The pictures here follow the construction of a conventional industrial building constructed during 1998 on the Silverlink Industrial Estate, North Tyneside. In particular, the installation of the concrete floor is explained. The pictures are intended primarily for students at Newcastle University taking courses in engineering & architecture. These notes were written in March 1999 at which time no tenant had been found. The building was constructed speculatively for mid-tech use i.e. most of the building is for warehousing or workshop use and part is for office use.
This shows the silver steel cladding from the rear. No attempt has been made to upgrade the appearance from this aspect.
The structure comprises propped portal frames supporting cold rolled steel purlins and side rails. Sag rods are provided at mid-purlin spacing.
By providing a prop, the designer has reduced the bending moment in the rafters, but has introduced the inconvenience of columns in the building.
Note the haunch at the eave. This is provided to take the large bending moment which occurs there and also to provide the depth required to take all of the bolts transferring the moment between the rafter and the column.
Note the bolts connecting the column to the rafter and the 45 degree stays which help to develop a moment between the purlins and the rafter.
The eaves detail on the left is the simple one as used on this case study. The one on the right is often used to hide the roof pitch and thereby to give the impression of a higher specification building. Care has to be taken with the possible build-up of snow in the valley. Note the relationship between the main steel sections, the purlins, the cleats connecting the purlins to the main steel and also the two layers of cladding.
The roof sheets have been fixed. Note the proportion of roof lights. Too much and the building will overheat, even in winter, too little and it will be dark. In time, the sheets will become dirty and the illumination will diminish.
Note the way in which the roof sheets are overlapped in such a manner that water cannot enter. Also, notice the fillet beneath the rafters which provides room for the moment connection bolts. The sheets are held to the purlins by J bolts. Care has to be taken with the waterproof washers since a common problem is for water to penetrate through the holes drilled through the sheets for the J bolts
Tubular steel is used as the wind bracing in the end roof bay. A plate is welded onto the ends of the tubus to allow each tube to be bolted to a cleat fixed to the rafters.
A concrete block wall has been constructed to first floor height around the building to form ground floor offices. This gives the occupants the feeling that they are in a substantial building and increases levels of thermal and sound insulation. The blocks are constructed between the columns and the side cladding will be installed outside the columns to hide the blocks.
It is important to isolate the floor from the steel structure. One of the centre props has been boxed out with concrete and an expansion joint has been placed around the concrete box. Flexible material, which can be easily squeezed with the finger and thumb has been fixed to the concrete. A rough 20mm square section timber fillet has been placed over the compressible material. After the floor has been cast, the timber will be removed and replaced with a bituminous waterproof material to keep the compressible material dry. This is a cost effective method of forming a full movement joint.
Joints have to be formed in the floor to control cracking resulting from restraint to shrinkage and warping of the slab as it dries. Dowel bars are included in the slab to spread applied load between neighbouring bays. The dowel bars need to be held firmly during concreting. If just one of the dowel bars becomes miss-aligned, the bars will lock the slabs together and cracks will form in the floor. Note the polythene sheet which separates the concrete from the underlying crushed rock sub-base. The polythene acts as a water barrier so preventing the concrete drying out too quickly whilst setting. It also reduces the friction between the concrete and the crushed rock, so reducing the chance of cracking.
Concrete is usually delivered as shown. A readymix truck can hold 5 or 6 cubic metres of concrete. For a long time, the concrete industry avoided the word "readymix" because it was registered by the company Ready Mix Concrete (RMC). The industry tried to introduce the term "premixed" - fat chance.
Most floors are reinforced with steel mesh or steel fibres (the Sunderland Nissan plant is a notable exception). Mesh of size 4.2m x 2.4m m is placed into the fresh concrete and is trod into place. Most designers prefer to have the mesh placed near the upper surface of the concrete to help to control surface cracking. The designer has a choice. Either, lots of mesh is specified with few or no joints or little or no mesh is used and joints are formed at spacings as close as 5m.
The concrete is compacted with a twin beam vibrator which is pulled by ropes. At the far side, it is supported on the formwork and on the near side, it is supported on previously installed concrete. This is known as long strip construction. Often, alternate strips are constructed using formwork at each side, then the infill strips are constructed using the previously constructed concrete as the formwork.
At the perimeter of the building, an isolation joint is formed using compressible board. Note the tie bars in the formwork. They connect neighbouring strips. In a large building, one side of the tie bars on every fourth longitudinal joint is coated with debonding compound to avoid locking the long strips together.
The transverse joints are formed by sawing a 40mm deep narrow groove in the surface of the concrete over the spot where dowel bars have been placed. This is like creasing paper before tearing it. Essentially, we are managing the cracking, rather than eliminating it. We let the slab crack in its own good time, hopefully at the groove. The groove is sealed with polysulphide or bituminous sealant to prevent dirt and water entering the "induced joint".
See how the ties are placed at mid-depth by inserting them into holes in the formwork.
Higher quality cladding materials are used at the front of the building. The project is completed by the lanscaping, including hot rolled asphalt for the access roads and concrete block paving for the car parking.