Civil Engineering Reference
In-Depth Information
which can govern the design where unpropped construction is used. Floor
structures subjected to dynamic loading (e.g., as in a dance hall or gymna-
sium) are also susceptible to excessive vibration (Section 3.11.3.2).
The width of cracks in concrete needs to be controlled in web-encased
beams, and in hogging regions of continuous beams (Section 4.2.5).
Excessive stress in service is not itself a limit state. It may however
invalidate a method of analysis (e.g., linear-elastic theory) that would
otherwise be suitable for checking compliance with a serviceability criter-
ion. No stress limits are specified in EN 1994-1-1. Where elastic analysis
is used, with appropriate allowance for shear lag and creep, the policy is
to modify the results, where necessary, to allow for yielding of steel and,
where partial shear connection is used, for excessive slip.
If yielding of structural steel occurs in service, in a simply-supported
composite beam for a building, it will be in the bottom flange, near mid-
span. The likelihood of this depends on the ratio between the character-
istic variable and permanent loads, given by
r
=
q
k
/
g
k
on the partial safety factors used for both actions and materials, on the
method of construction used, and on the ratio of the design resistance to
bending for ultimate limit states to the yield moment, which is
Z
=
M
pl,Rd
/
M
el,Rk
(3.82)
where
M
el,Rk
is the bending moment at which yield of steel first occurs.
For sagging bending, and assuming
γ
A
for steel is 1.0, the ratio
Z
is
typically between 1.25 and 1.35 for propped construction, but can rise to
1.45 or above, for unpropped construction.
Deflections are usually checked for the characteristic combination of
actions, given in Equation 1.8. So, for a beam designed for distributed
loads
g
k
and
q
k
only, the ratio of design bending moments (ultimate/
serviceability) is
.
135
g q
gq
.
+
+
15
. .
135
+
+
15
r
µ
=
=
(3.83)
1
r
This ratio ranges from 1.42 at
r
0.8 to 1.45 at
r =
2.0
.
From these expressions, the stress in steel in service will reach or exceed
the yield stress if
Z
=
. The values given above show that this is unlikely
for propped construction, but could occur for unpropped construction.
Where the bending resistance of a composite section is governed by
local buckling, as in a Class 3 section, elastic section analysis is used for
>
µ
