Intermediate Ties in Short Footing Pedestals

[tchiggins]

thread507-168397

I have a question of whether or not intermediate lateral ties are required for square foundation pedestals (30" sq. x 4' ht.). I have referenced a closed thread above because it's sort of a similar question. Our office is split on the subject - some of us feel that the true purpose for intermediate ties in columns per ACI isn't realized in the type of pedestals we usually see.

My particular footing:
24" dia. drilled pier
30"x30"x4' ht. pier cap, 18" above grade, 30" below grade
Supports a W12x65 T-post, 25 ft. tall with a fixed-base-plate(independant cantilever column).
Base Plate loads - 8 kips vertical and a 50 k ft moment.
Seismic design category C, which is due to being in a Site Class E.
Reinf. in Cap: 12-#5 Vert.@ 8" sp., #4 Ties.

Does anyone feel that ACI318 directly applies to this question? Ch.22 refers to plain concrete pedestal being allowed for Horiz/Vert ratio less than 3. So, no intermediate ties needed?

 

[JAE]

I always use ties in short pedestals. There could still be bursting stresses. Also, short pedestals many times have anchor bolts and I like the fact that the ties are there to resist App. D concrete conical failures to some degree.

 

[msquared48]

Why would you not put confinement steel in what is really a concrete column? If it seel compression, confine it.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vandede427]

you said the base plate had moment, but you didn't list a shear. unless i've missing something, you'll have a shear on that base plate if moment is present. this didn't really partain to your question, and the concrete shear strength alone is probably 100x the applied shear.

just put the ties in. it's good measure. no contractor will ever complain about them. and with a 30"x30" dimension, you'll have no trouble fitting them in.

if you still have serious heartburn about it, maybe you could analogeously (is that a word) justify providing half the ties of what is required for a true column seeing as how pedastal's are only required to have half the vert reinf ratio of a true column with the same dimensions (vert reinf ratio of 0.005).

but seriously, just use the ties.

 

[asixth]

It is possible for no shear to occur, if the 8kips is at 6ft eccentricity without any lateral loads on the cantilever. However I expect that for some load case shear force will be present.

What I have done in the past is create the reinforcement cage from U-bars each way top and bottom and on the sides. This eliminates the need for ties.

Attached is the reinforcement cage and drilled pier for a light tower approximately 35' high. The pier cap is a bit larger than the one you described in your posts.

 

[ToadJones]

I would apply ACI 7.10 and/or 7.11.
Compression could be due to axial load or flexural compression.

 

[tchiggins]

Thank you for your replies. I can understand the majority opinion, interm. ties should simply be included as "good practice", and as ACI318 requires it in Chapter 7.

There will be a shear load at the top of the pedestal from temperature, wind and/or seismic, but less than 2 kips. In other jobs, our lateral loads will be as much as the vertical, and sometimes we have uplift with laterals in the double figures.

I thought however that there may be a section in some code that covers this subject specifically for foundations. I have cases when these intermediate ties interfer with the Anchor Rods. The pedestals are usually sized for accomodating the base plates, and not from the strength calcs. So they are usually bigger than what is required for strength.

What do you think if it were a rectangular pedestal supporting equipment saddles? Just for example - a 14" x 8'w x 5' ht., would you think of it as a wall, sect 14.5.2, would you use intermediate ties?

 

[ToadJones]

The way I look at it is, on most pedestals there is a shear force at the anchorage (baseplate etc.) that induces bending in the pedestal, which creates flexural compression and a need for ties.

 

[tchiggins]

I just realized after re-reading through my initial post, I didn't mention that I'm refering to "NON-CORNER" bars. That's what I meant by "intermediate" ties.

I'm asking about the requirements of tieing longitudinal bars that are more than 6" away from the corners or not adjacent to a tied bar. Sorry if I was too obscure, hopefully all understood my question.

 

[ToadJones]

From what I gather about ties in walls is the reason the provisions are different is because in many walls there simply is no room for tie steel. If there is room, I'd use ties.
Also, Even if you are treating the pedestal as a wall and the vertical reinforcing is more than 0.01, ties are required.
Our foundations contractor often prefers the ties as they sometimes actually aid in tying up the steel and help to keep it in place.
Personally, if they don't interfere with the anchors or anchor
plates that we often use, they may actually help to contain the anchors, which is also nice.

 

[KootK]

Based on your revised question:

In your particular situation, I suspect that the only important function performed by the intermediate ties is providing lateral support to longitudinal compression steel that might otherwise buckle.

Were you so inclined, you could calculate the maximum strain that would occur in your compression steel. Using that value you could work out the acceptable unbraced length of the compression steel. If it's greater than 4', you're good to go. Since your "column" is very short and probably significanly oversized, this may well be the case.

Of course, you'd be completely nuts to go to all that trouble. Just include the ties. I like to draw pier details in plan with any anchor bolts included. You can usually work something out to avoid the interference problems. It's usually the hooks that screw me up.