Moment Capacity - Direct Embedment of Steel Bridge Column into Spread Footing

[kapoA]

I am evaluating the design of an existing pipe bridge (single pipe layer). Bridge is 20 feet high and 50 foot span.

Contractor did not construct foundation as per design. Instead of constructing pedestal and anchoring the column using anchor bolts, he embedded the bridge column to the bottom of the foundation rebar and poured the concrete. Footing is 2 feet in thickness.

How do I evaluate the shear and moment capacity of the embedment. Part of my scope is to determine if it is adequate to sustain the wind loads and not have concrete breakout or pull out failure. I assume anchor bolt equations are not valid in this case.

Any ideas/help is welcome.

Thanks.

KapoA

 

[DCBII]

So if the column is at the rebar level, what's between it and the soil? 3" of clear cover?

You could (with caution) calculate your breakout prism (for tension) much the same way you'd calculate it for a bolt with a plate washer on it. Just treat the base plate as a plate washer. You could check shear breakout similarly. A large embedment such as a column will have the tendency to make edge distances more critical.

 

[briancpotter]

What type of column is it? How large is the baseplate?

Assuming it's a steel column w/ a baseplate, I agree with DCBII - this thing will act essentially like a giant HCA, and you should use those provisions as a tentative guide. Start with ACI 318 Appendix D and go from there.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[kapoA]

The column is a W 10 x 33.

DCBII: Yes, the column is at rebar level, so there is 3" clear cover between soil and column.

DCBII and Brian: I don't think they used a base plate. I am planning to use ACI 318 APP D - treating the column like a giant bolt with 21 inch embedment; however i don't feel very confident about the accuracy of my calculations because of the large size and different shape - very different from a typical anchor bolt.

 

[briancpotter]

Without any baseplate I'd be wary of using concrete breakout - I'm not familiar with any applicable references for calculating pullout of smooth steel, though perhaps someone else is.

Unless your uplift is very, very low, I'd say this needs some angles welded to the W-section and bolted into the concrete, or some other repair to give you a guaranteed uplift capacity.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[BadgerPE]

What type of gravity load is on this column?

 

[briancpotter]

Also, don't neglect your gravity load check - with only 3" below the bottom of the concrete, I don't see how it could work for gravity punching. You're going to need a repair to transfer the gravity load at the top of the column instead of the bottom, and it needs to be done before the column is loaded to establish a proper load path

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[kapoA]

Gravity Loads are as follows:

bridge has a 52 ft span. Loaded with one 16 inch, one 12 inch, one 10 inch, one 8 inch, one 6 inch and a 12 inch cable tray.

supported on combined footing at each end of the span. Total columns is 4.

Gravity Loads per column are (after multiplying by a factor of 1.4) :
Vertical Axis is "Y".

X (kip) 0.899
Y 22.674
Z 0.837
Mx (kip in) 68.822
My -0.213
Mz 19.56

 

[BadgerPE]

That is what I was getting at Brian...If you are designing using ACI code, according to 15.7 Minimum Footing Depth:Depth of footing above bottom reinforcement shall not be less than 6 in for footings on soil.... Therefore, like Brian said, a means of transferring gravity loads to the top of your footing will be necessary. You may be able to design it to take the gravity and wind loads as well by using post installed anchors.

 

[theonlynamenottaken]

My two cents...

I agree with everything said above, however, I'd stay away from using App D. Furthermore, use chapter 22 "Structural Plain Concrete" for checking the 3" of concrete you have below the column. As a quick pass:

bo = 2(8") + 2(9.75") = 35.5 in
h = 3"
k = 4/3 + 8/3(1/1.2188) = 3.521 > 2.66 => 2.66
f'c = 3000 psi (assumed)

Vn = 15.5 kips
phiVn = .55(15.5) = 8.53 kips

Grossly fails vertical punching, and that's without considering your 69 k" moment. I think the only force you may be able to resist with this situation is your 20 k" torque.

 

[LOKSTR]

download the report from attached link.
This will help you in calculating moment capacity for embedded wf shape.
This report is for piles but you can assume your W section column as pile embedded into pile cap( in your case spread footing)

 

[kapoA]

All: Great pointers from everyone. Thanks so far.
LOKSTR - I will download the report.

Btw, the bridge has been standing for over a year. Hasn't failed in punching yet. My guess is it must be barely passing in vertical punching.

 

[DCBII]

With only 3" of concrete between the bottom of the column and the soil I wouldn't be surprised if it has already "punched" through the bottom of the footing. Unfortunately you won't see this type of failure since it's on the under side of the footing. You've basically got a column bearing directly on the soil with no footing.

Appendix D's breakout prisms assume load is transferred at the head of a bolt. Without a base plate you have no "head", so you're left with friction between the steel and concrete resisting pullout. Why wouldn't there be a base plate? How else would you plan on attaching the column to the concrete?

 

[kapoA]

DCBII - Agree with you, it's mostly friction between steel and concrete resisting pullout. How do I calculate that value in this case? Any reference?

About the base plate: Not sure why they didn't use a base plate. All I know (based on what I'm told) that they haven't used one.

 

[DCBII]

I wouldn't rely on any friction to resist pullout. Frictional resistance is a function of normal force. How much normal force could there possibly be in this situation? I'd recommend what I read in briancpotter's post. A pair of angles field welded to the column flanges with epoxy anchors into the footing should give some uplift resistance if your loads aren't too big.

The bearing problem, rather than the uplift problem seems like the bigger issue here.

 

[kapoA]

Decided to do something similar. Weld base plates to the outer edges of the column and anchor it to the footing using bolts. That should transfer the loads to the top and also resist uplift.

 

[theonlynamenottaken]

I'm sure you have this covered, but by your description I'm envisioning two rectangular base plates sitting flat on top of the footing, with a single line of fillet weld along each of the two column flanges. Whatever you're welding on will become a cantilever, which is why I think briancpotter recommended a couple of angles. That would provide a weld group with some moment capacity If you're taking this "wing plates" approach you'll need groove welds at the very least, and in all actuality will probably need a couple of gussets.

Maybe I'm misunderstanding your approach.

 

[kapoA]

Thanks. I went with the Gusset.

Thankful to all for the input and help in resolving this.