STRUCTURAL DESIGN FOR ARCHITECTURE

Table 5.1 Data for elements in low-rise loadbearing masonry structures

Walls single and >
2 storey 2 storey
External wall (cavity) Inner leaf 102.5 mm 215 mm
Outer leaf 102.5 mm 102.5 mm
Internal wall Loadbearing 102.5 mm 215 mm
Non-loadbearing 102.5 mm 102.5 mm

Floors Timber joist Depth range 75 mm to 300 mm (see Table 6.10)
Reinforced concrete slab Depth range 150 mm to 440 mm (see Table 4.1)
Roof Rafters and joists Depth range 75 mm to 300 mm (see Table 6.11)
Trussed rafter See Table 6.8
Truss See Table 6.9

Masonry structures

the span capacity of the floor system. A span
range of 3 to 5.5 m is normal for timber floors
and 3.5 to 8 m for reinforced concrete. It is
advantageous to keep all principal floor spans
in a building (i.e. those of the main spaces)
more-or-less the same by spacing the
loadbearing walls equal distances apart,
because this avoids the need for floor units
which are of different depth and therefore
simplifies the detailing of the building. It also
maintains the stresses in the walls at a
constant level and avoids the need to specify
different strengths of masonry. Some spaces,
such as corridors, in which spans are markedly
different from the principal spans of a building
are, of course, unavoidable. In buildings which
have one-way-spanning floors and roofs,
openings in walls are spanned by lintels; these
are normally of precast concrete or steel.
The two-way-spanning floor system which is
most commonly used in masonry buildings is
the in situ reinforced concrete two-way-
spanning slab, which is sometimes called a
flat-slab. Individual slabs are supported by an
arrangement of walls which is square on plan
and this type of floor structure therefore works
best in conjunction with cellular plan-forms in
which each cell is approximately square. The
high statical indeterminacy^6 of this form of
structure allows it to be used with irregular
plan-forms, however, and this is one of its

advantages over one-way systems. The high

statical indeterminacy also means that the

structural material is more efficiently used

than in the one-way system, particularly if

continuity is achieved over a number of

approximately equal spans. This system is

therefore capable of larger spans than are one-

way systems and gives smaller slab depths for

equivalent spans. The statical indeterminacy

also makes it possible for walls to be designed

with fairly large numbers of openings, without

the need for local thickening of the slab or for

lintels to be used to bridge the gaps.

It is desirable that the structural walls in

multi-storey masonry buildings should be

continuous throughout the height of the build-

ing and so the same basic plan-form must be

adopted at every level. Where this is not feas-

ible, as in certain building types in which

larger interior spaces are required at ground

floor level (for example, ground floor shops

with houses above or an hotel with a restau-

rant and bar on the ground floor and

bedrooms above) a special structural arrange-

ment must be used at the level where the

floor-plan changes. The most common

solution to this problem is to use a reinforced

concrete or steel frame at ground floor level;

the reinforced concrete floor slab which forms

the top of this is then used as a base for the

loadbearing-wall structure above (Fig. 5.25).

Alternatively the masonry itself can be

reinforced by incorporating steel reinforcing

6 See Macdonald, Structure and Architecture, Appendix 3. bars into the bedding planes. This gives it 167