Rib and Timber

Rib And Timber

Formstress rib and timber infill flooring system consists of precast prestressed concrete ribs spaced generally at 900mm centres with timber infills placed between them. The ribs have variable depth to suit the load/span requirements of the project. This multipiece system is tied together with a minimum 75mm in-situ concrete topping and mesh reinforcing. This system is suitable for those tricky sites where access is a problem or poor foundation conditions dictate the use of a comparatively lightweight floor. The Formstress rib and timber infill system is a well proven economic method of floor construction used worldwide. Typically the Formstress ribs are laid at a standard 900 centres and 25mm thick timber formwork placed between. A minimum 75mm thick insitu concrete topping is cast over producing a composite, versatile flooring solution.


Rib Hangers

The Formstress rib and timber infill system is now available with end hanger brackets cast in. This allows the designer to top mount the ribs and minimise the overall floor thickness. This detail can be used at one or both ends of the rib and is ideal for steel structures.

Designed to sit on steelwork, the precast ribs are placed at a standard 800crs/900crs with 25mm thick timber formwork placed between. A minimum 75mm thick insitu concrete topping is cast over producing a composite, versatile solution.




Product Specifications

Rib depths can vary from 110mm to 350mm in height, and up to 12m spans. All Formstress precast ribs are cast from 45 MPa (at 28 days) concrete. Rib strength = 45 MPa at 28 days. Topping strength = 25 MPa minimum at 28 days (as required by NZS 3101:2006). A fire rating of 60 minutes is achieved by the Formstress ribs in accordance with the NZS 3101 concrete code. Fire rating of 90 minutes can be achieved, contact Formstress for design. Another parameter affecting fire rating of the floor system is the topping thickness, 60 minutes for 75mm thick and 90 minutes for 100mm 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 strand based on your load requirements and span. Provide a minimum of 75mm end seating to precast ribs landing on concrete supports and 60mm seating on steelwork.


Infill Timbers

Formstress typically provides H3 treated 150x25mm and 200x25mm rough-sawn timber. We can also provide timbers dressed one side and with arrised bottom corners for exposed situations. The infill plank depth is typically 25mm although this does vary depending on preferences. Timber infills should be dampened prior to placing the concrete topping. Ribs are easily adapted to circular floor plans, where the use of other suspended flooring systems is difficult. Large floor openings of up to 700mm between ribs are easily accommodated.


Installation And Construction

Erection - It is recommended that ribs be seated 75mm onto concrete supports or 60mm onto steel beams with bearing strips to ensure an even bearing at the concrete level.

Ribs must be handled and stacked at two points, at, or directly beside the lifting eyes.

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.


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.