Base Plate w/ Only Moment Loads 3

[Brad_S]

I am designing a column that supports a glass railing on a roof. The column has very little axial load but has a large horizontal load applied from the wind causing a moment.

I have a standard base plate connection with 4 bolts and am attempting to determine the tension in the two bolts in tension. I have been going through codes and have been struggling to find information for when there is no axial load on the base plate. In the AISC Base Plate and Anchor Rod Design it has examples but when looking to determine e (Mu/Pu) I get a value of over 72".

The current proposed base plate is 5" x 12" x 1/2" with a moment of 10 in-K and only .138 K axial force.

 

[rb1957]

on a 12" plate I suspect your bolts are spaced 8" apart (maybe 9") then bolt load is M/(d*n) or 10000/(8*2) = 625 lbs ... suggest 1/4" bolts (as the smallest to use)

another day in paradise, or is paradise one day closer ?

 

[WARose]

In that situation, the center line of your (assumed) compression zone (i.e. the 2/3 rds point of the pressure triangle) would be the center line of a flange. Your T/C couple would then be the distance from the center line of a (tension) anchor bolt line to this point. Your pressure triangle would be 3 * the distance from the center line of the flange to the edge of the plate. (You wouldn't want to make that distance too long because then you'd wind up with a compression zone that goes into the location where the tension anchor bolts are.)

In any case, you'd check the bearing stress (knowing your contact area) and the flange forces.

 

[BridgeSmith]

Not sure of what's up with the AISC equation (looks like spread footing calc with no anchorage), but tension in the anchor bolts is M/S - P/A, where S is the distance between the bolt lines (if you have leveling nuts) or from the bolts in tension to the compression edge of the plate (if the base plate bears directly on a grout pad) and A is the number of bolts.

Assuming 1-1/2" edge distance and the base plate oriented with the 12" dimension perpendicular to the rail, without leveling nuts, the tension in each bolt would be 10k-in/10.5in/2 bolts - .138/4 bolts = .442k = 442lb

For a base plate that is thin enough and heavily loaded enough to bend, the distance would be to the center of the compression zone (for simplicity, it's often conservatively taken as the edge of column). In your case, the moment is so small, the 1/2" plate is essentially rigid and the compression zone is such a narrow strip along the edge opposite the bolts in tension (about 1/32"), the centroid of the compression zone can be taken as the edge of the plate.

 

[BAretired]

If you don't use grout, you need leveling nuts under the base plate. In that case, the maximum compression is P/4 + M/2d where P is the axial load, M is the moment and d is the c/c distance between bolts in the direction of wind. The maximum tension is M/2d - P/4. With a large wind load, the bolts will be subject to localized bending between base plate and top of concrete.

It might be better to grout under the plate to avoid bolt bending, but with a 5" minimum dimension of plate, the spacing between bolts in one direction is going to be about 3" or less. Bolt forces under wind load depends on wind direction. Is it parallel to the 12" dimension or the 5" dimension...or both?

BA

 

[oldrunner]

I am a little confused. Your handle indicates that you are a "structural" something. You question is a very basic issue. I am wondering if you have had a course in statics?

 

[TEDstruc]

oldrunner,

The question may be basic, but the OP may be a young engineer or even an intern. I don't think there is anything wrong with asking a question, no matter how basic. They may have taken a statics course and still be learning how to apply free body diagrams to real world applications. After all, didn't we all ask basic questions at some point in our education?

 

[BridgeSmith]

All too often, we go looking for an equation in the spec in order to solve a problem, instead of applying what we learned in Statics, Mechanics of Materials, and Structural Analysis. We get so buried in the code books, we forget to look at basic principles. Many of us need to be reminded to take a step back and just look at the physics and mechanics of what we're doing, rather than blindly plugging numbers into what we think is the right equation. That's my 2 cents worth for today.

 

[WARose]

Actually, I've rarely seen this addressed in most steel texts. (A notable exception being Jack McCormac's book on steel design.) Especially the part about the compression zone. Where that zone is.....is pretty tricky once you think about it. If the base plate is large enough, saying the distance from the center line of the compression flange to the edge of the plate is one-third of the zone may not be accurate. (And that is critical for bearing stresses.)

 

[rb1957]

don't forget to size the post for the moment.

you're seeing two different ways to react moment ...
1) simplistically, on the bolts as a couple ... not precise but near enough (and simple), or
2) the tension load (of the couple) is reacted by the bolts, and the compression is a triangular distribution between the plate and the ground. You can sanity check the two by seeing how close the "compression" bolt is located to the centroid of the triangle.

another day in paradise, or is paradise one day closer ?

 

[oldrunner]

Concerning the analysis - this is a classical structural engineering problem. I took Strength of Materials in my sophomore year and FBD diagrams were in the book. (Strength of Materials, Cox, Germano and Bateman. 1951 - pages 10 - 15). A good explanation of Free-Body Diagrams is on page 36 of Shigley's Mechanical Engineering Design - 5th Edition.