Pre-Engineered Canopy Reactions Forces for a Mat Foundation (Also includes equipment skids) 1

[oengineer]

I am working on the design of a Mat Foundation. The Mat foundation is supporting 4 large equipment skids and a Pre-Engineered Canopy. The Canopy is composed of 12 columns with a rain skirt roof.

This link contains a sketch of what the project looks like:

https://files.engineering.com/getfi...8fe-893f-33a24e91fd25&file=overall_sketch.pdf

The canopy will not have a siding, just columns and a 3’ rain skirt at the top of the prefab building.

I cannot obtain the loads for the pre-engineered canopy from the manufacturer because it will be going out to bid and they cannot specify the building manufacturer. So I have to calculate the lateral reaction loads for the pre-engineered canopy (i.e. wind lateral reactions & dead lateral reactions)


I have some questions:

[ul]
[li]How would one go about to determine the lateral dead load for a pre-engineered building/canopy? Are there any technical guides that give examples of this?[/li]
[/ul]


[ul]
[li]How would one go about to determine the lateral wind load for a pre-engineered building/canopy? Are there any technical guides that give examples of this? I ask because pre-engineered metal buildings typically have much greater wind forces at their reactions than a conventional steel building. [/li]
[/ul]


[ul]
[li]Would hairpins still be necessary for a Mat Foundation supporting a pre-engineered building/canopy at the column supports?[/li]


[/ul]

[ul]
[li]Are there any potential issues to consider/beware of for using a mat foundation to support a pre-engineered metal canopy?[/li]
[/ul]


Suggestions/comments are appreciated.

 

[r13]

The logic is correct.

 

[oengineer]

The logic is correct.

Sorry, but whose logic?

[ADD] Or I should say, which logic?

 

[EDub24]

You’ll need to estimate the column reaction loads. In the past I’ve done some simple 2-D analysis of the frames to guestimate the loads and make clear the fdn would need to be checked based on the PEMB submittal. You should also know the general type of bldg. Typically for a canopy type housing equipment the owner doesn’t want braces and you would need portal frames in bith directions. You basically need to tell the contractor how you want the bldg to look and they will design it to those standards.

 

[r13]

Every statement follows a good reasoning, so logically right.

 

[Brad805]

A 2'-6" thick mat slab in this case is a large volume of concrete, and at some point you are going to start pondering the minimum rebar like another poster not long ago. Your rebar quantity will not be insignificant to meet minimums. I would start with a few simple volume calculations comparing conceptual solutions. Skilled trades can whip up pad/piers or form walls quickly. If the formwork becomes complicated, that changes, but here we seem to be talking about rectilinear shapes that are easy to build.

I would expect the cover indicated by the geotech is to achieve a certain bearing capacity. If so, compacted granular material is normally more cost effective at achieving the bearing capacity your floor requires.

If the building is delivered and erected before the skids, pad/piers are not ideal for the crew that slides the skid into place unless they are using skates tall enough to roll over the anchor bolts. I would talk to someone that understands the work sequence if you are unfamiliar. We have rolled much larger compressors into place, and it takes a skilled crew to do that.

Yes, the skid would need to be designed to span with a pad/pier solution. You would need to confirm with the vendor. Most vendor drawings show the minimum bearing points when in place. If they need continuous support pad/piers would not work, but I think a bearing wall would.

Yes, you would need to resolve all the forces from the pre-eng using the thickened edge combined with the slab. Your geotech should be able to point you in the right direction for the soil spring values to use.

You know I am not saying there is anything wrong with your original design. I was merely musing a thought out loud about something I thought was unique. If this is a part of a much larger project, it is unlikely the bean counters will notice the foundation cost. If not, you need to be able to defend your design if someone suggests your design is overly conservative.

I agree with Ed about the bracing for this type of bldg.

 

[oengineer]

You know I am not saying there is anything wrong with your original design. I was merely musing a thought out loud about something I thought was unique. If this is a part of a much larger project, it is unlikely the bean counters will notice the foundation cost. If not, you need to be able to defend your design if someone suggests your design is overly conservative.

I feel that the underline portion of the quote is where I am at now regarding my proposed foundation design.

 

[oengineer]

Every statement follows a good reasoning, so logically right.

I am sorry, but was it my statements or another posters statement?

