baseplate design for axially loaded column

[senthil83]

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.

 

[BAretired]

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

BA

 

[TehMightyEngineer]

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.

 

[hokie66]

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.

 

[CELinOttawa]

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.

 

[mike20793]

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.

 

[BAretired]

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

BA

 

[BAretired]

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

 

[malikasal]

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.

 

[BAretired]

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.

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

 

[oldrunner]

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.

 

[msquared48]

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)

 

[BAretired]

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

BA

 

[msquared48]

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)

 

[jayrod12]

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.

 

[steellion]

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

 

[Ingenuity]

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.

 

[senthil83]

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

 

[BAretired]

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

 

[senthil83]

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.