Oxford Method Research Programme

Intermediate Floor Construction

Original 6M intermediate floor construction generally comprised:

• purpose-made twin lattice beams; as described above and dimensionally coordinated with the overall building grid giving pre-determined zones for services penetrations. Coordinating structural zone depths were variously 900/1200/1500mm, depending on design requirements for clear spans and for accommodation of major service runs.
• Richard Lees Ltd "Holorib" permanent troughed steel formwork, with V-shaped longitudinal trough profiles at nominal 150mm centres, and "Holowedge" accessories were used for suspending service pipework etc. from the trough profiles.
• cleat connectors shot-fired through the "Holorib" formwork into the lattice beam members, to achieve a composite structural action linking beams to slab, and allowing calculation of greater spans for the given slab depth.
• mesh-reinforced in-situ concrete floor slab, with power-floated finish, and giving 150mm overall slab depth
• purpose-formed galvanised steel slab edge pressings, incorporating 25mm foam plastic insulation, acting as edge/level formers, as well as providing fixing points for the "Multibeam" vertical cladding rails.
• sub-divisions of the ceiling void, to form compartment fire barriers or cavity smoke-stop barriers, were formed at both faces or at one face respectively of the twin beams. These subdivisions were formed by vermiculite spray onto a wired clay lath. Ductwork penetrations of fire barriers were sleeved in wired mineral fibre; plastics drainage penetrations were locally sleeved by a GRP wrap designed to melt down in the event of fire and form a seal.

Note:

• the twin lattice beams were fabricated with a calculated pre-camber, allowing for a level slab finish after concrete loading
• de-watering techniques were usually employed for intermediate and roof slabs, prior to power floating
• slab laying was carried-out to BCA recommendations in intermediate strip bays using temporary formwork
• in general. Structural fire protection was provided by the suspended metal ceiling, giving nominal one-hour protection. In local exceptions, eg in plant rooms, additional fire protection was spray-applied to mesh shot-fired to the structural soffit.

Local exceptions to the intermediate floor slab construction apply, generally as noted for the ground floor slab.

Matching construction in substituting 6M components; in extending 6M buildings

• Record drawings, design principles and supporting calculation procedures for the twin lattice beams, remain available via the original 6M consultant engineers, Messrs. Roughton of Southampton. However, despite achieving economy of materials, the twin lattice beams were relatively complex and slow and expensive to fabricate; in 6M construction the structural steelwork package was normally let to a specialist nominated subcontractor.
  In later (brick clad) variants of 6M construction, the twin lattice beams were replaced by more conventional standard profile rolled steel beams. For reasons of economy and ease of sourcing, this approach is recommended in any replacement or extension of 6M construction. Standard structural steel profiles will intrude deeper into the ceiling void depth otherwise available for the distribution of M & E Services and this factor should be allowed-for at an early stage of design development.
•  "Holorib" steel formwork remains available from Richard Lees Steel Decking, in profile references Z28 and Z35, generally matching profiles originally used in original 6M construction. Holorib is available in a range of gauges (0.9, 1.0, 1.2mm) affording a range of structural spans, according to slab depth and loading conditions (indicative 3.15m for 1.0mm gauge, slab depth 150mm and imposed load 5.0KN/mm²). Test-based fire ratings of 1-2 hours can be achieved, for mesh-reinforced composite slabs, depending on mesh type, slab depth and span.
  Revised design guidance and Codes of Practice have been published by British Standards and Steel Construction Institute, since original 6M design development, and should by checked by consulting structural engineers in respect of composite action and fire resistance of slabs formed on Holorib formwork.
• the power floated slab construction to an overall depth of 125mm remains appropriate for matching construction; together with the insulated slab edge pressing
• ceiling void fire/smoke-stop barriers in clay lath with vermiculite spray remain commercially available from Mandoral Coatings Ltd. "Mandolite CP2" has been tested in accordance with current BS476: 1987. A comparable alternative construction, also currently available comprises wire mesh reinforced mineral fibre slabs, wired directly to structural steelwork and to subsidiary light metal angel frameworks as necessary; available from Messrs. Rockwool, Lancaster Fibre Technology and others.

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