Holdfast Training Services Trial Blog

Issue.  So the concrete frame sub-contractors TWD made a slight error in his backpropping calculations.  Unfortunately when interrogating the structural loading document for the permanent design he forgot to subtract the selfweight of the slab and therefore overestimated the capacity of the slab in the temporary state (Figure 1).

Figure 1 – Backprop design

In the permanent design the intermediate slab is designed for a dead load of 7.4kn/m2 and an imposed load of 2.5kn/m2 (Figure 2).  The dead load includes the selfweight of the slab, therefore the spare capacity of the slab at present is 2.5kn/m2 of imposed load and 0.5kn/m2 of dead load as the services are yet to be installed.  Therefore, by inspection the intermediate slab is overloaded as the TWD requires 5.88kn/m2.

Figure 2 – Loading details

After a bit of research I found ‘The Temporary Works Toolkit: Part 4 – An introduction to backpropping of flat slabs‘ published in The Structural Engineer.  This details how for flat slabs below 350mm a simple percentage of load transfer method can be used for calculation of loads in the slab and props (figure 3).  This is the method used by the TWD in the calculations above.

Figure 3 – Method 1 for slab and prop calculation

Due to the limited capacity in the permanent case design, this method of backpropping is not acceptable.

Options.  In order to solve the problem we could either change the permanent design or the TW design.

Change the permanent design.  This option isn’t really possible due to the additional re-design costs and the fact the intermediate slab is 30% complete.

Transfer load to the ground bearing slab.  If the backpropping is set up leg for leg with the formwork above then the load could be transferred to the ground bearing slab (Figure 4) .  However, as the load is applied the props would shorten and therefore the slab will deflect and it will therefore take some load also.  In order to ensure this load is not excessive could you calculate the elastic shortening of the prop and then pre-tighten the prop to minimise the intermediate slab load?

Figure 4 – Backdrop redesign

Grateful for any thoughts or observations on the logic above.  Also, from others experience, is it normal that there is so little capacity in the permanent design to allow for the temporary construction state?