Civil Engineering Reference
In-Depth Information
Vertical Loads
Total applied
horizontal loads
(Wind plus EHF)
Wind F
w,k
Figure 15.2 Horizontal loads applied to bracing
x'
recommended that the designer carries out sufficient hand cal‑
culations to validate the computer output.
Building Weight F
bldg,k
L
15.2.5 Building Regulations
All building construction in England and Wales has to comply
with the Building Regulations (HM Government, 2000). (Similar
regulations exist for other parts of the UK.) Part A Structure of
the Regulations stipulates the requirements for stability, namely:
Figure 15.1 Ultimate limit state (ULS) of equilibrium - overturning
moment should be distributed between shear walls (refer to
example in 15.3.2.3).
Loading
15.2.3 Sway effects
When a structure is loaded vertically it should have sufficient
stiffness not to cause excessive lateral deformations or sway.
The sway can be caused by geometrical imperfections in the
framing, i.e. out of plumb. The Eurocodes (BS EN1992:2004,
BS EN1993:2005, BS EN1995:2004, BS EN1996:2005) rec‑
ommend an
equivalent horizontal force
(EHF) is determined
and added to the other horizontal actions applied to the build‑
ing. For example, total horizontal action applied to the build‑
ing is the sum of the wind plus EHF (
Figure 15.2
). Refer to
section 15.4.5 below.
A1 (1) The building shall be constructed so that the combined
dead, imposed and wind loads are sustained and transmitted by
it to the ground:
(a) safely; and
(b) without causing such deflection or deformation
of any part of the building, or such movement of the
ground, as will impair the stability of any part of an‑
other building.
(2) In assessing whether a building complies with sub para‑
graph (1) regard shall be had to the imposed and wind loads
to which it is likely to be subjected in the ordinary course of
its use for the purpose for which it is intended.
15.2.4 First‑ and second‑order structural analysis
A first‑order structural analysis is an analysis where the frame
deformation is calculated on a single pass basis and bending
moments and shear forces are determined on that basis. In a
second‑order analysis account is taken of the frame deformation
in the analysis and the frame deflections, bending moments and
shear forces are amended accordingly. This is the P‑δ effect.
If the frame is sufficiently stiff then the second‑order
effects can usually be ignored. But in some frames such as
multi‑storey moment frames, the additional moments can be
significant and should be taken into account. EC3 Cl 5.2.1(2)
gives recommendations when second‑order analysis should
be adopted. If the frame is sensitive to second‑order effects
then all the lateral actions (e.g. wind + EHF) are increased or
amplified.
Many modern‑day computer analysis programs carry out
first‑ and second‑order analysis including automatic allow‑
ance for frame imperfections. Consequently, the designer
is not required to calculate and apply EHFs. However, it is
15.3 Low‑rise buildings
Low‑rise buildings are generally accepted to be one, two or
three storeys in height. In the UK, most single storey buildings
are either of steel or masonry construction.
15.3.1 Steel framed single storey buildings
Lateral stability of single storey steel framed buildings should be
provided in two orthogonal directions by the following means:
rigid frames (e.g. portal frames) and/or
■
■
braced bays (plan bracing in the roof acting in conjunction with
vertical bracing in the walls).
15.3.1.1 Portal action and bracing
Wind is the principal horizontal action requiring to be resisted
by a single storey building. Wind pressure and suction on op‑
posite sides of a building cause a resultant transverse action on
