Construction

We have designed a number of reinforced concrete floor slabs to support Storax loads with the rails cast in. There are a number of considerations when doing the design the main one being the moving point loads from the wheeled racks. – We use finite element analysis with plate elements and a spring support at each node to model    the subsoil stiffness and insulation underlay stiffness. – The rail stiffness is included in the model to reduce the maximum slab moments and hence  reduce the amount of rebar without effecting the factor of safety. – The sequence of construction is critical to a good installation which is as follows: – Note this is for a cold room slab and would be similar for a standard warehouse excluding the  insulating layers. – Level, compact and test the subsoil – Excavate strips under the rail positions – Cast the reinforced strips on an insulation layer with rebar protruding along the edges – Fix the rails to the strips by drilled anchors. – Adjust the rail alignment to the very fine tolerances using the adjustable rail clamps – Check that all the rail clamp bolts are tight – Recheck the alignment with a laser. – Place the under slab insulating layer – Fix the remaining rebar – Employ an expert flooring contractor to cast and power float the slab to tight tolerances – The strips are bonded into the final cast with the protruding rebar and the final slab acts as a  single monolithic unit taking advantage of the thicker slab under the rail point loads. The following list of items can easily trip up the designer: – Expansive clays in the subsoil-if there is any doubt in an experienced designer’s mind then a  Geotechnical report must be called for. – Concrete shrinkage-these slabs are generally large and have no expansion joints-they shrink  towards the centre point of the slab and if there is any restraint a crack will occur at the restraint-  i.e. all existing steel columns (and any other restraint) must be wrapped with compressible layer  e.g. jointex. These slabs are often cast on top of existing slabs which then require access ramps-  care must be taken to ensure the ramp is not anchored and can shrink with the slab. – There are surround reinforced concrete kerbs and the starter bars for these kerbs must be cast  into the slab. In cold rooms the moisture content of the concrete must be below specified levels before the temperature is drawn down-excess mixture will expand on freezing and cause problems. Extensions to existing freezer slabs have the problem that the new concrete touches frozen concrete and will not set-propriety products must be used in locally around the frozen areas or an igloo must be built locally to allow the temperature of the existing slab to be raised locally when casting. Rod Holmes CONSPEC cc CONSULTING CIVIL ENGINEERS 7 SURCINGLE AVE, HOUTBAY 7806 TEL/FAX: 021 790 5638