The permanent works design of the slab will have been carried out to relevant standards, and is outside the scope of this guide. The Hy-Rib, during construction of the slab, has to support the full weight of the concrete, together with the additional load from the operatives placing the concrete, known as the construction operations load. (See BS 5975 on Falsework (Ref. 17)). The safe span of the Hy-Rib, i.e. the clear distance between the supports, should not exceed the value given in the table below.
Although in soffit applications the deflection between the falsework supports is rarely critical, the likely maximum mid span deflection of the Hy-Rib is stated in table below. At end supports the Hy-Rib should have at least 50mm of bearing when seated on steel or concrete, increasing to 70mm on masonry (CIRIA C558 (Ref. 21)). Note that this value is the deflection of the Hy-Rib between supports during construction, and not the final deflection of the completed slab after the falsework is struck / removed.
Indicative clear distance between supports for Hy-Rib used as permanent soffit formwork
| Equivalent depth of the solid concrete slab | Grade 2411 |
Grade 2611 |
Grade 2811 |
|||
| clear distance | midspan deflection | clear distance | midspan deflection | clear distance | midspan deflection | |
| mm | mm | mm | mm | mm | mm | mm |
| 75 | 950 | 5.4 | 800 | 3.8 | 725 | 3.5 |
| 100 | 875 | 5.2 | 725 | 3.6 | 650 | 3.3 |
| 125 | 825 | 4.8 | 675 | 3.3 | 600 | 3.1 |
| 150 | 775 | 4.5 | 625 | 3.1 | 575 | 2.9 |
| 175 | 725 | 4.2 | 600 | 2.9 | 550 | 2.6 |
| 200 | 700 | 3.9 | 575 | 2.7 | 525 | 2.5 |
| 250 | 625 | 3.4 | 525 | 2.4 | 475 | 2.2 |
| 300 | 575 | 3.1 | 475 | 2.1 | 425 | 1.9 |
| 400 | 500 | 2.5 | 425 | 1.7 | 375 | 1.6 |
| 500 | 450 | 2.1 | 375 | 1.5 | 350 | 1.3 |
The Hy-Rib is used in horizontal direction with the ribs pointing upwards and spanning in the strong direction between supports. In considering the permissible span of the Hy-Rib, the following loads have been included in the design:- the Hy-Rib self weight, It is assumed that the Hy-Rib is simply supported. Even if the Hy-Rib is spanning over several of the falsework supports, its lower bending strength at the support is such that it will yield as the full pressure is applied, making critical the bending at mid-span as a simply supported member. The anticipated midspan deflection of the Hy-Rib has been conservatively estimated using the self weight and the equivalent weight of solid concrete as simply supported; in practice the deflection is reduced by continuity. Where more than one Hy-Rib sheet is used to make up a span, there should be at least 75mm projecting past in both directions at the intermediate support with the lap. The minimum bearing of the Hy-Rib at end supports is 50mm for steel or concrete, increasing to 70mm when seated on masonry.
