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)

Analysis of framed structure (BS 8110) 407
11.4 The analysis of framed structure (BS 8110)
11.4(a)
General comments
Although the now normally accepted method of section design is based on
ultimate conditions, the stage of development of corresponding methods
of structural analysis for concrete structures is such that further work is
needed before they can be accepted (see Section 4.9 and References 17,
18). In current design practice, elastic analysis is normally used to obtain
the member forces and bending moments in structural frames. A redistribution
of these moments as explained in Section 4.9 is permitted, to
make allowance for what may happen under ultimate conditions.
In the analysis of the structure to determine the member forces and
bending moments, the properties of materials (e.g. the modulus of
elasticity-see Table 2.5-6) should be those associated with their characteristic
strengths, irrespective of which limit state is being considered.
According to BS 8110: Clause 2.5.2, the relative stiffness of the members
may be based on the second moment of area/, calculated on any of the
following sections (but a consistent approach should be used for all
elements of the structure):
(1) The concrete section: The entire concrete cross-section, ignoring
the reinforcement.
(2) The gross section: the entire concrete cross-section, including the
reinforcement on the basis of the modular ratio ac, which may be
taken as 15.
(3) The transformed section: the compression area of the concrete crosssection
combined with the reinforcement on the basis of the modular
ratio ac, which may be taken as 15.
The I.Struct.E. Manual [20] additionally gives the following recommendation
for calculating the stiffness of Hanged beams: the flange width ofTbeams
may be taken as the actual flange width, or 0.14 times the effective
span plus the web width, whichever is the less; the flange width of L-beams
may be taken as the actual flange width, or 0.07 times the effective span
plus the web width, whichever is the less.
BS 8110 allows a structure to be analysed by partitioning it into subframes.
The sub-frames that can be used depend on the type of structure
being analysed, namely braced or unbraced, since a rigid frame's reaction
is different for the two cases. A braced frame is designed to resist vertical
loads only; therefore the building must incorporate, in some other way, the
resistance to lateral loading and sidesway. Such resistance can be provided
by bearing walls, shear walls or cores, truss or tubular systems [16]. It is
obviously better to provide a regular and symmetrical system of stiffening
walls [16], as otherwise lateral loading may also induce undesirable
torsional effects in the frames that are being assumed to resist vertical
loads only. The unbraced frame, where the building incorporates none of
these stiffening systems, has to be designed to resist both vertical and
lateral loads.
BS 8110 allows the moments, loads and shear forces in the individual