Eccentric footings and combined footings 5

[pattontom]

In our office, colleagues often use load reduction to design footings with the columns directly on the exterior portion of the lot (zero clearance) to avoid oversize footings. This often results in underdesigned exterior footings. Now I need to design one that needs to use the full commercial live load and dead load for 4 storeys. The footings that come out of SAFE are about 50% bigger. The lot has 12 meters distance end to end and 3 columns spanning them (or 6 meters beam span). I'm thinking of using combined footings for the 3 columns in a row. But haven't seen much book references that treat such for most combined footings often use 2 columns only. Do you recommend that I use 50% bigger spread footings for the eccentric footing or one combined footing for the 3 columns? Which would have better seismic resistance, for those with experience of familiar with them? I understand the combined footing is more sensitive to variable changes in loading affecting the shears and moments in the span. Thank you.

 

[msquared48]

Something looks funny there. Is "13.8" the area of the footing?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[pattontom]

Actually we tried the first idea already with strip footing in grid X1 and X3. But the improvement is only 0.5 meter smaller width. So instead of 3 meter strip footing in sides. We have 2.5 meter. And there is still eccentricity in the exterior columns. I think the best is your main proposed horizontal combined footing plan where there are 3 horizontal combined footing in rear, middle and front.. Don't you think the width of the 3 horizontal combined footings is enough to resist the north-south lateral forces? The dead load of the front and back would be sufficient to resist any overturning, isn't it so why do you worry about the north-south direction. I wonder whether to add grade beams to all the footings to make the moments distribute. Or would combined footings (horizontal) be sufficient to do it??

 

[pattontom]

msquared, yes. 13.8 is the area of the footing of the left middle. See the original pic. Although the footing only supports load of 730 kn. The reason it is oversize is to handle the fact the column is put at the extreme exterior end with zero clearance. The oversize is to avoid the footing tilting and many bars top and bottom to distribute the moments to each area of the footing. So this is really feasible, isn't it. But our problem is different sizes of footings can cause differential settlement so we the combined footings where the left, mid and right footing is connected as one.

 

[hokie66]

pattontom,

Your calculation gives the ultimate moment/metre in the North-South direction.

My calculation was for the total service load moment for the East-West direction, where you have the footing cantilevered off one side of the column. The footing is 4.6 metres wide in that direction, but the column would have to take the same bending moment, so it won't work.

 

[pattontom]

hokie, if one uses very strong grade beam or tie beam above the footing and the below the slabs holding all the columns together. this can take care of the bending moments of the columns, isn't it. I wonder who use this technique and what is the advantage, disadvantage.. remember tie beam can redistribute moments of all columns.

anyway. if combined footings are used, it can redistribute the moments and tie beams not needed, right?

 

[BAretired]

Actually we tried the first idea already with strip footing in grid X1 and X3. But the improvement is only 0.5 meter smaller width. So instead of 3 meter strip footing in sides. We have 2.5 meter. And there is still eccentricity in the exterior columns.

Why do you need 2.5m width of strip footing? A load of 730 kN requires an area of 4.9 m2 which could be supplied by a strip footing approximately 1 m in width which boils down to an eccentricity of 250mm assuming a 500 wide column. Where do you come up with these numbers?

If you do not start looking seriously and intelligently at our suggestions, I am going to depart from this thread with one last recommendation...find someone in your area who knows what the hell he is doing because you clearly do not.


BA

 

[BAretired]

4.9 m2 above should read 4.9 m[sup]2[/sup].

BA

 

[pattontom]

The problem with 1 meter string footing at the sides is the end of the footing opposite to the columns can lift up, this is because we have to put the columns on the left side of 1 meter footing so the walls can be at edge most location. That is why 2.5 meter was calculated to decrease the lift up of the other side of the footing with heavily reinforcement moment bars.

Of course I'm seriously considering your suggestion of combined footings that is why I told the seniors engineers to go ahead do the design of 3 horizontal combined footings at back, middle and front and they say load combination and calculations can support it and it is stable. So thanks to all your useful suggestions. You guys convinced me combined footings is the way to go for completely eccentric column positions.

 

[rjenyoy12]

... I told the seniors engineers to go ahead do the design of 3 horizontal combined footings at back, middle and front and they say load combination and calculations can support it and it is stable.

