Friday Afternoon Dilema!! 1

[ars001]

Ok, I'm working on a metal building foundation project. Obviously I have vertical and lateral loads. Well originally I designed my exterior pad footings for gravity loads only and was going to provide a grade beam/tension tie member across the building to take out my horizontal thrust loads. I ended up with 10x10 pad footings with 16"x16" grade beams w/(8) #8 w/mech splices and ties. I can get 14'x14' footings by taking both thrust and gravity loads into account. I can get a cost estimate on the different design. Will I be happy with either design? I'm affraid that if I design my footings for overturning that I might get unwanted rotation from my footings. An overturning moment of 650k.ft scares me!! Please advise.

 

[msquared48]

Sounds like one BIG metal building here - I would still go with the direct tension tie. The 14 X 14 could be approaching half of your bay spacing for the mainframes here.

Also, why do you need ties in a tension tie? These are for shear. Just embed the #8's and weld or connect them as you mentioned, providing 3" of concrete cover.

Mike McCann
McCann Engineering

 

[MattJM]

Hairpins or tension ties that engage the bolts also help prevent them from blowing out the side of your pier.

Mike
I have often seen (and even used as a company standard) a detail that provides ties in the tension bar splice region. I really can't see any reason for them off hand.

 

[shin25]

I believe there will be a slab on grade for the building. Why can't you cast this slab monolithically with the footing. You can use this slab to take out your thrust and get rid of the grade beam. On top of that, passive earth pressure on the side of the footing will provide resistance to the lateral thrust.

One more thing, why are you worried about the rotation in the footing? Most of the time, the connection between column and footing is considered pinned. which means the footing will experience only tension,compression and lateral thrust from the load combinations.

 

[msquared48]

shin25:

Normally, for smaller metal buildings, I do use #4 or #5 hairpins in the slab and do cast monolithically.

It's just that for the tension forces ars001 is describing, I think that hairpins (8#8 bars he said) would not work - too many bars, too big, in too little an area to develop the bar tension. I think a lot of cracks would open up in the slab unless they went a long way into the slab. Moreover, with #8 bars, I would really start thinking about a 6" thick slab for the development. Really, I would go with a direct tension tie here as I understand the situation.

ars001: I, too am a little confused by so much moment to the footing. Is the moment due to the footing not being concentrically loaded, or the lateral kick to the pilaster and on to the footing?

Mike McCann
McCann Engineering

 

[dik]

For high loads, I've often used a bearing plate with rebar welded to it to accommodate the horizontal reaction.

Dik

 

[JLNJ]

It seems to me that the weak point may be the mechanism which takes the shear from the column reation and transmits it into the 8 #8s. That's got to be 150kips of shear (service). I've never seen a shear key in a PE building but it sounds like you'll need one.

Maybe 2 Dywidag bars would do the trick in the tension tie.

 

[zaes73]

ars001
can you tell a bit more about your buildng ! is it single story, whats the span and bay spacing. whats the eave height and ridge height. I am just a litle curious that what size of building brings 650 K-ft momment to foundations

 

[jike]

We need more of the story! Is there a pier on the footing? Is the tension tie at the top of the pier or at the footing level?

 

[ars001]

Ok, I'll see if I can explain it a little better. The metal building is approx. 225'x358'. It will be in indoor soccer facility. The field is turf so there isn't a floor slab. I orginally designed the footings for gravity loads using a 10x10 footing and a tension tie member across the building. The tension tie has member with (8)#8's (4 sets of bundled bars. I will be wrapping these bars around my anchor bolts in my piers. So actually there is really not a problem with the main frames. The problem lies on one of the endwalls were I have 20' of grade against the building. So basically I have an inward thrust load of around 120k. With no floor slab I need to accomodate for this inward thrust. Here in MN we need to drop the footings during winter construction for frost protection. Therefore create the 650 k.ft overturning moment. So thats sort of my dilema.

 

[MarcbSE]

Can the endwalls be designed as cantilever retaining walls?

 

[ars001]

Originally the intention was to design the endwall as a retaining wall. The retaining wall needed to have a 24" thick wall with a 20' wide footing. So we went a different direction.

 

[MarcbSE]

Correct me if I'm wrong, but if the thrust is inward then you won't have tension ties at all. They will be compression members, and very long ones at that.

If that is the case the larger spread footings doesn't sound like that bad of an option. Are you thinking of putting a horizontal bond beam at the bottom of the wall to help distribute the lateral forces to the spread footings?

 

[shin25]

Now a days, besides the retaining wall systems, there are other soil retaining methods that are available. These are sleek and cost effective and virtually can eliminate retaining wall (depending application). Why don't you try something like these?

http://www.tensarcorp.com/index.asp?id=99