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Single-Storey Frames, Shells and Lightweight Coverings 249
Cross rib (or diaphragm)
bonded to brick leaves
to form a rigid I-section
Cross ribs (or diaphragms)
bonded to brick
leaves to form a
rigid box section
Figure 4.85 Brick diaphragm wall.
Diaphragm wall of
two brick leaves with
borded cross ribs
I-sections structurally, as illustrated in Figure 4.85. The compressive strength of the bricks
and mortar is considerable in relation to the comparatively small dead load of the wall,
roof and imposed loads. Stability is provided by the width of the cavity and the spacing of
the cross ribs, together with the roof, which is tied to the top of the wall to act as a horizontal
plate to resist lateral forces.
Construction
The width of the cavity and the spacing of the cross ribs is determined by the size of the
box section required for stability and the need for economy in the use of materials by using
whole bricks. Cross ribs are usually placed four or five whole brick lengths (with mortar
joints) apart and the cavity one-and-a-half or two-and-a-half whole bricks (with mortar
joints) apart so that the cross ribs can be bonded in alternate courses to the outer and inner
leaves, as illustrated in Figure 4.86. Loads on the foundations are relatively slight, thus a
simple strip foundation can be used in good ground conditions.
The roof is tied to the top of the diaphragm wall to act as a prop in resisting the overturning
action of lateral wind pressure, by transferring the horizontal forces on the long
walls to the end walls of the building that act as shear walls. The roof structure is tied to a
reinforced concrete capping beam by bolts, as illustrated in Figure 4.87. Care is required
at this junction to ensure that thermal bridging does not occur across the capping beam.
Roof beams are braced by horizontal lattice steel wind girders, which are connected to roof
beams, as illustrated in Figure 4.88.
Door and window openings should be designed to fit between the cross ribs so that the
ribs can form the jambs of the opening. Large door and window openings will cause large
local loadings; thus double ribs (or thicker ribs) are built to take the additional load, as