I was just wondering why your senior engineers need to be told what to do? Aren't they experienced in doing this kind of work? Based on this thread, it seems you are designing a commercial building in your country that you are not capable of doing so. Sorry for the words. I hope that you find a good mentor to teach you not just relying on commercial softwares. A master's degree in Structural Engineering in a local university may also help.

 

[pattontom]

I'm just a new graduate and the building is supposed to be my first design. Also I happeedn to be the one assigned to communicate with the client. This is why with the project getting complicated. I let the seniors handle them and I'm the middleman or communication channel between what the client and architect wants and the senior engineers. They don't directly contact the client because they are working full time on structural design.

 

[hetgen]

You seem to get along with the client, ask him/her to buy a small land for centric foundation.

 

[johnbridge231]

You should also consider the effect of shear forces at the top of the footings to the total bending moment.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[wth]

When I've encountered difficult combined footing with eccentricity I've built a FEM-model of the foundation, on a spring area support to validate stability and allowed soil pressure. This also lets you see if it's stable for multiple load combinations..

Disclaimer: Didn't read the whole thread.

 

[civeng80]

Without reading the whole of the thread, I think its very important to do appropriate designs of eccentic footings. They can be very dangerous so consider combined or strap footings to suit.

 

[MJB315]

Oh.. he was referring to the biaxial bending moments of the columns.. of course, it's taken care of. We mostly use staad and etabs in getting the moments of the structures. We don't do manual calculations anymore here (so many structural engrs. even forgot the formulas). After we got the moments in the software. We enter them in either PCA columns, PCA beams, SAFE or other software to get the reinforcements required for shears and moments.

This is one of the most reckless assertions that I have read on these forums in sometime. I only skimmed this post, and I do not mean to preach or soapbox, but I think it's useful to address it.

Tom, as a self-described recent graduate, you need to recognize this statement as a huge red flag. If you pursue engineering as a career-- which will require dozens of calculations a day, for approx. 6500 days in your 25 year career-- you are guaranteed to make mistakes. If you follow the lead of your peer engineers who don't "do manual calculations anymore" and "forget the formulas", you are guaranteed not to catch them. And that's bad.

Strap footings, combined footings, moment frames, or whatever- these are just terms used to describe some thing that tries to balance some thing else. And balance is the same on that side of the Pacific as it is the other.

Equilibrium isn't a formula (even though you can develop formulas to describe equilibrium). So I would strongly suggest not following the path of your peers and forgetting-- it won't serve you well over time and in the end.

"We shape our buildings, thereafter they shape us." -WSC

 

[StruteK]

Use a Pile

 

[johnbridge231]

MJB315, your contibution was very useful.


Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[MJB315]

John,

I know- and it retrospect my post was a little too precious and heavy-handed. But after I read through the thread for the first time, I felt like ringing the bell.

Tom,

Sorry for the brusqueness of my past response. I could have been more helpful. But everything starts with the truth that: if you don't want your structure to move, it has to be in equilibrium. Sketching it out and putting a hand calculation to it will get you at least 90% of your way there. It's how the world was built.

I think that designing the right sized combined footing is the answer to your issue. I wouldn't try to resolve any unbalanced moments by trying to engage the anchor bolts and bending the column. It's too dicey in my opinion, especially when the foundation solution seems the most straight forward and time-tested.

"We shape our buildings, thereafter they shape us." -WSC

 

[Enhineyero]

hokie, if one uses very strong grade beam or tie beam above the footing and the below the slabs holding all the columns together. this can take care of the bending moments of the columns, isn't it. I wonder who use this technique and what is the advantage, disadvantage.. remember tie beam can redistribute moments of all columns.

anyway. if combined footings are used, it can redistribute the moments and tie beams not needed, right?

tom,

Using grade beams to take the lateral loads is common practice, it will provide additional lateral support at the same time it could carry your brick walls. for the columns I release the support moment parallel to where the beam will span, so if i have grade beams at both directions that will hit my column then that column will be pinned supported.

Anyway, it is always a good practice to do some hand calculations from time to time. And i usually prefer to do some hand calculations first before using a computer model to generate the complete calculations, that way i get to practice and at the same time check the computer generated results.

 

[ChrisMagadiaCom]

combined footing will have more resistance to seismic unless you tie the isolated footings with beams

chris magadia

http://www.chrismagadia.com

ChrisMagadia.Com - The Structural Engineers' Forum and Resources Website. Civilizations owe its existence to Structural Engineering. Do you Agree?