Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL THEORY 5.13
In ASCE-7-95 and 98, the basic wind speed changed from fast mile wind to 3second
gust wind speed in miles per hour. The wind speed values on the basic
wind speed map has changed. This change should not have any big impact on the
wind pressure. However, confusion is easily created because all the major building
codes including the IBC 2000 are still using old basic wind speed map based on
fast mile wind, and they repeatedly refer to ASCE-7 95 or 98. It is to be noted that
the reference from the building codes to the ASCE-7 are either adoption of ASCE-
7 as an alternative approach or for certain factors that are not related to the basic
wind pressure.
In ASCE-7-95 and 98, new factors such as wind directionality factor, topographic
factor were introduced, and gust effect factors were updated for rigid structures
as well as for flexible/dynamically sensitive structures. The calculation became
much more complicated than the approach in this book and the results should
be more accurate. We suggest that for complicated structures it is necessary to use
ASCE-7-98 method to check the results.
Snow, Ice, and Rain Loads. These, in effect, are nonuniformly distributed, vertical,
live loads that are imposed by nature and hence are generally uncertain in
magnitude and duration. They may occur alone or in combination. Design snow
loads preferably should be determined for the site of the proposed building with
the advice of meteorologists and application of extreme-value statistical analysis to
rain and snow records for the locality.
Rain loads depend on drainage and may become large enough to cause roof
failure when drainage is blocked (see Art. 3.4.3).
Ice loads are created when snow melts, then freezes, or when rain follows a
snow storm and freezes. These loads should be considered in determining the design
snow load. Snow loads may consist of pure snow or a mixture of snow, ice, and
water.
Design snow loads on roofs may be assumed to be proportional to the maximum
ground snow load p g, lb/ft 2 , measured in the vicinity of the building with a 50year
mean recurrence interval. Determination of the constant of proportionality
should take into account:
1. Appropriate mean recurrence interval.
2. Roof exposure. Wind may blow snow off the roof or onto the roof from nearby
higher roofs or create nonuniform distribution of snow.
3. Roof thermal conditions. Heat escaping through the roof melts the snow. If the
water can drain off, the snow load decreases. Also, for sloped roofs, if they are
warm, there is a tendency for snow to slide off. Insulated roofs, however, restrict
heat loss from the interior and therefore are subjected to larger snow loads.
4. Type of occupancy and uses of building. More conservative loading should be
used for public-assembly buildings, because of the risk of great loss of life and
injury to occupants if overloads should cause the roof to collapse.
5. Roof slope. The steeper a roof, the greater is the likelihood of good drainage
and that show will slide off.
In addition, roof design should take into account not only the design snow load
uniformly distributed over the whole roof area but also possible unbalanced loading.
Snow may be blown off part of the roof, and snow drifts may pile up over a portion
of the roof.