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Single-Storey Frames, Shells and Lightweight Coverings 165
Functional requirements
The functional requirements of framed structures are:
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Strength and stability
Durability and freedom from maintenance
Fire safety
Strength and stability
The strength of a structural frame depends on the strength of the material used in the
fabrication of the members of the frame and also on the stability of the frame, which is
dictated both by the way in which the members are connected and on the bracing across
and between the frames. Steel is most used in framed structures because of its good compressive
and tensile strength, and good strength-to- weight ratio. Hot rolled steel and coldformed
strip steel provide a wide range of sections suited to the economical fabrication of
structural frames. These sections are also relatively easy to recover and reuse at the end of
the building’s life. Concrete has good compressive strength but poor tensile strength and
so it is used as reinforced concrete in structural frames to benefit from the tensile strength
of the steel and the compressive strength of the concrete. The concrete also provides protection
against corrosion and damage by fire to the steel. Timber is often used in the fabrication
of roof frames because it has adequate tensile and compressive strength to support the
comparatively light loads. Timber tends to be used instead of steel to form lightweight roof
frames because of its ease of handling and fixing.
Durability and freedom from maintenance
On exposure to air and moisture, unprotected steel corrodes to form an oxide coating, i.e.
rust, which is permeable to moisture and thus encourages progressive corrosion, which
may in time adversely affect the strength of the material. To inhibit rust, steel is painted,
coated with zinc or encased in concrete. Painted surfaces will require periodic repainting.
Any cutting and drilling operations will damage zinc or painted coatings. Reinforced concrete
is highly durable and the surface will need little maintenance other than periodic
cleaning. Seasoned, stress-graded timber treated against fungal and insect attack should
require little maintenance during its useful life other than periodic staining or painting.
Fire safety
All loadbearing structures (including roofs) should be designed so that they do not fail
prematurely during a fire. Providing the structure with the necessary fire resistance helps
to reduce the risk posed by falling debris to building users, pedestrians and fire fighters.
Elements of the structure that give support or stability to another element of the building
must have no less fire resistance than the other supporting elements. Similarly, if a roof
provides stability and support to columns, then the roof must have at least the same fire
resistance as the columns. All roofs should have sufficient fire resistance to resist exposure
from the underside of the roof, remaining sound for a minimum of 30 minutes. The same
provision also applies to roofs that form part of a fire escape (appendix A, table A1). Where the
roof performs the function of a floor, the minimum period of fire resistance is dependent on
the purpose of the building and the height of the building (Table 4.1). If the building is
constructed with a basement, this will also have an impact on the required fire resistance.