[TowerTalk] Tower base dimensions - why?

[Hank Lonberg]

T'Talkers and Jim:

Let's get some fundamentals out of the way first.

In regards to design wind velocities: The IBC uses V3sec, Velocity of a 3
sec gust; the UBC uses the fastest mile wind velocity Vfm.

The correlation is:

Vfm          V3sec
70             85
80            100
90            110
100           120
110           130

All in mph.

Foundations:

I am speaking to free-standing(cantilevered) towers here.

There are two common philosophies in regards to founding a cantilever type
structure. The first is the embedded post concept, think of a fence post.
The second is the shallow spread foundation aka mat and other names.

The post type foundation supports the tower by using both the vertical and
lateral strength of the soil it is placed in. The plan area resists the
vertical load of the tower structure and the depth it is place in the ground
resists the overturning moment and lateral force cause by the wind or
earthquake. This type is typically small in plan dimensions and large in its
embedment depth.

The spread footing (mat) supports the tower weight and overturning forces by
its large area. It resists the overturning moment by its size; it will try
to rotate about its center of gravity as the overturning moment is applied.
If it is sized correctly it will resist the overturning moment by the soil
resistance it develops at the bottom of the foundation. As long as the
vertical soil pressure is less than the allowable, all is well. The lateral
force is resisted by a combination of the friction between the bottom of the
foundation and the soil plus by the depth the foundation is in the ground.

As far as which is better, that's a project specific call. In general the
post-type foundation will use a little less concrete but require a
significantly deeper hole. This can lead to excavation difficulties with
small plan area and large embedment depth. The spread footing is easier to
excavate due to it being shallower but it has large plan dimensions. This
makes for a larger area of concrete which can be visually unattractive.

In practical usage the amount of concrete used for either is the same give
or take a few cubic yards.

In regard to the frost depth of a foundation. The frost depth in the US
varies from 8" to excess of 48" deep per the codes (UBC/IBC, others).
Depending  on where you live the bottom of the footing should equal or
exceed the frost depth for that area; in order to minimize any frost heave
dangers. Those in FL should cheer!

Regards and 73

Lonberg Design Group, Ltd.

Hank Lonberg, P.E.,S.E. / KR7X
President