Increasing Shear and Tension Capacity in Pier

[someengineer]

Hello,

Does anyone have any insight into practical methods to increase pier anchorage capacity?

We have a situation where there is an existing frame that supported a vessel that was laterally supported at the top. Now our client wants to replace the vessel with another one (using the same frame and foundation) but there will not be any top lateral support. As you can imagine now there are some pretty significant shear and uplift loads in the foundation (from seismic and wind). The frame is supported on 4 concrete piers that tie into a mat below grade.

With the new loading the piers do not have enough concrete breakout capacity in both tension and shear. Does anyone have any practical methods to reinforce existing piers to resist the above failure modes? We have a few ideas but they are difficult to determine the load distribution and effectiveness. The client wants to avoid having to replace the foundations since this needs to be done during a mill shut down.

A few ideas we are looking into -
- Pouring additional concrete around the existing piers
- Re-distributing the load into new piers using steel frame modifications
- plating or wrapping the existing piers

Any ideas or experience would be appreciated.

Thanks!

 

[KootK]

If the piers themselves work for tension and uplift, I'd try using plates etc to deliver the load from the frame columns to the piers directly, bypassing the anchor bolts. I think that would be the fastest solution. Maybe the least expensive too. It would just be ugly and inelegant but that may not be an issue here. If you post some sketches of the existing connection detail, we'll happily help you work through it.

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.

 

[someengineer]

Thanks KootK,

The piers themselves fail in breakout under both tension and shear (based on anchor rod edge distance). I'm a bit unclear about what you are suggesting, could you please elaborate. I have provided a quick sketch for information (NTS).

We want to do the reinforcement without removing the frame if possible.

 

[MacGruber22]

If you are in the States, maybe try contacting Structural Preservation Systems - they may have already done this many times using FRP sheet or plate.

I can certainly envision a solution, but what is in my head sounds expensive...

"It is imperative Cunth doesn't get his hands on those codes."

 

[SlideRuleEra]

I agree with MacGruber22, calling in a specialist is the best approach (but expensive) to get predictable results.

I consider the second best method, if the client can accept a "best efforts" result, is to encase the existing pier in full-height concrete. Anchor the new concrete to both the mat and the pier and take steps to get as good a bond as practical to both the pier and the mat.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[someengineer]

Thanks for all your input.

We have contacted an FRP specialist and have been told that is has been done before. We are still looking at other options however.

The challenge I see with encasing the existing concrete, is predicting how the shear and tension failure cones will behave between the old-new concrete interface. As well as how the dowels will behave across the interface. Does anyone have any experience with this problem? I imagine you can not simply just assume that there is increased edge distance due to the new concrete.

 

[KootK]

Are there braces delivering the shear? Or moment frames?

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.

 

[MacGruber22]

The challenge I see with encasing the existing concrete, is predicting how the shear and tension failure cones will behave between the old-new concrete interface.

To feel good about that, it would involve chipping back to pedestal vertical steel, creating a nice rough surface, doweling all over said surface, and installing new verticals ties around the reduced section. That is done often for building columns for seismic retrofit or repairs. It could be done, but it is a lot of work, and you need a contractor who knows what he/she is doing. An advantage of someone like Structural Systems is that they provide the solution and perform the work.

"It is imperative Cunth doesn't get his hands on those codes."

 

[someengineer]

KootK,

There are braces (both planes) that deliver the shear and uplift to the piers.

MacGruber22,

Thanks for the advice, I will be sure to get in touch with a specialist if we go that route. It definitely does sound like a lot of work, our client is concerned with schedule more so than cost. I imagine a repair like you suggest could take a fair amount of time.

 

[KootK]

I'm a bit unclear about what you are suggesting, could you please elaborate.

I was hoping for moment frames. The presence of the braces complicates a lot of the potential side plate options. Regardless, I've pitched something below. Since you were kind enough to post your sketches, I figured it was the least that I could do. Tension would be addressed through an effective lap between the pier verts and the pre-tensioned rods. Shear would be address through good old fashioned friction at the pre-compressed base plate / pier interface.

The challenge I see with encasing the existing concrete, is predicting how the shear and tension failure cones will behave between the old-new concrete interface.

I would share this concern. It would be difficult to establish a reliable capacity I would think. I have similar concerns with the FRP option. FRP would clearly increase the shear capacity of the pier but that's quite different from increasing the breakout capacity of the anchors.

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.

 

[KootK]

A couple more things:

1) It sounds as though you're using ACI 318 appD to evaluate this. Sensibly sized piers rarely work when evaluated that way. Before committing to a reinforcement strategy, you might consider an alternate assessment methodology such as this one: Link. It probably still won't work but it's worth a go.

