Checking a footing for a steel tower

[Robbiee]

A 160 ft self supporting tower with three legs is shown in the attached sketch. The unfactored forces are due to wind and ice. A 16’x16’x24” footing was provided with #8 bars at 12-1/2” O.C each way bottom only. Don’t you think this pad needs top reinforcing?

 

[hokie66]

Yes, without doubt.

 

[abusementpark]

Yes, based on the forces you have shown, the footing will certainly see negative moment.

 

[Ron]

Ditto. Wind reversal.

 

[msquared48]

Yes, just in order to pick up the soil over the footing to resist the OTM.

Mike McCann
MMC Engineering

 

[Robbiee]

Thanks all. Now, I had absolutely no doubt that it needs top reinforcing, but wanted to use this case to ask the following question: How often a senior engineer misses basic things like this one?

 

[BAretired]

Depends on the senior engineer and how long a time frame we are talking about. But it probably happens more often than we care to admit.

BA

 

[Ron]

It happens, though this should be obvious to an experienced engineer. Is it negligence or lack of technical competence? Neither is good for the engineer or the company, in the event of failure.

This is a good example of why it is necessary to have another engineer review the initial work. It's basic Quality Assurance.

 

[DaveAtkins]

I am not convinced it needs top reinforcing. The ultimate unreinforced capacity of a 24" thick footing is 14 kf per foot width, assuming 4000 psi concrete (I assumed phi = 0.55, which I think is correct). That may be enough to support the overburden during overturning, but a computer model of the mat should be run to be sure. Also remember this is not a point load at the middle of the footing, but three point loads. This will reduce the bending in the footing.

DaveAtkins

 

[Robbiee]

Dave,
The Canadian concrete standard A23.3-04 doesn't allow using the concrete tensile strength to resist bending. However, I checked to see if Factored Bending moment is smaller than (Mcr/1.2), in which case I would be comfortable without top rebars, but it wasn't the case.

 

[csd72]

I would put in top reinforcing.

A general rule:
If the footing mainly resists wind overturning then top and bottom reo.
If the footing mainly resists downward load from gravity then bottom reo should be okay.

By the way, you have not shown the horizontal reaction, those pedestals over the footing will be cantilevers in bending.

 

[Robbiee]

csd72,
Yes, Thanks you. There is 6.7 K horizontal reaction at each pier.