 

[oengineer]

You’ll need to estimate the column reaction loads. In the past I’ve done some simple 2-D analysis of the frames to guestimate the loads and make clear the fdn would need to be checked based on the PEMB submittal. You should also know the general type of bldg. Typically for a canopy type housing equipment the owner doesn’t want braces and you would need portal frames in bith directions. You basically need to tell the contractor how you want the bldg to look and they will design it to those standards.

I honestly have no control or input into what the pre-engineered canopy will be like.

 

[r13]

oengineer,

I meant your statements.

I honestly have no control or input into what the pre-engineered canopy will be like.

You need to talk to the owner (mechanical engineer, or ..) of the equipment, ask for his input, then go from there.

 

[AK4S]

OP, When you are checking with the Owner/Mechanical I would also recommend to check if there is any system which would control the maximum building drift requirement. Since it is an open structure it can be fairly flexible and if no direction is provided in the drawing or Specs, the PEMB contractor might go for H/100 or worse.
For enclosed structures we have gone for H/240 or H/300, to avoid issues with Operable openings (while also limiting the local member deflection to L/360 for members framing the Operable opening).

 

[EDub24]

I honestly have no control or input into what the pre-engineered canopy will be like.

I agree with retired13. You need to know what type of lateral system it will be if you want to design the foundation. Or as others have said you can just design something ridiculously conservative and then trim it down during construction.

 

[Brad805]

"I honestly have no control or input into what the pre-engineered canopy will be like." Someone does and you need to talk to the decision makers. Either the owner will or will not allow cross bracing. It is a simple question based on access req'ts or piping. Pre-eng suppliers can build it either way, but one option is marginally more expensive. You cannot be expected to complete your design in the dark. The owner or your client has a role to play. If you are not asking questions, start. If nobody knows, you have a group that is unfamiliar and will need some education. Last resort, just use something heavy.

For the reactions call Behlen or any of the other large suppliers, and send them a sketch. In half the time of this thread you would have had a reaction package from them. Techs selling the bldgs can do this, and their software is very good at giving reactions. Add a fluff factor to those values and you have done what you can.

 

[EDub24]

In half the time of this thread you would have had a reaction package from them

I wouldn't bank on this. In my experience I've sent drawings to them and they hesitate to spend too much BD time/effort to send reactions considering they're not guaranteed to get the job. They also don't want to take liability for sending preliminary reaction loads to be used on the foundation design. I can picture it now. Engineer modifies foundation using submitted reaction loads. Contractor issues change order because the foundation got more expensive. Owner goes after Engineer. Engineer points to Behlen saying that's who gave them the preliminary reactions etc etc. It can get messy real fast and Behlen will want to keep their hands clean.

I do think you should still send your drawings to them because what they will do is give a quick review and let you know if they see any fatal flaws as well as giving guidance on what the drawings should show that could facilitate the construction/bidding. That's just as useful. At the end of the day no matter what you use to design the foundation you will have to check it again and verify once you get the actual submittal with reaction loads. Because of that I suggest being conservative on the foundation design and making it loud and clear that the design is preliminary, only for bidding purposes and that the foundation needs to be verified with the approved PEMB submittal. What I suggest doing is making sure the contractor provides a unit bid price for the foundation so that any increase in costs would be scaled from the bid set. For example if you show 100 yd3 of concrete in the bid set and they give a total cost of $1000 which is $10/yd3. Then if in construction you need to increase the concrete to 150 yd3 they would have to use the $10/yd3 cost given in the bid set.

 

[Brad805]

Ed, I have had much better luck. We work with some regularly and most times a safety factor of 1.15 - 1.25 is adequate. These reactions are never reviewed by their engineer because that requires a signed contract, but it is still better than nadda. As for the liability discussion, I think it would need to be clear where the project is at. This is the stage where one needs to be careful about the term budget v. quote. Based on what we know, best anybody can provide an owner is a Class 2 or 3 cost estimate. I find supplier information very helpful, and we make it clear where the information resides.

 

[JLNJ]

It seems like the OP needs to seek out a local senior engineer who can help him discern which items are important to fuss about.