2) I find your proposal of dumping the lateral loads into new piers quite attractive, particularly given the effort that would be involved with some of the other schemes that we've been kicking around.


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.

 

[SlideRuleEra]

The challenge I see with encasing the existing concrete, is predicting how the shear and tension failure cones will behave between the old-new concrete interface.

Agree completely, that's why I qualified the approach by calling it a "best effort" (lower cost, but no predictable result), AND having the client's concurrence. An industrial project's Owner may be willing to go along with that approach - depends on how much improvement in performance is needed.

KootK - I would be reluctant to use exposed steel in the fill, between the mat and the slab. Again, depending on the situation, would be common to have wash water, spilled process water, etc. getting into that area to promote corrosion.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

KootK - I would be reluctant to use exposed steel in the fill, between the mat and the slab.

I was hoping that OP's sketch was so NTS that the reinforcement might be able to be accommodated above the SOG. Wishful thinking no doubt.

If the reinforcement needed to extend below the SOG, I'd encase everything in a concrete blob as part of the SOG replacement. Hopefully that would help out with durability concerns.

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.

 

[SlideRuleEra]

KootK - Good idea. The fill may be fairly deep, sometimes process piping is embedded in that fill as a way to keep it out of the way and protect it from damage, yet remain reasonably accessible (compared with embedding the piping in the concrete mat).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

What if we:

1) removed the braces and made it cantilevered columns instead.

2) poured 3'-4" square concrete columns right from the top of the footing to the tops of the steel columns.

3) pretended the baseplate and anchor bolts didn't exist.

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.

 

[dcarr82775]

I think something along Slide Rules method could work. Expose the surface of the pier, roughen the surface, cast a new 'sleeve' around it. The key in my mind is provide enough ties in the new sleeve to prevent the new concrete from moving differential with the original pier. I would then consider the side face breakout a moot point. You could use....wait for it Koot....the shear friction equations to come up with an area of steel required. We have done something similar to add capacity to existing pier caps when we have added supplemental support piers.

 

[someengineer]

Some great discussion on this! Thank for all your help.

To respond to some of the inquiries - the pier is about 1'-4" up from the slab on grade so if we went with rods tied into the side of the pier, they would likely need to pass through the fill. This is a process area, so there may be some leakage into the fill but it's a bit unclear at this stage to what extent. Maybe duplex stainless steel rods could be used in this instance...?

There are certainly a few good ideas here. Sliderules & dcarr82775s suggestion seems feasible but would entail a significant amount of work.

Modifying the steel and encasing the column is very interesting. Has anyone done this before? How would the loads be resolved? I imagine at some height along the pier the loading would have to be completely taken up in the concrete. Would the loading just transfer back down to where the original anchorage is located? Maybe a combination of Sliderules idea and Kootks idea is a possibility (however still a lot of work)..?

I have found ACI 318 App D very limiting in the past, thank you for that guide on using reinforcement to assist with anchorage loading. Our new loading is quite a bit higher than what existed previously so I don't suspect that it will be able to be resolved without some kind of reinforcement, but worth a look regardless.

 

[SlideRuleEra]

Our new loading is quite a bit higher than what existed...

I suggest removing the slab and fill around each pier to determine the actual field conditions. This probably will not affect plant operations. Without this exploratory work there are too many unknowns that may (will) drive up costs.

Then bring in the specialist to get a cost estimate for necessary modifications. If an industrial plant can avoid an outage, as the client has indicated is preferred, even what seems like a "high" cost can be easily justified.

If the client want to pursue another solution, having the piers / mat exposed is needed information anyway.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

the pier is about 1'-4" up from the slab on grade so if we went with rods tied into the side of the pier, they would likely need to pass through the fill. This is a process area, so there may be some leakage into the fill but it's a bit unclear at this stage to what extent. Maybe duplex stainless steel rods could be used in this instance...?

You could use stainless/HDG, apply a bituminous coating, add sacrificial steel sectional area, and encase it all in concrete if you like. The pre-tensioned rods would only really need to go far enough down to effectively lap with the existing pier reinforcing. 1'-2' max below the SOG depending on the size of the pier verts and whether or not they're hooked at the top (unlikely). I'd only take the concrete encasement down maybe 4" lower than the steel.

Modifying the steel and encasing the column is very interesting... How would the loads be resolved? I imagine at some height along the pier the loading would have to be completely taken up in the concrete. Would the loading just transfer back down to where the original anchorage is located?

I would design this as though it were a square, hollow concrete tube column extending from the top of the steel column down to the top of the footing where it would be considered fixed. I would ignore everything within the tube and assume that the frame lateral load got delivered as a horizontal shear at the top. Obviously, the footing would need to be able to handle whatever moment was delivered by the new concrete column at the base. The big advantage here would be no doweling into the existing pier and no surface prep.


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.