CRANES IN GENERAL
Tower cranes are less prone to blowing over than
what one could think by looking at them.
It is important to note that at wind speeds above 40
- 50 MPH, depending on the make and model, all hammerhead tower cranes
must cease operation and allowed to weathervane, i.e. left free to swing
with the wind. Because the front jib is much longer than the cw jib,
the jib will follow the wind direction, and the cw jib will point against
the wind, so the actual wind area is a lot smaller than the broadside
surface. The wind sees the jibs from the end, not the side.
Furthermore, tower cranes are substantially counter
weighted. A typical tower crane will with zero hook load have a
reverse moment which is equal to the forward moment when the crane has its
full design load at max radius. As a consequence the tower will be
in balance, i.e. there is zero moment, when the crane has half its full
design hook load at max radius.
The substantial reverse moment when out of operation
comes into helping the tower crane in high winds. It takes some wind
to overcome the reverse moment, it takes more wind to equal full load
moment, and it takes yet more wind to overcome the strength of the tower.
Tower cranes in the sizes used for building
construction are by European Codes required to withstand a wind speed of
100 MPH. When such cranes are at their maximum allowed free-standing
height, they are unlikely to withstand winds much above 100 MPH.
Winds above 100 MPH are, however, extremely rare. We are aware of
only one tower crane which went down solely because of high winds.
The K-10000 tower crane is most unlikely to
be toppled by high winds, because of its very large reverse moment and its
enormous tower strength.
It takes a wind speed of 115 MPH to cancel
out the reverse moment, i.e. in a wind speed of 115 MPH there is zero
moment at the tower base. Purely as a mental exercise, solely to
illustrate the point, one could unscrew the nuts of all the anchor bolts
securing the crane to the footing. They are not needed in a 115 MPH
wind; but they are needed as soon as the wind subsides, so don't try it.
It takes a wind speed of 175 MPH to cause a
moment at the base which is equivalent to the moment in operation with
max. design hook load at max radius.
Consequently it will take a wind speed
greater than 175 MPH to topple the K-10000.
Much of this numerical data can be visited
from the DETAIL page by selecting CONCRETE FOOTING and go to the bottom of
the page, then click on:
CRANE BASE REACTIONS - U.S. UNITS