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Archive for May, 2016

Detailing Precast – 7 top tips

Sunday, May 22, 2016 @ 10:05 AM
Author: Spanwright Flooring

Good detailing is an integral part of good construction.  Where offsite construction is used for some or all aspects of the building, this becomes increasingly important.  The nature of the detailing process is such that it is almost never a single individual’s task, instead someone begins the process and it evolves by iteration.  Architects, Engineers, Product Suppliers, Fabricators and Construction Personnel all contribute to the final details.

In Precast Detailing, the responsibility to produce the drawing used to communicate specifics from the design team, through to manufacture and installation, is very often drafted by the precast manufacturer.   This allows the details to be fine tuned by the party with the most relevant experience, namely the precaster.  It is however vitally important that those who precede the precaster in the design process, have an understanding of the types of details which should be outlined and considered at the earliest stages of the design phase.



What they are: These are formed at the end of the floor unit and reduce the top level of the slab over a set distance from the end of the unit. A typical detail would be reducing the depth of a 200mm deep unit by 50mm to 150mm over a distance of 150mm from the end of the unit.

Purpose:  This detail typically occurs where floor units are to be placed into a steel beam or shelf angle.

Things to know:  When detailing slabs into steel beams or shelf angle, slabs may be reduced but tolerance should be left for installation. A good rule of thumb is that the slabs may be reduced at the ends by ¼ of its depth but fitting still requires a tolerance of 20mm i.e. for slab depths of 200mm they may be reduced by ¼ (50mm) giving a depth of 150mm. In this situation a minimum space for installation of 150mm + 20mm = 170mm should be left. Tighter tolerances may be feasible on deeper products but this should be discussed with the precast manufacturer and installer.



What they are:  Units manufactured with hollow cores can have the core end filled solid for a short distance from the end.

Purpose: Solid ends can be specified for a few reasons.  One reason is so as to plug the ends so as to stop air movement through the cores, plugging the cores also prevents travel of vermin through cores.  The 2nd reason for filling cores is to increase the compressive load carrying capacity from live or point loads above.  A 3rd reason for specifying solid ends is in cases of high shear in slender units.  This will normally be checked by the precast manufacturer in his design calculations.

Things to know: While some scenarios may require solid ends to increase compressive strength even a hollow core unit is extremely strong in compression given its constituent materials. This generally makes it unnecessary to provide solid ends for this reason.



What they are: These are manufactured steel components with one of a number of recognised heads which can be cast into the precast unit. A variety of cast in lifting mechanisms are available from most construction accessories providers.

Purpose: Cast in lifters make it easier to position units that are awkward shapes and in complex structural arrangements. They are an alternative to lifting units with slings.

Things to know: While widespread use of cast in lifters will increase cost of a precast installation, the benefits may still outweigh the costs. Lifters may also be used strategically in only some units in order to control costs. It is important to coordinate the use of cast in lifters on site so that units such as stairs and floors, which may come from different suppliers, can still use the same lifting equipment.



What they are: They are precast concrete slabs of the same production as full width units and are cut in the factory to produce a narrower unit.

Purpose: Ripped Slabs are used to fill out bays which are not the right width to take another full width slab.  They are also occasionally specified where slabs side bear onto the structure and a square edge is required for continuing the build.

Things to know:  Normally slabs may only be ripped (or cut) along the hollow core.   Because of the production limitations on rip widths it is normal to expect a tolerance up to 100mm when filling out bays with slabs.



What they are: These are machine formed cores that are filled with concrete post manufacture while the cast slab is in a semi set state.

Purpose: Solid cores provide a solid section into which fittings for wind posts can be post fixed on site. They also prevent collapse of the hollow units under high point or line loads. It is important to specify solid cores in locations where the edge of units are expected to carry a point load such as suspending a flight of stairs on an angle support

Things to know: When visually inspecting hollow core units, solid ends in a slab may give the impression of the cores being solid throughout the slabs. A core should never be assumed to be solid unless it has been specified as such on manufacturer’s drawings and/or trial bored onsite. It is possible that in wet cast slabs a void former in the slabs provides a core in an otherwise apparently solid unit.



What they are: These are usually 2 No hollow cores evenly spaced across the slab with the top flange of the slab removed over the width of the core and a distance of apx.600mm at the end of the unit.

Purpose: Open cores in hollow core units are created where there is a requirement to provide a structural connection with adjacent units or another part of the structure across the bearing.  The open core is formed to receive a steel bar and infill concrete during the grouting process.

Things to know: Considerable planning must be given when considering continuity slots and tie steel.  Where continuity steel is to be fixed to the structure and bent into
the open core it is advisable to post position the continuity bars in line with open core positions.  Where the tie steel is across a bearing from open core to open core, it is necessary to align bays so that open cores consistently fall opposite each other.



What they are: These are holes created in the floor area by means of cutting, coring or specialist casting.

Purpose: Openings and Notches are used where precast units conflict with other construction items such as: Plumbing, Electrics and Steel Posts.  Most precast units can tolerate some cutting and coring to accommodate services etc. but this gets more limited as units approach the limit of their design.

Things to know: Openings and Notches should be considered at the design stage, and where they are required to be post formed on site advice should be sought from the precast designer.  It is important when considering openings in units to allow for temporary state impacts such as handling and installation on the units integrity.  Small nibs of concrete and multiple perforations in the same zone are not generally feasible.  In the case of the later, it is often a better idea to completely omit an area of floor and introduce a steel support hanger.

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