Oxford Method Research Programme

Introduction
6M buildings dated from the early-mid 1980's were typically flat roofed and clad with horizontally profiled lightweight metal cladding; these notes relate to buildings of this period.
The structural design principles of 6M Oxford Method were developed in the late 1970s, in collaboration with consulting engineers Roughton & Partners, based in Southampton. As for other aspects of 6M design and production information, considerable and advanced use was made of computer-aided design techniques; software was developed to allow the rapid calculation of column thicknesses and of lattice member sizes, according to proposed loadings, beam spans and beam depths.
Later variants of 6M were brick-clad with pitched roofs, and whilst retaining most of the structural principles and construction outlined below, also incorporated a number of changes. These mainly comprised supplementary lightweight steel framing to support the pitched roof construction, and latterly the substitution of UB/castellated beams for the open lattice beams described below.

Columns
Columns are formed in RHS (rectangular hollow section) steel, and have a consistent overall section size 150mm x 150mm. The wall thickness of the RHS columns varies according to the local structural loading arising from the particular building design.

Bases
Each RHS column is resin-anchored via a base plate to an individual pad foundation, sized according to design loadings and local ground conditions, and typically of the order of 900mm or 1200mm square. Ground floor partition and general design floor loadings are taken by a 150mm mesh-reinforced ground floor slab, laid on a sub-base usually comprising 150mm weak-mix concrete with DPM over, and with a powerfloated finish. Cladding loads are accommodated by local perimeter thickening of the 150mm slab.

Lattice beams
The RHS columns carry welded angle cleats for the bolted connection of the steel lattice beams. The beams are fabricated in a "twin lattice" arrangement; this is devised to allow vertical service runs (typically 110mm foul or surface water drainage pipe stacks) to be made on the structural grid line, which is centred on the general 600mm planning grid.
(In Oxford Method, this 600mm planning grid sets out the suspended ceiling and is also used to predetermine a limited range of 'permissable' locations for hollow stud partitions and related concealed service drops)

Structural zone
The twin lattice beams will fall into one of a predetermined and limited range of structural zone depths. The depth of zone applicable to any particular building will be determined principally by the structural spans between columns. Zone depth may also be affected by non-structural local factors, such as the need for a particular floor-ceiling height, or space required for the distribution of major services such as large diameter ductwork adjacent to an air-handling plantroom.
The typical notional structural zone depth encountered in the majority of 6M buildings will be 1200mm; zone depths of 900mm or of 1500mm may also be found.
The notional zone depth is taken from the soffit of the suspended ceiling below the beam, to the finished floor surface of the floor slab above the beam. Thus within a notional zone depth of 1200mm; the upper 125mm comprises the floor slab depth, and the bottom 75mm comprises the suspended ceiling depth; the actual depth of the lattice steel beam will be 1000mm. Overall width of the lattice beam is nominal 300mm, with top and bottom booms generally nominal 125mm x 75mm RSA (rolled steel angles); leaving a central clearance of 150mm for vertical services drops.

Space for services distribution
Irrespective of beam depth, the small-section steel members comprising the diagonal lattice of the beam will be laid out to a consistent geometry, determined by the 600mm planning grid (see drawing). This geometry creates predetermined spaces for the horizontal distribution of M&E services and of drainage runs, through the beams.
The notional beam depth and lattice geometry therefore dictates the maximum diameter of ventilation ductwork that can be accommodated. As an indication, circular-section ducts of nominal diameter 450mm can be accommodated between the diagonal lattices of a beam of notional structural depth of 1200mm; and of nominal diameter 600mm within a notional structural depth of 1500mm.
The positioning of smaller services - such as drainage runs, cable trays, conduits, piped services etc - are similarly predetermined both on plan and by level, within the, geometry of the lattice beams.

Structural fire protection
For typical interior applications, and where exposed within the room space, the RHS columns will be encased in proprietary (non-asbestos) fire-resisting cladding. In exterior applications, for example at entrance canopies, the RHS column may be left exposed and directly painted, with structural fire resistance being achieved by filling the void of the column with concrete.
Structural fire protection is provided within the structurallceiling void by the pressed steel tiles and mineral fibre inlays forming the standard 6M suspended ceiling construction, which is deemed to achieve one-hour fire resistance. Accordingly, no additional fire protection is applied to the lattice beams themselves.

Fire compartmentation
Fire compartment and sub-compartment boundaries are planned to coincide with column/beam lines. At these boundaries, and within the structurallceiling zone, fire resistant construction is fixed to both sides of the lattice beams, and comprises lathing formed in wired clay, with sprayapplied 'vermiculite' finish. The same construction, fixed to one side only of the lattice beam, is also used to form intermediate cavity fire barriers.

In-situ slabs & composite action
Intermediate floor slabs, and flat roof slabs, are formed using in-situ concrete with light mesh reinforcement, laid on 'Holorib' profiled steel permanent shuttering. Self-tapping stud fixings with oversized heads are used to secure the Holorib shuttering to the lattice beams, achieving a ( 'composite slab' structural interconnections between slab and beam, and thereby allowing reduced slab thicknesses and lattice beam member sizes. Overall concrete slab thicknesses are nominal 125mm for intermediate floors, and nominal 100mm for roof slabs.
The twin lattice beams are fabricated, to incorporate a pre-calculated camber, both for intermediate floor slabs and for roof slabs. In the case of intermediate floors, the camber calculated to be fully taken by the designed floor loadings, giving flat floor construction.

Roof slabs
In the case of roof construction, the pre-camber is calculated to leave some residual fails, from mid-span of beams towards columns; rainwater outlets and drops will be located adjacent to column positions. Roof falls are achieved entirely by the fall of the roof slab; roof screeds to falls are not used. Asphalt finishes are applied directly to the power-floated roof slab, and overlaid by constant-thickness insulation, and holding-down pebbles or similar finishes.

Services in floors
Because intermediate floor slabs are directly finished by power floating, without the use of any additional screeds. the floor construction cannot be used for the distribution of services pipework or conduits. 'Holo-wedge' fixings, engaging in the soffit profile of the Holorib shuttering, are used to support suspended horizontal service runs.
Because intermediate floor slabs are directly finished by power floating, without the use of any additional screeds. the floor construction cannot be used for the distribution of services pipework or conduits. 'Holo-wedge' fixings, engaging in the soffit profile of the Holorib shuttering, are used to support suspended horizontal service runs.

Further contacts & detailed technical advice
Roughton & Partners carried-out the particular structural design of the large majority of individual 6M buildings, and have retained microfiche record drawings and calculations for most of these projects.
Engineers Peter Burrows and John Horton, who were closely involved with earlier 6M projects, are no longer available, but current Roughton Associate Bob Snowden, and CAD technician John Beattie, provide a track record of 6M knowledge and experience extending from the mid1980s.
Contact details are:
Roughtons, 321 Millbrook Road West Southampton S015 OHW
Tel:   023 8070 5533
Fax:  023 8077 5864
e-mail. bobsnowden@roughtons.co.uk