F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

11.4(c) Unbraced frame analysis
Unbraced frame analysis 419
The task remaining is the consideration of the frame, shown in Fig.
11.4-3, as unbraced and therefore to be assessed under the action of
lateral loads. It will be remembered that BS 8110: Clause 3.2.1.3.2 states
that, in such a frame, the design moments for the individual members may
be obtained from (a) or (b) below, whichever gives the larger values:
(a)
(b)
those obtained by simplified analyses of the types given in Examples
11.4-1(a) to 1(e) above; or
the sum of the effects of:
(1) single storey analyses, idealized as in Fig. 11.4-4 and loaded
throughout with 1.2 x (gk + qk); and
(2) the analysis of the complete frame loaded with 1.2wk only (Fig.
11.4-3), and assuming points of contraftexure at the centres of
all beams and columns (Fig. 11.4-2).
The single-storey analysis under a uniform loading of 1.2 X (gk + qk),
i.e. 1.2(36 + 45) = 97 kN/m, is similar to that given in Example 11.4-l(b)
and results in the bending moment diagram as shown in Fig. 11.4-9.
If the wind loading ( wk) is not given, then recourse has to be made to CP
3: Chapter V: Part 2 [5], which gives two methods for the estimation of
the forces to be taken as acting on the structure.
The procedure for the first method is as follows:
(1) Determine the basic wind speed V (m!s) for the appropriate UK area
from a given table of values (e.g. Aberdeen 49 m/s; Nottingham
43 m/s; London 37 m/s).
(2) Determine values for three factors, from tables:
S1-Topography factor. Usually 1.0.
S2-Ground roughness, building size and height above ground factor.
This varies with the various parameter values (e.g. open country,
town or city) and has a range from 0.47 to 1.27.
S3-Statistical factor. This depends upon the degree of security
required and the number of years that the structure is expected
to be exposed to the wind. The normal value is given as 1.0.
(3) Calculate the design wind speed Vs (m/s) where
Vs = V X S1 X S2 X S3 m/s
105 50
105 50
Fig. 11.4-9 Bending moment diagram (kNm) for unbraced frame with vertical
load 1.2( Gk + Qk)