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baseplate design for axially loaded column

baseplate design for axially loaded column

baseplate design for axially loaded column

(OP)
we have axially loaded basepalte with a small shear force as per the attached sketch.

i have doubt that, while designing the thickness of the baseplate for the cantilever portion as shown in the sketch
can i consider the anchor bolt as a support in the middle.

as i consider the anchor bolt as support in the middle my baseplate thk is coming lesser.

but in the books i found they are not taking it as support.

please clarify.

RE: baseplate design for axially loaded column

  • The anchor bolt is not a support. Why would you think it is?
  • The section to the right is incorrect.
I think you are required to have four anchor bolts, not two.

BA

RE: baseplate design for axially loaded column

I'll second what BA said. If you can, look at AISC design guide 1. It's a great reference for base plate design.

Maine EIT, Civil/Structural.

RE: baseplate design for axially loaded column

BA is correct. The anchor bolt is for overturning/uplift, not for gravity loading. His comment about 4 anchor bolts is, I think, based on North American safety rules for erection. Not sure where you are, or if a similar rule applies.

RE: baseplate design for axially loaded column

This is the third time I've seen Mech Eng posting very basic Structural Engineer questions in as many weeks. Strange things are afoot...

I would strongly advise you to get some local help. This is a very, very, basic detail which you don't understand. That said, there are innumerable design guides which will show you exactly how to do this, most of the for free.

Also, in your defence, the anchor bolts are supports until (and in some ways unless - many traffic poles and light standards are designed this way) the base plate is grouted.

It is really quite rare for a wide flange column such as what you've drawn to go without grout for anything other than erection purposes.

RE: baseplate design for axially loaded column

Nicely put CELinOttawa, The OP really should buy AISC Design Guide 1. A concentrically loaded baseplate is quite easy to design by hand. Try to use a square baseplate, so it doesnt get installed in the incorrect orientation; and, as a quick size rule, add 8" to the depth of the column to get a trial baseplate dimensions. Also, 4 anchor bolts are indeed a requirement as mentioned above.

RE: baseplate design for axially loaded column

Four bolts are a requirement in USA, but possibly not in India (I don't know).

BA

RE: baseplate design for axially loaded column

Quote (CELinOttawa)

Also, in your defense, the anchor bolts are supports until (and in some ways unless - many traffic poles and light standards are designed this way) the base plate is grouted.

Further to CEL's post, I would agree that anchor bolts can be supports when leveling nuts are used below the plate, but they were not shown on the sketch. I don't think steel shims can be regarded as permanent supports.

BA

RE: baseplate design for axially loaded column

Take it easy on the guy!

Some designers tend to treat the bolts as a support in limited cases, but my advice just ignore the bolts when dealing with compression loads and go with the procedure outlined in AISC design guide 1 - you will find everything you need there.

the bolts are only considered when dealing with tension loads.

I didn't have the chance to check your detail, but as BAretired said 4 bolts is a minimum.



“If you don't build your dream someone will hire you to help build theirs.”

Tony A. Gaskins Jr.

RE: baseplate design for axially loaded column

Quote (malikasal)

Take it easy on the guy!

The OP should be more interested in the correct answer to his question than whether he is treated with kid gloves on this forum.

You responded, in part: "Some designers tend to treat the bolts as a support in limited cases" but you did not indicate what those limited cases are. My point is that anchor bolts can NEVER be considered a support unless they have leveling nuts bearing upward on the underside of the plate. Leveling nuts were not shown.


Quote (senthill83)

i have doubt that, while designing the thickness of the baseplate for the cantilever portion as shown in the sketch, can i consider the anchor bolt as a support in the middle. as i consider the anchor bolt as support in the middle my baseplate thk is coming lesser.

If the anchor bolt is centered on the projection of the plate, your baseplate thickness will not be less...it will be more because you would be carrying the entire load on the bolts, 200 kN per bolt, whereas uniform bearing pressure under the plate results in a smaller force operating at the same eccentricity, namely m/2. If that is not clear, please say so.

BA

RE: baseplate design for axially loaded column

The bolts are not carrying any vertical loads - but may resist some lateral load. Basically friction would probably do between the bottom of the base plate.

This is a simple bearing plate sized by P/A = fc which should be lower than the allowable fc. One should be able to find the method in a 2nd year strength of material text book. Like others I have no idea what the detail on the right of the drawing is, but in order for the plate to be effective, you will have to design a portion outside of the column flange section as a cantilever. The AISC manual mentioned is a free down load.

As far as designing double nutted base plates without grout, we do this very frequently. Our biggest problem presently is correctly designing the cantilevered bolts (as they are sometime described) for lateral loads. Presently ACI 318 Appendix D does not consider this but some European codes do. Some post installed programs do consider this, but they have a nut and a washer bearing on the concrete plus the standoff distance. Also some recommendations discuss the tightening of the double nuts is given. Another recommendation is the base plate should be at least as thick as the anchor bolts.

RE: baseplate design for axially loaded column

I have to look it up, but I have run across an exception to the OSHA 4 bolt rule. Two are allowed in certain conditions. Certainly not on a major structural element though.

