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Rib depths can vary from 100 to 350mm in height, and up to 12m spans.
All Formstress precast Ribs are cast from 45 MPa (at 28 days) concrete.
STC 55 rating can be achieved with a rib and timber infill system with 75mm topping and plaster board ceiling.
A fire rating of 90 minutes is achieved by the Formstress Ribs in accordance with the NZS 3101 concrete code. The overall rating of the floor system is dictated by the topping thickness, 60 minutes for 75 thick and 90 minutes for 100 thick.
Insitu topping concrete should be a minimum of 75mm thick and achieve 25 MPa at 28 days.
Our Formstress engineers will produce the final design for your Ribs, specify the number and size of the prestressing stands based on your load requirements and span.
Refer to NZS 3109:1997 Table 5.1 for precast dimensional tolerances.
Provide a minimum of 75mm end seating to precast Ribs landing on concrete supports and 60mm seating on steelwork
Formstress typically provide H3 treated 150x25 rough-sawn timbers cut to lengths. We can also provide timbers dressed one side and with arrised bottom corners for exposed situations if required.
INSTALLATION & CONSTRUCTION
Before placing Ribs, install props to the required levels. This should be achieved to within ±3mm. The propping system (designed by others) must be adequate to carry all construction loads. Props should not be removed until the insitu topping has reached 75% of the specified concrete strength.
Insitu concrete toppings should be water or membrane cured for at least 7 days.
Floor systems with smaller spans may be installed without props on consultation with a Formstress engineer.
Drilling holes through or cutting precast Ribs is not recommended and should only be undertaken with the written approval of a Formstress engineer.
GENERAL NOTES
Contact Formstress about any specialist needs you may have. Formstress can in some cases improve fire ratings or arrange special concrete mixes to suit your situation.
Before manufacture can commence the contractor shall sign off the Formstress shop drawings certifying that the shop drawings are dimensionally and structurally correct.
All Formstress precast units are provided with lifting eyes. Chains and strops must be of the correct length and not more than 30 degrees of vertical.
For temporary storage of precast Ribs provide bearers each end on level ground
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