Equipment Pedestal - Elongated Pedestal or Wall?

[Point_Load]

I have had a discussion come up a few times and have yet to come to a clear consensus between engineers on the correct approach for designing elongated pedestals to support equipment. I’m curious if anyone here has dug into this and could help clarify the best approach for this type of design problem. All the references below refer to section in ACI 318-14 unless noted otherwise.

For the purpose of having a reference, and a situation where I’ve seen this before, I’ll imagine I need to support a horizontal vessels with a saddle support about 7’-6” in length. For this, assume the pier supporting the saddle will be 8’ long x 1’6” wide x 4’ tall. The saddle bears continuously across the pier so the gravity loads will be, for simplicity, evenly distributed over the pier’s cross-section.

Initially, my first instinct is to jump into ACI Chapter 10 which includes provisions for the design of columns and reinforced concrete pedestals. Section 10.6.1.1 requires an area of longitudinal steel to be a minimum of 0.01Ag unless the gross area of the pedestal is larger than necessary to resist the loads in which case the minimum area can be reduced to 0.005Ag per 10.3.1.2 which is common for these types of structures. Finally, 10.7.3.1 requires the longitudinal reinforcement be at least (4) longitudinal bars within rectangular ties.

So based on this I would need at least 0.005Ag longitudinal steel which would result in several vertical bars spaced along each face of the pier within a rectangular tie. The additional transverse reinforcement requirements would then kick in per 10.7.6.1.2 which leads to 25.7.2.3. 10.7.6.1.5 goes on to say all longitudinal reinforcement shall be laterally supported with ties unless analysis shows otherwise. Ultimately to limit going into additional detail the pedestal would need to have several vertical bars with a rectangular tie around the perimeter, ties providing confinement at the anchor bolts per 10.7.6.1.6, and interior cross-ties to satisfy the tie spacing requirements for laterally supporting vertical bars per 25.7.2.

Looking through the code, it appears to me that these types of support structures in many cases could qualify as a wall which might be designed without the perimeter and cross-ties to support longitudinal reinforcement in the wall. “Wall is defined in ACI as a vertical element designed to resist axial load, lateral load, or both, with a horizontal length-to-thickness ratio greater than 3, used to enclose or separate spaces. So the definition with the exception of enclosing/separating a space is met. If we then went to Chapter 11, we could find that the minimum longitudinal reinforcement could be as low as 0.0012 from Table 11.6.1 if the axial loads are below the calculated Pn value. This chapter goes on to state in 11.7.4.1 (errata 9th printing) if lateral reinforcement is required for compression and if Ast exceeds 0.01Ag, longitudinal reinforcement shall be laterally supported by transverse ties. Since we would not meet this criteria the addition of lateral ties per 25.7.2 wouldn’t be required. So from this approach you could have quite a bit less required reinforcement in the wall since the area of longitudinal steel would be less, the perimeter tie is not called out and the cross-ties are not required. I would still assume the ties for anchor confinement would need to be provided per 10.7.6.1.6.

My approach has always been to treat this as a column designed to Chapter 10. Any thoughts from anyone here on reasons the second approach can/should not be followed?

 

[r13]

The pedestal is a compression member, but not a column. I would design it as a wall, but check reinforcement requirement for deep members too, for better crack control.

 

[Point_Load]

So I guess this means you'd have to rely on the chapter 2 definitions and lean on the ratio for length to thickness to determine if you have a column or wall - thus leading to your minimum reinforcement requirements?

column—member, usually vertical or predominantly
vertical, used primarily to support axial compressive load,
but that can also resist moment, shear, or torsion. Columns
used as part of a lateral-force-resisting system resist
combined axial load, moment, and shear. See also moment
frame.

wall—a vertical element designed to resist axial load,
lateral load, or both, with a horizontal [highlight #FCE94F]length-to-thickness
ratio greater than 3[/highlight], used to enclose or separate spaces.

 

[r13]

I probably will add "a structural member with length much greater than cross sectional dimensions" to the definition of "column", and the word "usually" before "used to enclose or separate spaces", for the inclusive of large pedestals.

But, no matter how good the wording is, engineering judgement must be applied when special condition arise, such as, if there is large later load applied to the long face of the pedestal, then you have to design it as a cantilever beam-column rather than wall, and reinforced as such.

 

[dauwerda]

For what its worth, my previous firm (myself included) always designed these types of foundations as walls for the reasons you outline above. I believe that it is well within the code definitions as well as the performance/strength requirements to do so.

 

[dauwerda]

To follow up on my previous post, here is what PIP STE03360 - Heat Exchanger and Horizontal Vessel Foundation Design Guide has to say about it:

4.7.3.1 Piers should normally be designed as tension-controlled members
(cantilever beams) with two layers of reinforcement. If the pier is a
compression-controlled member, the pier should be designed as a
column.

4.7.3.3 Minimum reinforcement for piers is #5 at 12 inches on each face
with #4 ties at 12 inches. A minimum of two #4 ties (or three ties if
moderate or high seismic risk) should be placed within 6 inches of
the top of concrete of each pier (not including grout) to protect
anchor bolts. All ties should encircle the vertical reinforcement,
unless special tie reinforcement for boundary elements is required.

4.7.3.4 For tension-controlled piers, as is normally the case, intermediate ties
are not required.

 

[Point_Load]

That's a good find from PIP, dauwerda. I remember reading those sections at some point in the past after seeing it again but it hadn't come to mind when writing the original post. I should have thought to look back through the PIP standards. They often have good direction and guidance for some of the more oddball cases. Appreciate the feedback.

 

[dhengr]

Point_Load:
You are trying too hard to find a perfectly worded code para. which exactly fits your idea of the situation at hand. The codes still don’t cover every possible condition and detail in the whole universe, for all times. But, they are working hard on doing that, and will soon issue volumes 79 and 80, each 2” thk., with the next ed. You have to use some engineering judgement and experience on some of these problems. What’s the same about columns and walls, what’s different and how does that show up, as accounted for, in the code sections? What load and force conditions are you trying to protect against, or design for. I don’t happen to have the latest eds. of ACI, but there must be some area where they kinda blend or overlap, and that’s the case with your “elongated pedestals to support equipment.” I’ll bet a length-to-thickness ratio greater than 2.8, or 3, or 3.3 act almost the same and the pedestal doesn’t know which section you were just reading, as it works from season to season. You’ve left out all the important details, so we know very little about your actual problem, but I’ll bet it’s a large LP tank or some such, and its fabricated saddle sits atop your pedestals. One end should be a longitudinally sliding saddle surface/joint, while the other is fixed longitudinally.

Once you have to do some/any forming and installing of rebar, it makes little difference whether the bars are #4’s vs. #5’s, spaced at 12” or 10”. It’s a little more steel, but not much else changes in time, field effort or cost. Good rebar detailing, for easy field assembly will be worth real time/labor savings. It’s out in the wild, partly buried, and partly exposed and you don’t want it to crack due to expansion and contraction or loading, so pop for a couple extra rebars. Look at how steel saddles actually work for both vert. and lateral loadings and you’ll find that the two outer ends of the saddle see some heavy loading under various conditions, and the same goes for your conc. pedestals. So, at the very least provide some confining steel to match the temp. steel in the two faces, with a few larger vert. bars, maybe a lightly reinforced col., if you like, at each end of the pedestal. Then, don’t worry too much about what some people call it, ‘pedestal’ is quite a nice name.