use of prestressed and precast concrete.
Precast New Zealand Incorporated
Precast New Zealand will promote, aid, foster, research, develop and protect the interests of the precast concrete industry within New Zealand.
