Beam structures

Structures consisting of beams supported on columns are simple and commonly used, especially where the minimum internal volume is required.

Trussed girder

Trussed girder

 

Beam structures, consisting of beams supported on columns, are of common usage both in single span and multi-span form. A major advantage is that they lend themselves to prefabrication in elements for ease of transportation and site erection.

An early example of this was the Crystal Palace erected for the Great Exhibition in 185 l covering an area of 7.3 ha (18 acres) with prefabricated components of cast iron and timber on a 24 ft module. None of these weighed more than 1 ton so that they could comfortably be lifted with a system of horse drawn or hand-winched block and tackle

This form of flat beam structure is most appropriately used where the minimum volume of space is required within the desired clear height. The achievable span is then directly related to the depth of the beams which, for normal loadings, would require a depth to span ratio of approximately 1:15 for solid web steel beams.

Beams may be solid web or truss forms, offering a variety of opportunities for architectural design.

Although solid web beam structures have the depth to span advantage they have a high self weight and they often lose to the trussed solutions in terms of services co-ordination where the open webs provide valuable routes for the services.

Mies Van der Rohe's Crown Hall building on the Illinois Institute of Technology campus defined a new architectural vocabulary in the 1950s with the black painted solid web steel beams and columns expressed on the outside.

This refined use of structural steelwork as part of the architectural vocabulary was progressed in the 1960s by the American architects Skidmore Owings and Merrill on a series of industrial buildings and in the UK was pioneered by Team 4 on the Reliance Controls plant in 1965 at Swindon

Later, as the level of building services increased with the introduction of new technology, trussed beam solutions were more favoured, following the example of the SCSD (Southern Californian Schools Development) System for California School building where the mechanical and electrical services are co-ordinated within the roof trusses.

SCSD (Southern Californian Schools Development) System

SCSD (Southern Californian Schools Development) System

 

This Systems Approach was adopted by the Milton Keynes Development Corporation architects for the design of a standard 'Advanced Factory Unit' which was to be versatile enough to house almost any small industrial process. A 12m x 12m structural grid was chosen for maximum flexibility with secondary trusses at 4m centres to support the flat roof deck and services. The 1200mm deep lattice trusses were designed to carry services and air-handling equipment within the roof zone. Generally though, a structural grid of 12m x 12m would be considered too restrictive on the internal layout and column spacings of 18m-20m are preferred for this kind of multi span form.

There has been an increasing demand for longer column free spaces to provide maximum flexibility of the interior space.

The need for column free space may require much longer spans, such as the Boeing Assembly Building at Everett near Seattle which has five bays of 488m x 9lm with bridge beams spanning onto braced steel framed core zones, allowing space to assemble and manoeuvre the huge jumbo jets.

Similarly the British Pavilion at the Seville Expo 92 designed by Nicholas Grimshaw and Partners provided a large, column free space with 40m long tubular steel roof trusses pin-jointed onto trussed steel columns with a clear span of 32m.

The structure was fabricated in Britain with the beams in two sections and bolted together on site. Here the exciting large volume space provides a spectacular home to the British exhibition but it could equally be dismantled and re-erected elsewhere to suit some other purpose.

British Pavilion at the Seville Expo 92

British Pavilion at the Seville Expo 92

 

Modified truss forms can provide improved design solutions.

In some instances the use of triangular beams may increase the efficiency of the longer span structures as was found to be the case on the Sainsbury Visual Arts Centre at the University of East Anglia by Foster Associates.

Here the 2.5m deep triangular trussed beams span 35m onto triangular towers of the same depth. This kind of structure has higher fabrication costs but has the advantage of being inherently stable making erection easier and reducing the need for secondary bracing.

Additional advantages are that potential buckling due to compressive forces can be shared between two (usually top) booms, and spans for cladding support are reduced.

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