Stability Check for Penstock Pedestal Support 1

[aakalim152]

Hi.

I am new to this forum and this is my first post/questions.

We are basically in the process of designing reinforced concrete pedestal or saddle support for a penstock.

I have almost completed the design including the checks for sliding, stability and bearing pressure (with more than adequate factors of safety). But our Senior Engineer suggested that i should also include the check for resultant force lying between middle third of the base.

Now in order to satisfy this check I will have to increase the size of the pedestal by quite a bit. The question is that is this check really necessary? I mean i am already getting adequate FOS for overturning (2.6) and also sliding (22). So what if i do not include this check and how can i justify it.

Also please clarify if this check is only for vertical forces or we have to include horizontal forces as well because when we include horizontal forces, the resultant force acts at an angle which takes it completely out of the middle third of the base.

Sorry for such a long and such a basic question.
Any answers/views would be really appreciated.
Regards,

 

[cvg]

bearing on rock or soil?

 

[aakalim152]

@cvg
Its bearing on rock.

 

[JoshPlumSE]

What load combinations did you use for calculating your factor of safety for overturning? Did you use the 0.9DL or the 0.6 DL?

I've seen a number of projects have this as a design requirement. But, I personally feel it would be overkill when combined with the 0.6 DL factors.

 

[aakalim152]

Well basically i haven't used any load combinations or load factors in my calculations.

But I have checked that taking 0.6D gives a factor of safety less than 2 (1.83 to be precise). So i guess i will have to satisfy the check for resultant force.

 

[JStephen]

I think the middle-third requirement is to maintain positive bearing over the whole bearing surface, which may or may not be a requirement in a particular case.

The 0.6DL is from ASCE 7, and may or may not be applicable to your project. You wouldn't normally apply a factor of safety over and above the 0.6DL overturn check, though.

 

[cvg]

high pressures at the toe can cause bearing failure in soil , less likely in rock. keeping eccentricity within the kern will minimize the pressure at the toe. However, the standard of care is generally to keep it in the middle third.

 

[msquared48]

aakalim152

"So what if I do not include this check and how can I justify it."

I agree with the others that you need to check the location of the resultant, especially if your boss told you to do just that. Your question above concerns me with the "so what" statement. If that persists, eventually it will get you in trouble, if not fired. DO not try to second guess your boss's authorithy in the office. It is OK to ask questions though, always. Just watch how you phrase it. You might be misread. I am probably one of those.

Just trying to help. Cheers.



Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

Our standard is to keep the resultant withing the middle 1/3 of the footing. We will only allow the resultant to fall outside of the middle 1/3 for what we consider extreme short term loading conditions (0.6D + 0.7E).

If you are concerned about the size of your base then maybe you should express your concerns to your boss. I am sure he/she is well aware of how the numbers work and what this requested change will mean to the size of your base.

 

[CANPRO]

I've wondered about this many times...if your factors of safety are all working out, and your bearing pressure at the toe is within the allowable..why can't you just add top steel to the footing and make sure it works when the footing goes into negative bending?

 

[hokie66]

Keeping the resultant within the kern just assures a trapezoidal stress distribution. There is nothing inherently wrong with a triangular distribution if the overturning, sliding, and maximum stress limits are observed.

 

[aakalim152]

Thank you all for you prompt and useful replies.

I finally increased the size of the pedestal which satisfied the middle third requirement.

Regards,

 

[DST148]

aakalim152: hokie66 has already answered your original post to the point. SteelPE's suggestion is what we normally follow in our office. Keep the resultant within the middle third for sustained loading. It could fall outside the middle third for short term loading.