Steel Adjustable Single Post Design 1

[Bridge100]

Could somebody tell me how to design a single post adjustable steel column? I would like to use a steel pipe with threaded rods on each end of the pipe for adjustment. How do you treat the different sections (pipe and rod) over the length of the column?

 

[DaveAtkins]

If your loads are small enough (residential type loads), there are premanufactured adjustable columns available. This type of post has a short, threaded rod at the bottom, which gets buried in the concrete slab after the post is installed (note--many homebuilders install them upside down!). The manufacturer publishes allowable axial loads.

DaveAtkins

 

[msquared48]

How much vertical load is this column assembly to take?

The rod cannot be continuous through the steel column or it becomes the column. You will need to weld two thick plates, one to each end to receive the threaded rod, and somehow, the rod will have to be stabilized from lateral movement. It will have to be sized to take the same load as the steel column, so the length protruding from the steel column will be limited.

Mike McCann
MMC Engineering

 

[apsix]

"It will have to be sized to take the same load as the steel column, so the length protruding from the steel column will be limited."

That's what Bridge100 is asking about, how do you design this column with a varying cross-section?
I'd be interested to know also.

 

[msquared48]

KL/r

Mike McCann
MMC Engineering

 

[Ron]

Assuming you will have lateral stability by making the threaded receiver long and integral to the pipe (you can use several nuts welded together and inserted with tight fit into the pipe, then weld in place, or have a cylinder machined with internal threads, then weld inside the pipe),then the problem becomes one of multiple columns, each bearing the axial load and designed for buckling as Mike noted (kl/r). Check critical load by Euler's buckling equation.

 

[apsix]

OK, but what 'r' value is used? It must be a function of the different cross section properties, and the length and position of those cross sections.

I'm assuming that there is no restraint at the rod to post connection points, otherwise it is a reasonably trivial exercise.

 

[IFRs]

Usually the threaded rod is very short in comparrison to the length of the column. Is this not the case for you?

 

[Bridge100]

The threaded rods can be up to 12" long and the interface where the rods connect to the pipe are not laterally restrained. Most concrete form supply companies sell these "pipe braces" for concrete form support and they publish allowable load charts for the product (without maximum extended thread length). How do you use the kl/r formula with 2 different sections over 1 unbraced length? You would obviously never put the screw adjuster in the middle of the column but for what reason is it accpetable to place them at the ends?

 

[Bridge100]

It appears that this thread has been removed. I would like to continue to discuss this topic with everybody.

 

[RHTPE]

Bridge100:
Why are you trying to design something that several manufacturers of formwork and shoring equipment have offered for years (and have tested and have established load tables)? Most all of this equipment is available in lengths up to 16' or so (some even longer).

If you are trying to come up with an analytical procedure to check these load tables, you would still want to be sure your theoretical results matched the performance of the tested items.

I have been involved in testing many of the commercially available post shores. Most do not offer the factor of safety that the manufacturers claim.

 

[Bridge100]

RHTPE,
I have not found any heavy duty pipe braces that can support higher loads at longer lengths (beyond 16'). The pipe braces sold by concrete forming companies have high capacities in the short range but the rated extended lengths have impractically small load capacities. We have the means to fabricate our own braces but I cannot find analytical examples. The application is for concrete form bracing, therefore, the braces will be checked for combined bending (self weight) and compression.

 

[RHTPE]

Bridge100

Have you looked at braces for concrete tilt-up walls? They come in much longer lengths and much more substantial construction.

 

[Bridge100]

I will relook at that product. However, I'm still very interested in designing them myself.

 

[RHTPE]

Bridge100

There are also 2 other products out there that could serve your purpose. I am reluctant to name them in this forum for fear of it being considered a plug or advertisement.

See attached link.

 

[RHTPE]

Bridge100

Check out this link also.

 

[DTGT2002]

As RHTPE has stated, much of the material in this market segment is rated by testing. While some portions of the various pipe members and proprietary sections are designed by conventional analysis, it is a challenge to resolve each portion of the member.


From the comments above, an effort was discussed to analyze the members over the three segments, designing the top screw over its length, the main member over its length and the lower screw over its length. I see merit in this approach, but it still leaves the challenge of designing the connection or joints between each area.

In most testing I am aware of, the screw/main body interaction is the common failure mode, though most test setups are made such that the main body of the shore is well braces and translation or buckling of the adjustment screw is more likely to occur. That said, I am a designer of formwork, not a testing lab, so I won't claim to know the ins and outs of all test rigs and procedures.


The design of these members is not trivial, not that you imply that. That said, the last two suppliers listed above have got a lot invested in their product and their products have a much more substantial capacity than any tilt up brace commonly used for wind bracing of formwork. Much of that investment covers the testing of the product in ways it is difficult to design for and much easier to test.

What is your application? If you are bracing for wind, I have not found a scenario where tilt-up braces could not be arranged for satisfactory purposes with the stated load capacity, with maximum 40' tall x 25' wide ganged forms. If you are resisting direct concrete loads, such as from one sided formwork, I would not expect anything from the pipe brace catalog to be of service and would undoubtedly expect to find myself working around the capacity of one of the suppliers being linked to. I have used both brands with fair success, though the yellow guys have more of my market.


My company has worked through bracing from other steel members for formwork in the past, however, we have always avoided the inclusion of the screw for adjustment. Generally, we will work to a frame type solution with wedges at various locations to provide the adjustment.

Making the adjustment location easy to turn, but adequate to take the load is no small task. Its also a little more ME than CE/SE in my mind.

There are formulas that can be found to give strength across various threads and fasterners from mechanical references. Even then, I would not want to use such a brace without considerable testing to verify the adequacy of the member.


If you review literature for some commercial shoring members, they will note the type of thread used in their member.


Keep on discussing.

Daniel

 

[BAretired]

The critical load for such a column can be calculated using numerical procedures. There is a similar example in "Theory of Elastic Stability" by Timoshenko and Gere. My copy is Second Edition and the example is on p. 122.

You may want to try googling "Newmark's numerical methods" for some additional articles on the subject of iterative solutions to problems.

I thought this one might be of interest in your case:

http://www.esdep.org/members/master/wg06/l0500.htm

Best regards,

BA

 

[BAretired]

Bridge100,

For a symmetrical column with I₁ at each end and I₂ for a length 'a' in the middle, Timoshenko and Gere provide a formula P_cr = mEI₂/l² and a table with v values for varying I ratios and varying a/l ratios.

Also included is an approximate method with a formula for the factor m.

I do not have permission from the publisher to copy this material, but you may be able to find it at a bookstore in your area.

Best regards,

BA

 

[Bridge100]

Thanks BA! - I will purchase the book that you referenced.