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Wide Flange Column Embedment Into Pier Footing

Wide Flange Column Embedment Into Pier Footing

Wide Flange Column Embedment Into Pier Footing

(OP)
Hello,

Need some help in figuring out the required embedment length of a wide flange column into a pier footing to fully develop axial and flexure load to the pier.  Thanks

RE: Wide Flange Column Embedment Into Pier Footing

This sounds like a trick question I was asked 45 years ago as a student.  If you are a student, you are not allowed to post homework questions.

RE: Wide Flange Column Embedment Into Pier Footing

Try to get the book by N.Subramanian, Design of Steel Structures on page 1092 it describes "Pocket Bases". It should help you with the elastic steel analysis he uses.

RE: Wide Flange Column Embedment Into Pier Footing

(OP)
I haven't been a student for six years...why would it be trick question? cap4000..i'll look into that book you recommended.   

RE: Wide Flange Column Embedment Into Pier Footing

My apologies, then.  Socketing steel columns into pier footings is not a common solution...I've never seen it done.  But I defer to the book mentioned by cap4000.

RE: Wide Flange Column Embedment Into Pier Footing

(OP)
no apologies needed,I work in the solar industry and it seems solar racking manufacturers prefers this method of construction.   

RE: Wide Flange Column Embedment Into Pier Footing

If your configuration and design will allow it, I would avoid it for constructability and design issues. You are mixing trades, and it makes it hard to level the steel, among other things.... This is a fixed base column, so you are trying to develop moment into the footing via the pier?

RE: Wide Flange Column Embedment Into Pier Footing

Like Hokie, I never heard of this before. There are a couple of ways that come to mind:

To form a void in the footing drop the column in, level and plumb externally, and fill with grout.

To externally level and plumb the column with connectors, and then place the concrete.

I don't like either of them, all columns would have to be done simultaneously.  

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Wide Flange Column Embedment Into Pier Footing

Can you provide a sketch? I imagined a pad footing with a concrete pier, with the steel column embedded into the concrete pier. I assume this is to protect the steel because this is underground but...

Sounds like others interpreted your situation differently.

The more info you give us the better help you will get!

RE: Wide Flange Column Embedment Into Pier Footing

While this is not common, it is done in telecommunications, where a steel pipe (up to 24" diameter) is "wet-set" into very fresh concrete with no spiral or vertical reinforcing.  
The concrete is simply cover for the steel and an enlarged bearing surface against the soil.  An oversized post socket.

Very quick and cheap, since the base plate and anchor bolts are eliminated.  Okay for single, small monopoles.

However, not the appropriate solution if normal structural steel tolerances (horizontal and vertical control, plumb...) are expected.  

Your drawings will need to be more specific about tolerances and be more involved in "means and methods" than usual.

RE: Wide Flange Column Embedment Into Pier Footing

The sketch attached seems to show a two step concreting process, with some site welding in between.

I have seen lighting poles done in the manner described by ATSE, but they were concrete poles, not steel.  

RE: Wide Flange Column Embedment Into Pier Footing

I think you'd be better off designing this as an auger cast reinforced pile with CIP anchor bolts, like road signs, traffic lights, and light poles. This is a very common and standard design method, allows leveling and adjustment via the nuts, and you'd have plenty of design guidance from AASHTO/DOT type publications...

RE: Wide Flange Column Embedment Into Pier Footing

Agree with all of the above comments.

It is done sometimes but it does have its problems. When I have used this methods for small signs I have designed it as ATSE has suggested - with the concrete just acting as a cover.

RE: Wide Flange Column Embedment Into Pier Footing

I've seen large galvanized transmission towers where the pole was just sticking out of the concrete instead of using a base ring with anchor bolts, but I have no idea how they are arranged under the surface, or what analysis goes into them.

RE: Wide Flange Column Embedment Into Pier Footing

(OP)
Thanks for all the help guys, typically I would use a base plate and anchor bolts but the client prefers to just embed the column into the footing.  From what I understand this is very common in the solar and carport industry.  In any case I got some good advice from my former empolyer and have it figured out now.

RE: Wide Flange Column Embedment Into Pier Footing

And it always seems to start rusting where the steel goes into the concrete.  Very hard to fix - but will take years - usually

RE: Wide Flange Column Embedment Into Pier Footing

I routinely use embedded columns for aluminum structures.  We use styrofoam blockouts for the concrete.

Analysis is straightforward, with resolution of the moment (and shear) at the column insertion point and applying a triangular stress distribution on the grout block.  Required depth of embedment can be computed algebraicly from an assumed stress block.  You can mechanically anchor against pullout by inserting a dowel through a hole in the column or compute the shear-bond strength of the grout/column interface.

RE: Wide Flange Column Embedment Into Pier Footing

How do you remove the styrofoam?

RE: Wide Flange Column Embedment Into Pier Footing

Well, before the environmentalists got all touchy about it, we used an aromatic solvent to melt the styrofoam.  Now we just dig it out.  We don't oil it because we don't want to compromise the grout-to-concrete bond.