To me this is much ado about nothing. The skids are light and the canopy reactions are small. The canopy anchorage will need to be re-examined when the vendor is selected and the real reactions and AB geometry are supplied. Don’t sweat it now. One page of quick hand calcs should suffice for now.

I would check the need for a mat. 30” of concrete seems unreasonably thick.

 

[oengineer]

To me this is much ado about nothing. The skids are light and the canopy reactions are small. The canopy anchorage will need to be re-examined when the vendor is selected and the real reactions and AB geometry are supplied. Don’t sweat it now. One page of quick hand calcs should suffice for now.

The project manager has requested a design for this foundation based on the information that I have provided in this board. They are looking for a design that would be acceptable for construction. I am not able to obtain the loads for the building from the manufacturer because the foundation will be going out to bid and they are unable to specify the pre-engineered building manufacturer.

I would check the need for a mat. 30” of concrete seems unreasonably thick.

The two options for the type of foundation to use for the condition described original post is a "Downturn slab/slab with haunch" or a Mat Foundation (slan-on-grade w/drilled piers or spread footings are out of the question).

The more I look into my two options, it appears that the "Downturn slab/slab with haunch" is not recommended. Please read the image below:

It appears that the mat foundation can be costly, but more functional. It seems like a mat foundation can be more economical compared to a Downturn slab depending on the siutation. Please see image below:

Here is some more discussion on the Downturn foundation

The Foundation and Anchor Design Guide for Metal Building Systems book has a procedure to design the downturn slab fdn. I will run the numbers to determine if the downturn slab is more economical than the mat fdn.

 

[KootK]

This assignment doesn't really seem fair to you. The cost of your client providing you with crap information is going to be you providing them with crap/costly information in return. But, if that's what they insist upon, I'd toss something out there that you know is going to work as shown below. Is that cost effective or likely to get built? Hell no. But, then, generating something cost effective requires decent information to work with at the outset.

This is one of those situations where you may have to hand hold your client a bit to coax them into asking reasonable things from you. That, both as a favor to your client and to prevent from you having to redesign this thing 20 times. If the down turned slab is on the table for discussion, and structural concern is the only thing driving that, prepare to be pushed hard to do the down turned slab. And, with the down turned slab options, your need to know something about the superstructure is going to go up an order of magnitude because anchorage issues will become a lot more demanding.

 

[oengineer]

This assignment doesn't really seem fair to you. The cost of your client providing you with crap information is going to be you providing them with crap/costly information in return. But, if that's what they insist upon, I'd toss something out there that you know is going to work as shown below. Is that cost effective or likely to get built? Hell no. But, then, generating something cost effective requires decent information to work with at the outset.

This is one of those situations where you may have to hand hold your client a bit to coax them into asking reasonable things from you. That, both as a favor to your client and to prevent from you having to redesign this thing 20 times. If the down turned slab is on the table for discussion, and structural concern is the only thing driving that, prepare to be pushed hard to do the down turned slab. And, with the down turned slab options, your need to know something about the superstructure is going to go up an order of magnitude because anchorage issues will become a lot more demanding.

I have been told by the project manager that a mat foundation or downturn slab is fine, whatever is best. So, I feel I have some leeway. But I also feel both avenues should be investigated.

Here is an example picture of a potential downturn slab detail:

Once I run the calculations, I can determine how long and thick my haunch will be. My foot print is rather large, so the downturn slab may look like an economical option at 1st, but it may be more labor intensive for a 84 ft by 38 ft area. In that case, the mat foundation would be, in my opinion, easier/quicker & cheaper to build. Due to the anchorage of the equipment skid, I expect my slab to be thicker than 12".

 

[oengineer]

It appears that I am able to make a downturn slab work with the applied loads I have generated based o the wind speed for the area of the project.

I am still concerned with the use of the downturn slab for the pre-engineered & equipment skid foundation. The Foundation and Anchor Design Guide for Metal Building Systems says the following:

Based on what the image above says, it seems that the slab with haunch foundation should not be used for a pre-engineered canopy. I wouldn't think that a pre-engineered canopy would have girt inset.

 

[KootK]

Based on what the image above says, it seems that the slab with haunch foundation should not be used for a pre-engineered canopy. I wouldn't think that a pre-engineered canopy would have girt inset.

I'm reading that the other way. When you have the bypass condition, which you may well, the slab haunch is okay.