Civil Engineering Reference
In-Depth Information
In beams for buildings, it can usually be assumed that recommended limit-
ing span/depth ratios are sufficiently conservative to allow for shrinkage
deflections; but the designer should be alert for situations where the prob-
lem may be unusually severe (e.g., thick slabs on small steel beams,
electrically heated floors, and concrete mixes with high 'free shrinkage').
In EN 1994-1-1, the effects of shrinkage need not be considered when
the span/depth ratio of the beam is less than 20. For dry environments, typ-
ical values of the free shrinkage strain are given as 0.0325% for normal-
weight concrete and 0.05% for lightweight concrete.
Composite beams also deflect when the slab is colder than the steel
member. Such differential temperatures rarely occur in buildings, but are
important in beams for bridges.
3.9
Vibration of composite floor structures
In British Standard 6472,
Guide to evaluation of human exposure to
vibration in buildings
[35], the performance of a floor structure is consid-
ered to be satisfactory when the probability of annoyance to users of the
floor, or of complaints from them about interference with activities, is
low. There can be no simple specification of the dynamic properties that
would make a floor structure 'serviceable' in this respect, because the
local causes of vibration, the type of work done in the space concerned,
and the psychology of its users are all relevant.
An excellent guide to this complex subject is available [36]. It and BS
6472 provided much of the basis for the following introduction to vibra-
tion design, which is limited to the situation in the design example - a
typical floor of an office building, shown in Fig. 3.1.
Sources of vibration excitation
Vibration from external sources, such as highway or rail traffic, is rarely
severe enough to influence design. If it is, the building should be isolated
at foundation level.
Vibration from machinery in the building, such as lifts and travelling
cranes, should be isolated at or near its source. In the design of a floor
structure, it should be necessary to consider only sources of vibration on
or near that floor. Near gymnasia or dance floors, the effects of rhythmic
movement of groups of people can be troublesome; but in most buildings
only two situations need be considered:
•
people walking across a floor with a pace frequency of between
1.4 Hz and 2.5 Hz; and
•
an impulse, such as the effect of the fall of a heavy object.
