ACI 318-11 tension only grade beam

[nirias]

I need tension ties between footings of a pre-engineered metal building to resist outward forces of the portal frame's columns. Because these are reinforced concrete elements tying footings, they would seem to inescapably fall under "grade beam" requirements in section 21.12.3 of ACI 318, with specific requirements in 21.12.3.2 for minimum dimensions and tie spacing. For the structure under consideration, the minimum dimensions (18" x 18") aren't an issue but I would need ties every 9" when there is no obvious load that they would resist.

The code commentary looks like an improvised response after the dog ate the homework: "The cross sectional limitation and minimum tie requirements provide reasonable proportions".

Does anyone have any insights into the purpose of the ties? Given the nature of metal building column reactions, I cant imagine a scenario where these tension tie/grade beams are loaded in shear or axial compression prior to catastrophic failure of the structure in some unexpected manner (such as column being struck by a very heavy vehicle).

 

[JoshPlumSE]

The shear ties at 1/2 the smallest dimension is a basic shear design concept. It's almost like saying minimum shear ties are always required for grade beams, regardless of the actual shear demand forces.

To me there is a difference between continuous footings (for wood walls or such) and grade beams. In that grade beams are expected to take moment and shear. As such they require shear ties. For continuous footings, that would not be the case.... unless the shear force in the footing is greater than 50% Phi*Vn.

 

[dik]

Can you do this with 'hairpins' taking the load back into the slab?

 

[KootK]

The main seismic concern related to grade beams is their function in acting as tension and compression bracing to stabilize the bottoms of your columns. Given that your column loads are probably pretty light and that they'll likely always be producing tie tension (no need for confinement or longitudinal bar buckling restraint), I'd proceed on the the path of not calling these grade beams. Concrete tension tie members have their own unique requirements such as only being able to use mechanical bar splicing rather than laps etc. That wasn't your question though so I'll assume that you're already up to speed on that stuff.

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.

 

[JAE]

Thread hijack alert: KootK - did you get your joist chord question answered? I began to post but it disappeared.

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[KootK]

@JAE: I did indeed get it answered by the joist guys 30sec after my post. 1.5x1.5x0.125. Was confused by optics and my own failure to recognize that the area was for the angle pair. Thanks just the same though. You may well see the same info pop up again under the guise of "how the hell do I fix this".

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.

 

[JAE]

Reminded me of this:

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[nirias]

Thanks for the replies.

I considered calling these tension ties but was trying to avoid the cost of the mechanical splices. It is a 50 foot span and it would be difficult to get >20 ft rebar to the site - so that means lots of those mechanical splice sleeves.

I didnt consider hairpins into the slab because the portal span is 50 ft and the slab will not be post tensioned. With saw cuts and dowels at the cut lines it cant carry the tensile force.

I guess I was hoping for some code exception I had overlooked that would get me away from mechanical splices AND large numbers of closed ties. I will cost out the mechanical splices vs the closed ties and associated labor and take the cheapest path.

 

[dik]

Can you obtain long bars? In these environs, bars are typically rolled in 18m lengths.

I've not postensioned a slab using hairpins... and much greater spans.

Dik

 

[nirias]

It is a difficult site, in the middle of the forest. 20 footers can be dropped at the site. 40 footers could be brought within a couple hundred feet by truck. I'm pretty sure 60 footers would require helicopter delivery for the last mile.

 

[KootK]

I guess I was hoping for some code exception I had overlooked that would get me away from mechanical splices AND large numbers of closed ties. I will cost out the mechanical splices vs the closed ties and associated labor and take the cheapest path.

I generally try to be more diplomatic than I'm about to be but I fear that you're heading down a dangerous path here. The member that you've described absolutely is a mission critical tension tie. Code compliance and public safety demand the use of couplers regardless of the outcome of your pricing exercise. I'd seek to omit the ties which, in my opinion, are not required here.

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.

 

[nirias]

Lack of diplomacy is totally cool if it means helping someone avoid a mistake. In any case, the pricing exercise has already led to use of couplers.

My original question regarded the reasoning behind the code and whether I had missed something, since the code commentary did not shed any light. I am still curious as to why the code requires ties for a tension only grade beam - what credible failure mode do the ties address? And I am curious about where the code requires couplers in tension ties that require less than all bars be fully utilized (in this case only 2 of 8 rebars in the cross section would needed to carry stress from seismic loads.

If you are wondering why only 2 of 8 bars might be needed, it is because the ties are sized for strain under full snow loads (200 psf) rather than just the computed seismic loads. I think the code is a bit unconservative in the handling of snow loads during earthquakes by assuming that nearly all of the snow just falls off the roof.

 

[dik]

The ties resist the horizontal outward thrust of the rigid frames.

Dik

 

[nirias]

Sorry about the imprecise "ties" terminology. I was referring to the closed ties that the code requires on anything that is called a grade beam.