Stability - ICE Manual of Structural Design: Buildings - page 253

Stiff Floor Diaphragm

=

+

Vertical Load
( N id )

Wind Moment
( M wd )

Combined Load
Intensity

Shear Wall
Load Intensity
Diagram

Load Intensity Diagram

Figure 15.17 Shear wall acting as vertical cantilever

Three checks are normally carried out:
(i) Stability check - maximum wind, minimum DL (γ G = 0.9,
γ Q = 1.5).
(ii) Wall resistance at base of ground floor wall (γ G = 1.35, γ Q =
1.5, γ Q = ψ1.5).
(iii) Wall resistance at mid‑height of ground floor wall (γ G = 1.35,
γ Q = 1.5, γ Q = ψ1.5) where wind overturning moment and
vertical load are reduced but wall resistance is also reduced
at wall mid‑height by capacity reduction factor φ.
Where an intersecting wall is used as the shear wall flange
the connection between the two walls should be checked for
vertical shear. Where metal ties or similar connectors are used
to bond two walls together the appropriate characteristic shear
strength of the connectors should be used.

panels in two orthogonal directions. In addition, overall sta‑
bility checks should be carried out for the following:
Overturning

■

■

Sliding

Uplift.

■

Since timber structures are light in weight these checks are
particularly important and can be critical when the building
height to breadth ratio exceeds 2:1. Stability checks need to
be carried out not only for the final built condition but also for
the construction phase when for example roof trusses and roof
tiles are not in place.
Figure 15.18 shows lateral forces applied to the gable eleva‑
tion of a two‑storey timber framed building. The lateral forces
from the masonry or timber cladding are transferred to horizontal
timber bracing at roof truss tie level and to the stiff diaphragm
at first floor level. These horizontal loads are then transmitted
to the stiff wall diaphragms which in turn carry the loads to the
foundations. The wall diaphragms also support the vertical loads
from the roof and first floor in addition to providing the in‑plane
shear or racking resistance to the lateral actions. The wall panels
also resist wind loading perpendicular to the panels.

Horizontal shear resistance
At the ULS, the design value of the applied shear load V Ed should
be less than or equal to the design value of the shear V Rd ,
where V Rd = f vd t ℓ c
and f vd - design value of shear strength of masonry
t - thickness of wall resisting shear
ℓ c - length of compressed part of wall, ignoring any part of the
wall that is in tension

15.3.3.2 Stiff horizontal diaphragms
Flat roofs and timber panel floors are used to transfer lateral
actions to stiff vertical wall panels. Eurocode 5 provides some
simple guidelines based on experience and practice to dem‑
onstrate that a conventional floor or flat roof consisting of
wood‑based panels fixed with nails or screws to timber joists
can be assumed to have adequate strength and stiffness to act

15.3.3 Low‑rise timber structures
15.3.3.1 Stability of timber structures
Stability of low‑rise timber structures is provided in a similar
way to other forms of construction, notably the lateral actions
such as wind forces on the structure are resisted by braced