Civil Engineering Reference
In-Depth Information
contribution is proportional to
y
r
.
The laws of
equilibrium are unaware of the designer's discrimination between bars
labelled as 'web reinforcement' and those labelled as 'main
reinforcement'. Eqn (1.11) accepts any reinforcement bar (be it labelled
as web bar or main bar) provided it effectively helps to preserve the
integrity of the concrete web by restraining the propagation and widening
of the diagonal crack. It judges the contribution of an individual bar by its
area
A
r
, the depth
y
r
and the angle of intersection
θ
r
.
v)
As explained earlier (Kong, 1986b) 'Eqn 1.11 focuses attention on
the basic features of what in reality is a complex load-transfer
mechanism; it does this by deleting quantities which are less
important compared with the main elements—quantities whose
inclusion will obscure the designer's understanding of the problem at
the physical level. It is a useful tool in the hands of engineers who
possess a sound understanding of statics, geometry and structural
behaviour. Of course, the equation can be abused by indiscriminate
application—as indeed can Codes of Practice be so abused. Consider,
for example, a deep beam with a wide bottom flange, which contains
two large-diameter longitudinal bars away from the plane of the web.
These bars clearly do not effectively protect the integrity of the
concrete web, though they have a large product
A
r
y
r
sin
2
θ
r
; hence it
would be inappropriate to include such bars when using Eqn 1.11.'
vi)
The CIRIA Guide (1977) does not in effect cover
web openings,
unless
they are minor with little structural significance. Eqn (1.11), however, has
successfully been extended to deep beams with web openings; this has
been explained by Kong and Sharp (1977) and Kong
et al.
(1978). A
brief description of the method is also given in Reynolds and Steedman's
Reinforced Concrete Designer's Handbook
(1981 and 1988). For a
detailed discussion of web openings in deep beams, see Chapter 3.
1.5
Draft Eurocode and CEB-FIP Model Code
The (draft) Eurocode 2 (1984):
Common Unified Rules for Concrete
Structures
does not directly provide guidelines for the design of deep beams.
It refers instead to clauses 18.1.8 of the CEB-FIP Model Code (1978). The
CEB-FIP Model Code applies to simply supported beams of span/depth ratio
l/h
less than 2 and to continuous beams of
l/h
less than 2.5.
1.5.1
Flexural strength: simply supported deep beams
The area of the longitudinal reinforcement is calculated from the equation
(1.14)
A
s
=
M
/(
f
y
/
g
m
)
z
