USE OF TIE BEAMS (GROUND BEAMS) 1

[gotlboys]

I am designing a three-storey residential building where its rear side (at both corners) sit nearly on the property lines. A friend suggests using 'tie(ground) beams' connecting all the moment resisting frames. According to him, that way the moments both from seismic and gravity loads will be carried by the tie beams. Thus no moments are transferred down to footings and settlement are equal. Finally we can design those eccentric footings only for axial loads.

I am not sure if you are convinced with it. I need your advice and any helpful reference.

Thanks so much!

 

[KootK]

I agree with your colleague's recommendation. I'd be looking to make the grade beams stiff and sting enough to force hinges to form in the columns above the grade beams under seismic load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

I like my grade beams to have the equivalent to 12 hornet stings.

How are your other moment frames tied together? The loads are still transferred to the soil you will be stiffening up the foundation system. You should also have a grade beam which is perpendicular to the moment frame to control the eccentric moment.

 

[gotlboys]

Hi Kootk, I appreciate your advice. It would really be helpful if you could expand a bit of your idea for me to understand the mechanic of such analysis.

Sandman12, I attached the foundation plan where you can see how I planned to tie the moment resisting frames. Please advice if you have a better idea to account for eccentric loading.

I use manual calculation in spreadsheets. So, I use portal method to distribute lateral loads. By providing 'tie(grade) beams' as shown on the foundation plan, my portal analysis shows that I have no moment transferred to column below grade since there is no lateral force applied at ground level and thus the footing carries no moment. I am not sure if I have it correctly.
Moreover, I use subframes method for gravity loads.

 

[KootK]

It would really be helpful if you could expand a bit of your idea for me to understand the mechanic of such analysis.

I'll try. Under seismic load, your moment frame must eventually form a plastic mechanism. It can't do that unless a hinge is formed at the bottom of the columns. Otherwise, the mechanism would not be complete. If you don't provide the grade beam system, then your plastic hinges need to form at the footing level. And that means that, at minimum, your footings would need to resist the moment associated with the plastic hinge capacity of your columns.

On the other hand, if you provide a grade beam stiff enough to force the column hinges to form at the top of the grade beam rather than at the footing elevation, then those plastic hinge moments can be resisted by the grade beams instead of the footings. You'll still draw some bending to your footings even with the grade beams. It will be much less moment though and, most importantly, it won't be moment that you need to resist with the footing.

The sketch below shows what I'm envisioning for your system. I've drawn your tie beams very deep, like a basement wall. The shallower and less stiff your grade beam becomes, the less well the system will work to shield your footing from moments.

Like sandman, I'd also recommend perpendicular grade beams to help deal with that footing eccentricity. As it stands:

1) You'll struggle to make the grade beam/footing joint work and;

2) You may need to tie the slab on grade into the top of the grade beam to stabilize the sytem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

Your plan is what I would expect for moment frame columns along a property line footing. A couple of thoughts regarding the details, with the tie beam at slab level and the columns extending below the slab you are going to get backstory effects. The tie beams and column below the slab will act like a small moment frame and induce shear on the footings. I would setup my grade beam similar to what KootK has drawn and have them at the level of bearing and design the columns including that height.

 

[gotlboys]

Thank you Kootk and Sandman21. Your thoughts really help. I just found a reference dealing with tie beams that are used to create plastic hinges either at the column bottom or tie beam itself, however no detailing examples shown.

How would the column reinforcements be detailed if we wish to let plastic hinges occur just at the top of tie beams?

 

[KootK]

Can you share the name of this reference? I'd like to check it out myself. Regarding the detailing, you'll probably want to:

1) capacity design the tie beams and beam column joints for the overstrength flexural capacity of the columns. Pay extra attention to the detailing of the joints at the ends of the tie beam runs.

2) capacity design the shear capacity of the columns for the overstrength flexural capacity of the columns and rely only on the column ties for column shear capacity.

3) Detail your column ties to promote ductility with tight spacing and 135 degree seismic hooks rather than 90 degree hooks.

4) Keep your axial load to axial capacity ratio low for the columns.

5) Keep any non-mechanical vertical column bar spices out of the column plastic hinge zone.

6) Use ductile rebar for the vertical column bars in the column plastic hinge zone.

Other than #1, this is all typical special moment frame column to footing detailing. I'd recommend reviewing the relevant ACI requirements for that as I've probably missed some things.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[gotlboys]

Thank you for such a helpful advice. I am going to see ACI for more information on detailing. It's 'seismic design of reinforced concrete by Thomas Paulay'.