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Tension on bolts for moment resisting base plate

Tension on bolts for moment resisting base plate

Tension on bolts for moment resisting base plate

(OP)
If you have a moment resisting base plate set up, how would you include the rigidity of the base plate to determine your tension on the bolt? Normally, I would put the distance from the far edge of the beam to the anchor in tension in order to determine the upward force. One thing I disagree with however, is assuming that there is a couple on the bolts assuming that there is some perfect scenario that puts compression on a bolt. In reality there is no way the bolt is going in compression. Some engineers try to convince me of this theory. There should be a triangular distribution of compressive force on the concrete, however, it seems to be the thicker the base plate is... the less tension would be on the bolt (assuming the maximum distance used for tension would be at the end of the plate (which is practically impossible)).

RE: Tension on bolts for moment resisting base plate

(OP)
Wanted to make another question. Why assume that the base plate is rigid enough not to deflect?

RE: Tension on bolts for moment resisting base plate

To allow you to design the anchor bolts as a force couple. If your baseplate was flexible then you would have to do a much more difficult and iterative design process to ensure you're providing the moment capacity needed for your design.

Steel baseplates are cheap. Make it thick enough to act rigidly, design your anchor bolts for the force couple and be done with it.

RE: Tension on bolts for moment resisting base plate

Playswow:
I probably agree with you, but I don’t know what I’m agreeing to. Why don’t you put your convoluted description of your base pl. condition into a sketch with sufficient detail, several views and dimensions, etc. so we can have a meaningful discussion, and attach the sketch to your next post.. We can’t see what you’re looking at from here. I generally agree with the triangular compressive pressure from the conc./grout. Except, that compression won’t be a max. at the edge of the conc. or base pl., it will be zero, and it will fairly quickly increase to its max. as a function of the crushing, spalling, etc. of the conc. and/or the flexibility of the base pl. Its resultant will be approx. at the third point of the triangular pressure diagram, and from there to the opposite bolts (those in tension), is the lever arm for the couple. You might also have to consider the stiffness of the beam flg. plus a base pl. in determining how the pressures/forces are really applied. While the pressure diag. shape will be the same, the pressure (compression) will most certainly be greater under the beam web thna out at the flg. tips. The flexibility of the base pl. and beam flgs. could also allow the tension end of the beam to literally start to lift off the conc., thus moving the zero point of the triangular pressure diag. further from the tension bolts. Also, you could get compression on some of the A.B’s. if you applied leveling nuts to them at the underside of the base pl.

RE: Tension on bolts for moment resisting base plate

Agree with jayrod12...

RE: Tension on bolts for moment resisting base plate

Typical baseplates in these parts are done with shim plates and grout - no leveling nuts. Without leveling nuts I can't see how the anchor bolt could see any compression. Does anyone specifically require the use of nuts below the plate?

RE: Tension on bolts for moment resisting base plate

I would calculate tension in the anchor bolts in the same way I would calculate tensile reinforcement in a beam-column. The axial load in the column tends to decrease the magnitude of tension in the bolts; if the axial load is large enough, there may not be any tension in the bolts.

If the axial load is negligible, tension in the bolts can be treated in the same manner as reinforcement in a beam.



BA

RE: Tension on bolts for moment resisting base plate

I did some FEM modelling of a wide flange column base strong axis moment connection back in the day. Bolts outboard of the flanges. No matter the thickness of the base plate, all the way up to 5", the compressive reaction was always centred on the flange and considerably in excess of allowable concrete bearing stresses.

Consequently, I design my anchor bolts assuming that the compression reaction is centred under the column compression flange. I then check for bolt prying as recent investigations have shown that to be an issue at steel to concrete interfaces as well as steel to steel interfaces.

The rigid base plate assumption is a poor one that we make for the sake of convenience rather than theoretical rigour. The reinforced concrete analogy has validity so long as the bolts plastify prior to any brittle concrete failure modes. If the bolts cannot plastify then it is common to assume an elastic bearing stress distribution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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