Mike McCann, PE, SE (WA)


RE: baseplate design for axially loaded column

Mike, would you say a column carrying 400 kN (90 kips) is a major structural element?

BA

RE: baseplate design for axially loaded column

Absolutely BA. Definitely applicable in this case.

All I am saying is this is not a blanket rule. The requirement is for temporary election safety during construction, and not all types of structures are included in the stipulation.

Mike McCann, PE, SE (WA)


RE: baseplate design for axially loaded column

I've done a few buildings where a selection of the columns only had two baseplates. It required special erection bracing but it can and has been done.

RE: baseplate design for axially loaded column

Mike, the OSHA rule is you need 4 anchor bolts if the column supports a load of 300 pounds or more.

RE: baseplate design for axially loaded column

I think 29 CFR OSHA 1926 SUBPART R requires you to use 4 anchor bolts min per baseplate unless you have a "post".

§1926.755(a)(1):
All columns shall be anchored by a minimum of 4 anchor rods (anchor bolts).

§1926.755(a)(2):
Each column anchor rod (anchor bolt) assembly, including the column-to-base plate weld and the column foundation, shall be designed to resist a minimum eccentric gravity load of 300 pounds (136.2 kg) located 18 inches (.46m) from the extreme outer face of the column in each direction at the top of the column shaft.

From 1926.751:
Post means a structural member with a longitudinal axis that is essentially vertical, that: (1) weighs 300 pounds or less and is axially loaded (a load presses down on the top end), or (2) is not axially loaded, but is laterally restrained by the above member. Posts typically support stair landings, wall framing, mezzanines and other substructures.

From 1926.751:
Column means a load-carrying vertical member that is part of the primary skeletal framing system. Columns do not include posts.

RE: baseplate design for axially loaded column

(OP)
hi

thanks for your replies!!!!

i having been out of office for some days,

this is the baseplate we are using for supporting a continous rail which is supported to the ground at 1400mm intervals,

i have gone thru the AISC Design Guide Series -1, i understand the logic behind the base plate design and anchor bolt design.

but i still have the doubt that, when a outstand portion of baseplate is checked for bending, then some bending will be resisted by

the anchor bolt in the middle??/

thanks

RE: baseplate design for axially loaded column

If the bolt holes are 2 or 3 mm larger than the bolt diameter, how will a force or a moment be applied to the anchor bolt?

BA

RE: baseplate design for axially loaded column

(OP)
even though the bolt holes are 2 to 3mm higher than the bolt dia, we have nut and clip for holding rail above the base plate which will transfer the load to rods
and stop plate from bending upward due to reaction of the foundation.

RE: baseplate design for axially loaded column

You need a responsible, independent structural engineer in your group.

Your answers seem to imply "I want to design this connection this way because it suits my intentions and because it is my idea (as a mechanical engineer/designer), and I want you (the structural forum readers here) to back me up and support my conclusion."

Your replies are not accepting the technical structural criticism offered about this specific "work-around" non-standard anchor bolt design, but are arguing against it. Look, anchor bolts hold everything up, tie everything down, and are the ONLY link between your mechanism high above - and all of the forces and impulses and accelerations and jerks being applied up high - and the ultimate "dirt" and rocks underground.

Do you really, really want to "try something new" in that location?

RE: baseplate design for axially loaded column

If your base plate were very thin -- inappropriately thin -- you would indeed get a tendency for upwards curling at the overhanging edges of the base plate. I've seen this in a pre-engineered metal building with 1/4 inch thick base plates. And such a result would be predicted by structural engineering theory. So, with a very thin base plate, you're right, the bolts would clamp the base plate down to the foundation and provide a "support" of sorts.

All that theoretical fun aside, you should absolutely choose a base plate thickness that is sufficiently thick and stiff that it works without considering any clamping force provided by the anchor bolts. This is especially true if you're new to base plate design. Most structural engineers won't go any thinner than 3/4 inch for a serious base plate. Among other things, there's little savings to be gained by going super thin.

I hope that I've interpreted your question correctly and provided something useful to you in terms of advice. When you're done with this, you should consider reporting back with your solution. We can give it a reality check for you.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: baseplate design for axially loaded column

Quote (senthil (OP))

i have doubt that, while designing the thickness of the baseplate for the cantilever portion as shown in the sketch
can i consider the anchor bolt as a support in the middle.

I thought you were asking if the anchor bolt acts in compression, taking part of the load. It can do that if there are leveling nuts under the plate, but you did not show leveling nuts.

Instead, you are asking if the anchor bolts act in tension, preventing the base plate from curling upward. If the anchor bolts are in tension, then the total force between the plate and concrete exceeds 400 kN and is carried by an area less than the full area of the plate. The bearing stress would be extremely variable along the length of plate.

In the usual case, the base plate size is derived by dividing the load by the allowable bearing stress, so the full area of plate is needed to carry the load. To minimize deflection of the plate, the thickness should be not less than 1/5 of the overhang m or n.

The actual bearing stress varies slightly because the plate deflects, but the usual assumption is that the stress is uniform over the entire plate. If the concrete under the base plate is in compression at the location of the anchor bolt, then the bolt cannot be in tension unless it is prestressed.

BA

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