When the ground is poor and the load is heavy, how best to deliver a commercial floor that’s safe, cost effective and delivers on performance? 

Using a suspended post-tensioned slab on grade solution is becoming more popular as sites with better ground in the cities become scarcer. The poor ground will often require a ground improvement or structural piles to support the heavily loaded slab. The suspended slab then spans between the piles in a similar way to how slabs span between columns in a multi-story building. 

Contech undertakes the design of the suspended slabs on grade using specialist reinforced concrete and PT frame software Rapt. Rapt is preferred to other software for its ability to design the post tensioning and conventional reinforcement as well as assess the design criteria. 

Governing load cases for industrial warehouses tend to be racking post loads and warehouses with suspended slabs are no different. The slabs are often thicker than a conventional post-tensioned slab in order to span between the piles.  

The Contech design team utilised the RAPT software to assess a frame long section of the slab in both orthogonal directions, ensuring crack widths and stresses remained within code limitations. RAPT has some limitations as it was primarily developed as a design software for multistorey residential and commercial post tensioned slabs.  Therefore how the loads were applied to the frame section requires careful manipulation.  We use finite element modelling software to assess the equivalent moment created by the unique racking arrangements and then reverse engineer them into a loading configuration that RAPT was more accustomed to.  

The section is assessed with zero stiffness in the pile supports to negate load transfer to the substructure of the frame. Using assessed ground conditions lower and upper bound pile diameter models were developed to simulate any variances in load spread through the engineered raft. 

We have also designed and built warehouse slabs which have required a combination of traditional post tensioned slabs on grade connected to suspended slabs on grade. These designs are a lot more intricate, utilising the techniques discussed above alongside analysis of on-grade slabs using finite element modelling along with our post tensioning design systems. 

These techniques provide an efficient design for clients and with the increasing scarcity of developable land for large industrial warehouses in parts of the country, Contech will look to utilise this technique on many more projects to come. 

Some example projects where suspended post-tensioned slabs were adopted:

Tuapeka Gold Print

Placemakers Dunedin

Visit our Suspended Post-Tensioned Slabs page for more information on this innovative solution.