Allowable Bending Stresses for A325 Bolts

[JCWilson]

What is the proper allowable stress design method for A325-X bolts for simple bending stresses? Are the bolts designed per ASD Chapter F, section F2 with Fy obtained from Table I-C in Part 4 Connections?

 

[ScottD13]

I'm not sure that I can envision an application where the bolts are actually in bending (as designed, but I would think that the way to approach the problem would be to apply the interaction formulas given in Table J3.3, per Section J.3.5 (pg 5-72 of the "Green Book&quot.

Scott ~-)

 

[Watermelon]

Hello JCWilson:

I'm with ScottD13 on this one. I can't see any way to put a bolt into serious bending, unless you really do some serious fabrication. Normally bolts are either in tension, shear or a combination thereof. Connections can be complicated when loads are eccentric or prying action is involved.

All the best.

 

[JCWilson]

This is actually the opposite of serious fabrication, and I assure you these bolts are bending.

FYI, these structural bolts are being used as "shear pins" between exterior building panels. Take a light gage steel frame panel with tracks top and bottom, stick a bolt through a hole in the track web from the inside face and weld the head fast. The bolt then protrudes through a hole in the adjacent panel's track section with a nut welded loosely on the end. Add in a 3/4" expansion joint between tracks and you have a bolt with about one inch between a relatively fixed end and a shearing force.

This can add up to significant bending and shear forces when one panel is fixed and the other wants to move in or out due to high wind forces.

I have used capacities similar to ScottD13's suggestion - its conservative enough that I can live with it. Regardless of whether any of us heavy steel guys like it, I think this type of detail is something of an "industry standard" in light steel panel framing.

 

[Watermelon]

Hi JCWilson:

Okay, I'm with you now. The bolts aren't being used as one would expect. I don't think you will find a "proper" way to design for bending stresses, because the bolt people do not expect you to use A325 bolts in this manner.

Where I'm from, our book isn't green, but here's the approach I'd use.

First, they don't like to publish the yield strength (Fy) of high-strength A325 bolts. I'm not sure why. Probably because they'd have to carry out extensive expensive testing to certify a particular yield strength. The tensile strength is about 100 ksi, but it depends, to small degree, on bolt diameter.

Yield should be .5 of 100 (50ksi). Allowable bending stress on a pin is .9Fy. So say 45 ksi.

If there is too much uncertainty in this for you, then use a grade of steel with a certified yield stress. Use die for threading.

Take note that you do not have full diameter wherever threads exist.

Merry Christmas and all the best,

Watermelon

 

[CoryPad]

Watermelon,

Who are "they" that do not like to publish Fy data? The ASTM specification lists the "Alternative Proof Load, Yield Strength Method, min" in Table 4, with a value of 92 ksi or 81 ksi depending on fastener diameter.

Allowable bending stress on a threaded fastener should incorporate the stress concentration from the threads. I would simplify the bolt geometry to that of a notched bar.

I am aghast that welding and bending threaded fasteners would be considered standard practice.

 

[Watermelon]

Hi CoryPad:

I don't have acess to the ASTM specs, but isn't 92 and 82 the tensile strength and not the yield strength?

And yup, I think you're right about using A325 bolts in such a manner. I guess I missed the bigger picture, (shame on me) but the question was only with regard to bending stress. I would try to come up with something other than a welded A325 bolt.

 

[boo1]

ASTM A325 type 1
min tensile strength min yield
1/2-1" 120,000 psi 92,000 psi
1 1/8-1 1/2" 105,000 psi 81,000 psi

use the shear stress area to allow for the reduction for the threads.

Watermelon
shear values often are related to yield * 0.5

 

[1stproe]

I also have an application on my desk that is proposing to put bolts in bending. In this case, I want to use a bolt in a circumferential slot to lock out rotation of a bearing after positioning. The bolt will pass through a lower fixed flange, pass through a gap (approximately 3&quot into the top flange which is threadead. In the gap will be a nut to clamp the bolt to the bottom flange (providing a friction connection to resist torque) and a second locking nut the meets with the top flange. Between the two nuts is a residual gap of approximately 1/2" which results in a bending stress due to the eccentric shear at top and bottom of the gap. I'm currently searching for a reasonable stress concentration factor to apply at the thread root (currently I'm considering Kt = 3, but don't have reasonable data). I've had a look through Peterson "Stress Concentration Factors" which has a short discussion suggestion anywhere from 2.7 to 5 or 6 for a Kt. Please forward any suggested data on Kt you may have.

Thanks

 

[boo1]

Would not the thread type, loading reversals, material elasticity and fabrication method effect the stress conc. (IE fine rolled threaded being better)

Design of Machine Elements; Spotts 6th ed lists Kt=3.85

 

[juanon]

Dear All,

I have a situation where on removal of a blind flange on the vessel accsess hole it was observed that the stud ( 3/4-10UNC-2A)was bent approximately 1/16" ( measure at the tip of 5 1/8" long stud). Further review suggested that the the access hole stud holes got skewed.

The fabrication calls for angular deviation of +/- 1 deg.

My qeustion:

a)Is there a angular tolerance level of stud holes in the standatd ANSI B16.5 .

b) Is there any calculation that can be done to satisfy the code requirements for using the exisitng stud ( which in htis case has to include bending stress of the stud) ?

Any guidance on this.

regards