Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
External wall construction
The external skin of a multi-storey building is supported off the structural frame.
In most high quality commercial buildings the cost of external cladding systems
greatly exceeds the cost of the structure. This influences the design and construction
of the structural system in a number of ways:
• The perimeter structure must provide a satisfactory platform to support the
cladding system and be sufficiently rigid to limit deflections of the external wall.
• A reduction to the floor zone may significantly reduce the area and hence cost
of cladding.
• Fixings to the structure should facilitate rapid erection of cladding panels.
• A reduction in the weight of cladding at the expense of cladding cost will not
necessarily lead to a lower overall construction cost.
Lateral stiffness
Steel buildings must have sufficient lateral stiffness and strength to resist wind and
other lateral loads. In tall buildings the means of providing sufficient lateral stiffness
forms the dominant design consideration. This is not the case for low- to
medium-rise buildings.
Most multi-storey buildings are designed on the basis that wind and/or notional
horizontal forces acting on the external cladding are transmitted to the floors, which
form horizontal diaphragms transferring the lateral load to rigid elements and then
to the ground. These rigid elements are usually either braced-bay frames, rigidjointed
frames, reinforced concrete or steel–concrete–steel sandwich shear walls.
Low-rise unbraced frames up to about six storeys may be designed using the simplified
wind-moment method. In this design procedure, the frame is made statically
determinate by treating the connections as pinned under vertical load and rigid
under horizontal loads. This approach can be used on both composite and noncomposite
frames, albeit with strict limitations on frame geometry, loading patterns
and member classification.
British Standard BS 5950 sets a limit on lateral deflection of columns as height/300
but height/600 may be a more reasonable figure for buildings where the external
envelope consists of sensitive or brittle materials such as stone facings.
Accidental loading
Factors influencing choice of form 53
A series of incidents in the 1960s culminating in the partial collapse of a systembuilt
tower block at Ronan Point in 1968 led to a fundamental reappraisal of the
approach to structural stability in building.
Traditional load-bearing masonry buildings have many in-built elements providing
inherent stability which are lacking in modern steel-framed buildings. Modern
structures can be refined to a degree where they can resist the horizontal and