Civil Engineering Reference
In-Depth Information
functions with sensitive movement or vibration requirements
need to be understood. Often the structural issues arising can
be avoided by reorganising the building planning at an early
design phase, but sometimes there are other design issues, such
as restricted sites and required adjacencies, that will constrain
the overall design.
By way of an example, consider hotels, as shown in
Figure 7.5
.
The majority of the superstructure contains bedrooms which
have a regular, short-span rhythm of floor heights, walls, service
risers and corridors, easily allowing close-spaced vertical struc-
ture. However, hotels also need open function and dining spaces.
Thinking 'mono-disciplinarily', structurally it would be logical
to place these at the top of the building to avoid the columns
and risers from the bedrooms needing to be supported over them
by transfer structures. However, function rooms and restau-
rants contain large numbers of people who need access and fire
escape provisions. If placed at the top of the building the large
vertical circulation required would impact the design through its
whole height. As a result, these spaces are better placed close to
the ground. If space allows, they can be placed outside the bed-
room block's footprint, but that requires a sufficiently large area,
which is not often the case for urban sites.
There is also a trend worldwide to place hotels at the top
of tall multiple-use towers. The views from the rooms are the
best, the prestige the highest and hence the price that can be
charged increases. Physically the number of lifts to serve a
hotel is much lower than the number to serve office buildings,
where everyone has to go out to lunch during the same hour.
Placing hotels above offices reduces the size of the lift core
over much of the height of the building and hence increases the
amount of usable space.
This is efficient for the overall design but conflicts with a
key structural concern. Humans are most sensitive to the sway
of building when lying down and at rest as they often are in
hotels. The allowable accelerations from building sway are
much more restrictive for a hotel than an office. Thinking only
structurally, the top of a tower is the worst place for a hotel,
but increasingly they are put there. Thus, the desirable solu-
tion for the project as a whole may require the engineer to
design a much stiffer lateral stability system than the structural
optimum.
However, a key issue to strive for is having as many columns
as possible to run through the building from top to bottom.
Indeed, many seismic codes preclude discontinuous columns
without stringent studies and penalising precautions. A slight
deviation in the position of a column between floors can cause
very high local forces requiring careful detailing and restraint
forces from other members. A more significant displacement,
or indeed the deletion of a supporting column, will require a
transfer structure which is usually a member significantly dif-
ferent from those around it that requires special design and
construction consideration.
Changes in building function between various levels and in
particular the requirements of vehicle circulation at the base of
Figure 7.5
The structural engineer's and architect's preferred mix of hotel space
