Floors - Anatomy of Steel Structure.

As has already been discussed later, regular and continuous concrete floors are not usual in industrial steelwork structures. Floors can consist of any of the following types:

(1) In situ concrete (cast on to removable formwork)
(2) In situ concrete (cast on to metal deck formwork)
(3) Fully precast flooring (no topping)
(4) Precast concrete units with in situ concrete topping
(5) Raised pattern ‘Durbar’ solid plate
(6) Flat solid steel plate
(7) Open grid steel flooring.

Typical sections of these types of floor construction are shown in Fig. 3.6.

Typical sections of floor construction
 Fig. 3.6 Typical sections of floor construction

The selection of floor type depends on the functional requirements and anticipated usage of the floor areas. Solid steel plates are used where transit or infrequent access is required or where the floor must be removable for future access to plant or equipment. They are normally used only internally (at least in the UK) to avoid problems with wet or waterlogged surfaces.

Open grid flooring is used inside for similar functions as solid steel plate, but with additional functional requirements where the flooring is subjected to spillage of liquids or where air flow through the floor is important. It is also used for stair treads and landings. Consideration should be given to making, say, landings and access strips from solid plate at intervals to assist in promoting a feeling of security among users. It is also in common use externally due to its excellent performance in wet weather.

Concrete floors are in use where heavy-duty non-removable floor areas are necessary. It is not normally advisable to use beam-and-pot-type floorings in any heavy industrial environment due to the damage that can be suffered by lightweight thin- walled blocks. For similar reasons, where precast concrete floors require a topping for finishes or to act as a structural diaphragm, it is advisable to use a fully-bonded small aggregate concrete topping with continuous mesh reinforcement, with a typical minimum thickness of 75–100mm.

A particular advantage of both precast flooring and metal deck permanent form- work is that in many industrial structures the floor zones are irregular in plan and elevation and therefore cheap repetitive formwork is often not practicable.

Even where formwork can sensibly be used, the early installation of major plant items as steel frame erection proceeds can complicate in situ concrete formwork.

Floors must be able to accept holes, openings and plant penetrations on a random layout, and often must accept them very late in the design stage or as an alteration after construction. This provides significant problems for certain flooring, particularly precast concrete. In situ concrete can accept in a convenient manner most types of openings prior to construction, but it may be prudent deliberately to allow for randomly positioned holes up to a certain size by oversizing reinforcement in both directions to act as trimming around holes within the specified units without extra reinforcement.

Metal deck formwork is not so adaptable as regards large openings and penetrations, since it is usually one-way spanning, especially where the formwork is of a type that can also act as reinforcement. If there is sufficient depth of concrete above the top of the metal deck profile, then conventional reinforcing bars can be used to trim openings. Many designers use bar reinforcement as a matter of course with metal deck formwork to overcome this problem and also to overcome fire pro- tection problems that sometimes occur with unprotected metal decks used as  reinforcement.

Steel-plate flooring should be designed to span two ways where possible, adding to its flexibility in coping with openings since it can be altered to span in one direction locally if required.Open-grid flooring is less adaptable in this respect as it only spans one way and therefore openings will usually need special  trimming and support steelwork. Early agreement with plant and services engineers is vital to establish the likely maximum random opening required, and any structure-controlled restraint on location.

Other policy matters that should be agreed at an early stage are the treatment of edges of holes, edges of floor areas, transition treatments between different floor constructions and plant plinth or foundation requirements.

This last item is extremely important as many plant items have fixings or bearings directly on to steel and the exact interface details and limit of supply of structural steelwork must be agreed. Tolerances of erected structural steelwork are sometimes much larger than anticipated by plant and equipment designers and some method of local adjustment in both position and level must normally be provided.Where plant sits on to areas of concrete flooring then plinths are usually provided to raise equipment above floor level for access and pipe or cable connections.

It is convenient to cast plinths later than the main floor, but adequate connection, for example by means of dowelled vertical bars and a scabbled or hacked surface, should always be provided. It is usually more practical to drill and fix subsequently all dowel bars, anchor bolts for small-scale steelwork and for holding-down plant items, and similar fixings, than to attempt to cast them into the concrete floor.

Except in particularly aggressive environments, floor areas are usually left unfinished in industrial structures. For concrete floors hard-trowelled finishes, floated finishes and ground surfaces are all used: selection depends on the use and wear that will occur. Steel plate (solid or open-grid) is normally supplied with either paint or hot-dip galvanized finishes depending on the corrosiveness of the environment; further guidance is given in BS 4592: Part 11 and by floor plate manufacturers.

Steel-plate floors are fixed to supporting steelwork by countersunk set screws, by countersunk bolts where access to nuts on the underside is practicable for removal of plates or by welding where plates are permanent features and unlikely to require replacement following damage.

Alternatively, proprietary clip fixings can be used for open-grid flooring plates to secure the plates to the underside of support beam or joist flanges.

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