[TowerTalk] Tower base dimensions - why?

[Alan C. Zack]

Hank,
I bow to your knowledge and experience in these matters.  It certainly 
exceeds my knowledge.
I am just curious that if there is just a simple correlation 
comparison factor to converting from UBC-97 specs to IBC-2000 specs 
why can't U.S. Towers provide IBC-2000 spec 90 wind calcs to their 
customers?  I was on the phone and engaged on email with them for over 
two months.  I had a price quote and drawings to UBC-97, 70 MPH wind 
calcs for a HDX-555 crank up tower I wished to purchase from them.  I 
took their drawings to my Bldg Dept and was told they were fine, 
EXCEPT, they wanted 90 MPH wind calcs per IBC-2000.  I got back to UST 
and was told they have to out source the wind calcs to an outside P.E. 
because they don't have one on their staff.  They could get the wind 
calcs for me for a fee of $250.00.  I agreed and they contacted their 
P.E.  After many phone calls and emails I was finally told that NONE 
of their crank up towers would meet IBC-2000 90 MPH wind calcs.  I 
thought this to be strange but what could I do?  I shopped some more 
and both Heights Towers and Tashjian Towers Corporation were able to 
provide the IBC-2000 90 MPH wind calcs for their crank up towers no 
problem.
I was ready to order a $5000.00 plus tower package from UST if they 
could have provided my Bldg Dept the drawings they requested.  As it 
turned out Heights Towers got my order.
I then had my hole drilled IAW the drawings provided me by the P.E. at 
Heights towers that were then stamped by the Bldg Dept and my permit 
was issued.

Hank Lonberg wrote:

> 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
> 
> 
> 

-- 
__________________________________________________________________________ 

Alan Zack
Amateur Radio Station K7ACZ
Las Vegas, Nevada, USA
Quality Engineer, The Boeing Company, Retired
Aviation Chief Warrant Officer, U.S. Coast Guard, Retired
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