RE: Wide Flange Column Embedment Into Pier Footing

@samchrun
How did you design it?

Ron,

Can you expand some on how you design these?  what do you mean when you say grout block?  I guess I'm not visualizing this the stress distribution your describing.

Sorry don't mean to hijack the post.

EIT

RE: Wide Flange Column Embedment Into Pier Footing

(OP)
I essentially designed it per Ron's description. Triangular stress distibution from top of footing down to bottom of column embedment, resolved the moment with the T-C couple arm and check the actual bearing vs the allowable bearing of the concrete per the ACI and added a few additional ties at the top of the footing for tension.     

RE: Wide Flange Column Embedment Into Pier Footing

RF...the styrofoam blockout is larger than the column, so there is space to back-grout the hole around the column.  I deal with aluminum structures, almost always extruded tubes, so the bottom of the tube gets flared to give a mechanical key within the grout.  If uplift is very high, we use a stainless steel pin through the bottom of the tube to give additional mechanical keying.

Analysis is as samchrun noted.   

RE: Wide Flange Column Embedment Into Pier Footing

Stress on concrete is (Bd^2)/6 where b is the width of the steel and d is the embedment.

RE: Wide Flange Column Embedment Into Pier Footing

Okay, but how is level and plumb achieved? In slickdeal's link that is not a problem.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Wide Flange Column Embedment Into Pier Footing

I totally agree with the comments about constructability. It has ZERO tolerance. My link was only regarding how to engineer the problem. (link was originally provided by cap4000 in another thread).

RE: Wide Flange Column Embedment Into Pier Footing

Ahah - I see.  Thanks guys!

I got a nice dose of Aluminum design this year because someone wanted to put a 12' sail (canopy) on their existing building.  All I have to say is Weld affected this Weld affected that, can I have more strength, is there some way we can do this in steel.
It was a good learning experience though.
And although I was a little nervous about the design (being my first time and all) when I saw the manufactures product I thought - My support is designed for 40psf but that thing is going to fold like a deck of cards with 10psf.

EIT

RE: Wide Flange Column Embedment Into Pier Footing

Michael...the company that I do the engineering for has been doing this since 1960...I've been doing their structural design since 1984.....they have it down to a fine art with jigs and temporary supports.  Since it is aluminum, the weight is very low, so the extruded, rigid frame bents are relatively easy to erect.  For the cantilevered bents, it gets a little more tricky, but not bad.

RE: Wide Flange Column Embedment Into Pier Footing


I see no confining reinforcing in your pier.  What happens when the concrete cracks between the steel column and the face of the pier?  I have been doing similar designs for a solar installer here in the Northeast US.  They do not want to look beyond the life of the subsidy that helped pay for the solar installation.  We have yet to converge on a method that they like and that I can stand behind.  Rust will be the downfall of the method you depict, and at some point in the life of the column it WILL crack the concrete surrounding the column.  Confining reinforcing (column ties) in the pier is a must IMHO, no matter how you connect the steel column to the pier.

This must be an installtion in the lower latitudes.  Here in my area the pivot point for the array is much higher so that the array angle suits the sun's position during the winter.  Makes wind analysis much more interesting.

 

Ralph
Structures Consulting
Northeast USA

RE: Wide Flange Column Embedment Into Pier Footing

Samcrun:

This is called a ground-set foundation and is very common for flagpoles.  Page 14 of the Metal Flagpole Manual published by the National Association of Architectural Metal Manufacturers illustrates some generic dimensions for flagpoles up to 100 feet.  We have found these suggestions to be very conservative.

Your configuration is no different from a Pole type of structure and the methods to design footings for these kinds of structures have been around for years.  One of my references is Design Notes and Criteria Pole Type Buildings by H.J.Dengenkolb and Associates, San Francisco. (Published 1968).

I assume that you have a geotechnical report for the site and if conditions vary across the site, you will have various embedment depth requirements.  One of your biggest problems will be having enough mass of the steel column and the footing itself from pulling out of the soil.  Either you develop enough upward friction at the sides of the 24" pier or you add weight of some kind.  This kind of a structure is going to have a lot of movement due to the complexities of your wind load, so I would require a cage of stirrups in the hole.  I don't know what kind of fixity you have at the ground level. (Sketch shows asphalt?)     If this is not used as the point of rotation, you then do have to be careful where you select the start of the passive pressure of the soil for your lateral pressures.  (Get it in writing from the geotech!)   The design calculations for this kind of footing is easily set up in a spreadsheet so that you can hand calc the solution and be able to play with the different possibilities – such as depth, diameters, passive and maximum pressures.   You should also be checking the bearing pressure – 2 foot diameter of bearing plus whatever friction factor on the sides of the pier may not be enough.  You might end up with a bearing cap at the top of the footing to take care of this and also provide enough additional mass if the "wing" of solar panels tries to fly away.

Specifying stud anchors on the side of the steel column is easily determined - we don't count on any friction between the concrete and the steel.  There may be some other methods that are cheaper.

Others have already provided you with other good basic design information.  



www.nma-se.com
 

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