Introduction - Design of Reinforced Concrete - page 32

tends to slide off sloped roofs, particularly those with metal or slate surfaces. A
load of approximately 10 psf might be used for 45

slopes, and a 40-psf load
might be used for flat roofs. Studies of snowfall records in areas with severe win-
ters may indicate the occurrence of snow loads much greater than 40 psf, with val-
ues as high as 80 psf in northern Maine.
Snow is a variable load, which may cover an entire roof or only part of it.
There may be drifts against walls or buildup in valleys or between parapets. Snow
may slide off one roof and onto a lower one. The wind may blow it off one side of
a sloping roof, or the snow may crust over and remain in position even during
very heavy winds. The snow loads that are applied to a structure are dependent
upon many factors, including geographic location, the pitch of the roof, sheltering,
and the shape of the roof.
2. Rain. Although snow loads are a more severe problem than rain loads for the
usual roof, the situation may be reversed for flat roofs—particularly those in
warmer climates. If water on a flat roof accumulates faster than it runs off, the re-
sult is called ponding because the increased load causes the roof to deflect into a
dish shape that can hold more water, which causes greater deflections, and so on.
This process continues until equilibrium is reached or until collapse occurs. Pond-
ing is a serious matter, as illustrated by the large number of flat-roof failures that
occur due to ponding every year in the United States. It has been claimed that al-
most 50% of the lawsuits faced by building designers are concerned with roofing
systems. 20 Ponding is one of the common subjects of such litigation.
3. Wind. A survey of engineering literature for the past 150 years reveals many refer-
ences to structural failures caused by wind. Perhaps the most infamous of these
have been bridge failures such as those of the Tay Bridge in Scotland in 1879
(which caused the deaths of 75 persons) and the Tacoma Narrows Bridge
(Tacoma, Washington) in 1940. But there have also been some disastrous building
failures due to wind during the same period, such as that of the Union Carbide
Building in Toronto in 1958. It is important to realize that a large percentage of
building failures due to wind have occurred during their erection. 21
A great deal of research has been conducted in recent years on the subject of
wind loads. Nevertheless, a great deal more study is needed as the estimation of
wind forces can by no means be classified as an exact science. The magnitude and
duration of wind loads vary with geographical locations, the heights of structures
above ground, the types of terrain around the structures, the proximity of other
buildings, and the character of the wind itself.
Section 6 of the ASCE 7-02 specification provides a rather lengthy procedure
for estimating the wind pressures applied to buildings. The procedure involves
several factors with which we attempt to account for the terrain around the build-
ing, the importance of the building regarding human life and welfare, and of
course the wind speed at the building site. Although use of the equations is rather

20 Van Ryzin, Gary, 1980, “Roof Design: Avoid Ponding by Sloping to Drain,” Civil Engineering (New York:
ASCE, January), pp. 77-81.
21 Task Committee on Wind Forces, Committee on Loads and Stresses, Structural Division, ASCE, 1961, “Wind
Forces on Structures,” Final Report, Transactions ASCE 126, Part II, pp. 1124-1125.