by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : June 2008
?????????????????????????????????????? ?????????????????????????????????????? The use of permanent metal deck as formwork for post- tensioned structures is now widespread across Australia. The typical systems used are Bondek, Condeck and KingFlor. The PTIA Member Companies have literally completed hundreds of successful projects and the ease, speed and economics of this type of construction method are well recognised. The usual use of metal deck formwork is as a simple infill between conventionally formed band beams. The metal ribs are laid perpendicular to the band beams and allow the one way spanning tendons to pass between the ribs as shown in diagram below. This should only be an issue when the floor is supporting brittle partitions or there are tight surface tolerances. • Concentrated Loads The capacity of the slab due to concentrated loads needs to be accurately assessed. Because the slab thickness above the rib is less than that for a solid slab, the slab stiffness in the secondary direction is reduced leading to a reduction in the slab width supporting concentrated loads. • Fire Rating When considering fire rating the suppliers of metal deck formwork have written documentation on the effects of fire and its influence on the post-tensioned tendons. Generally fire ratings of 2 hours are satisfactory with the most common metal deck systems, but you should obtain job specific certification from the supplier. Metal deck systems that are shaped like ribbed slabs such that the slab thickness varies across the section will need special consideration and advice from suppliers should be sought. • Detailing Metal deck & band beams The soffits are quite clean and are often more attractive than a concrete off form finish. Painted finishes are available along with pre-coated metal decking. Apart from any direct material cost savings, the advantages for the Builder include much less waste, increased formwork laying productivity, elimination of formwork stripping, and less manpower required on the work site. When considering the use of metal deck formwork coupled with post-tensioning there are a number of things to consider as follows: • Strength The metal decking is bonded to the post-tensioned structure and therefore composite design actions are possible. However, it is usually deflection and “hogging” moment capacities that govern the design of post-tensioned structures and therefore any increased strength in the “sagging” moment regions may not lead to any direct advantage. Judicious redistribution of ultimate moments may result in an improved result, however, caution should be observed since service moments cannot be redistributed. • Deflection The use of metal deck formwork may lead to larger deflections under long term actions. The metal decking restrains the bottom surface of the concrete member and effectively over reinforces this section. This leads to a theoretical increase in deflections due to shrinkage warping. Detailing the slabs with the use of metal decking is particularly important, particularly at post-tensioning anchorage locations (slab edges, joints, etc). The critical factors are to get enough solid concrete around the anchorage and also to be able to physically fit the helical anti-burst in place. It’s the latter that generally governs. Generally the slabs need to be increased in thickness locally to 200mm minimum internally and 220mm to exposed slabs. There are other ways of achieving thinner slabs yet allowing anchorages to fit and it usually requires a small amount of conventional plywood to be used in lieu of the metal decking locally as shown below. Construction Joint and Stressing Pocket And don’t forget that the concrete framing plans should clearly show the conventionally formed sections and importantly must always indicate the laying direction of the metal deck ribs. More specific project by project information on the successful use of metal deck formwork can be obtained through a PTIA Member Company. PTIA Technical Committee Concrete in Australia Vol 34 No 2 35