Finding the tension in anchor bolts with a steel base plate.

[MiguelD]

I am a recent graduate and i am designing a T shaped pipe rack. I have gotten to the point where i need to specify the embedment depth for anchor bolts. However i am not sure how to find the tension in the anchor bolts. I believe i am supposed to use P/A - M/S but this equation will give me a stress. I suppose i could multiply by the area of the bolts and find the tension but i am not sure if this is right. Also if this is the correct equation to use, do i use the area and section modulus of the bolts or of the steel base plate? Please advise.

If this methodology is wrong then is there any other way of finding the tension in the bolts?

I am proposing to use a 14"x14"x1/2" base plate with 4-3/4" dia. anchor bolts 12" center to center.
P=2kips M=5kip-ft.

Thank you in advance for your input,
~MD

 

[patswfc]

if the base plate is stiff, then for a 4 bolt connection arranged in a square pattern at d centres, with the applied forces acting at the centre of the bolt pattern and the moment acting in an orthogonal direction to the bolt layout,
the max tension in a bolt will be (assuming P is a compressive force)

P/4 - (M/d)/2

M/d gives the forces in a couple which equals the applied moment. This value is divided by 2 as there are 2 bolts taking this force in compression and 2 bolts taking this force in tension.

 

[tngolfer]

MiguelD,

Not sure what steel code you are using but 1" is cutting it close for edge distance and may possible not be per code. Assuming you are using cast-in anchors, generally the base plate will come out with oversize holes for construction tolerances.

AISC steel manual 13th ed., Table J3.4 and J3.5, 3/4" bolt
Edge Distance
sheared edge = 1-5/16"
rolled edge = 1-1/16"

 

[chichuck]

another approach is to use the method in Design of Welded Structures by Omer Blodgett. I think its still available from the Lincoln Arc Welding Foundation. That method accounts for the compressive load in the column and the moment applied. It is much like analyzing a concrete bending member with compressive loads + bending. It does require an iterative process and is relatively straightforward. It also includes the effects of bending in the baseplate and the elongation of the bolts.


regards,


chichuck

 

[miecz]

MiguelD-

It's not clear to me whether or not your base plate bears directly on the concrete (or grout) or if it is raised above the support on levelling nuts. For a raised plate, the approach by patswfc applies. For the plate bearing on the grout or concrete, follow chichuck's advice.

 

[csd72]

Yes you use P/A +/- M/S but equate the area of each bolt to 1.

Then take d as the distance to the center of the farthest bolt and from that you can calculate the forces in the bolts.

This is an important method to understand as it can be used to calculate loads on piles as wells as a similar method can be used to calculate weld stresses.

You need to take relative flexibility into account as well, if some areas of plate are more flexible (due to a longer cantilever) then these will take less load. This probably doesnt apply to your connection, but you need to be aware of the issue for more complicated connections.

 

[MiguelD]

Thank you this has been helpful.

Tngolfer- your right i am going to increase the size to 16x16 plate.

the plate will be bearing directly on concrete(grout) so chichuck if you have a link for a little more info on the method of design of welded structures i would really appreciate it.


Thank you all again this was really helpful and i am glad i have joined the community.

 

[asixth]

I agree with patswfc,

you will need to resolve the moment into a push-pull force, the conservative approach would be to use the distance between the bolts as the lever arm.

In reality, this will not be exactly what happens, two bolts will develop tension to hold the baseplate down, and the compression force will be developed by the baseplate bearing against the grout pad, increasing your lever arm and therfore decreasing your push-pull force.

Just like considering compression steel to increase your lever arm in reinforced concrete beam design.

 

[Tmoose]

If the intersection of the T is smaller than 8 inch or so the anchors end up far away, and all the bending stiffness depends on the base plate thickness. Note in this heavily loaded application how the gussets/stiffeners reach out to get very close to the anchors.

http://www.lkgoodwin.com/more_info/baseplate_mounted_jib_crane/images/baseplate_jib_crane_pic1.jpg

When the bending loads are high (eccentric loading, or a fork truck bumping into the pipe rack) the "pivot point" may be where the edge of the T contacts the plate on the compression side, shortening the moment distance to the anchors in tension.

It takes a surprisingly stiff (thick) base plate to compensate for anchors more than 2 diameters away from the post or column.

Working on a big fan. the (4) 1 1/8 inch anchors are about 6 inches away from the "meat". The 1 inch thick base plate is acting like this.

http://www.atdamerican.com/images/large/TBF020-1.jpg

 

[edwin17th17]

you dont do this manually..

you will encounter this a hundred times..
get a hold of spreadsheet for this.. it's very common..

 

[civilperson]

If the horizontal forces are NOT orthogonal in the same direction as the axis of the plate/column then the 45 degree diagonal force gives the worst case since only one bolt is in full tension for a four bolt pattern.

 

[DRC1]

Or you can assume it is a simple beam and sum moments around the front row of bolts.

 

[asixth]

Better still, you can write your own spreadsheet/program, if you can't do the problem manually, then you really shouldn't be using a spreadsheet.

I remember a great quote from the Levy and Spillers textbook, "you never fully understand a method until you can code it into a routine", or something along those lines.

Spreadsheets are great, they can really increase your productivity at work. The biggest problem I have seen experienced engineers encounter when they use a spreadsheet is that they pigeonhole themselves to only using one approach. They are never looking elsewhere for new designs and ideas.