What is the shear strength of Grade 8 bolts?

A common question that we get here at Portland Bolt is in regards to the shear strength of bolts. The shear strength is the value at which the lateral stress on a bolt or screw causes it to fail. It is an important factor to take into consideration when designing structural steel or timber connections. Shear strength is also an easy value to roughly calculate when the ultimate tensile strength is known.

Below is a transcript of a recent live chat that we had on our website addressing shear strength. The chat was fielded by our very own Dane McKinnon and serves as a good example of what kind of response you can expect when you ask us a question. We’re substituting a generic name below for the customer to protect their privacy.

  • Visitor: Henry
  • Operator: Dane McKinnon
  • Company: Portland Bolt
  • Started: 26 Jan 2010 12:07:57
  • Ended: 26 Jan 2010 12:22:18

Henry:
What is the shear strength of grade 8 bolts 1/4″ – 1″

Call accepted by Dane McKinnon. Currently in room: Dane McKinnon, Henry.

Dane McKinnon:
Good afternoon Henry. Shear strength is typically 60% of tensile strength, and the minimum tensile strength of a grade 8 bolt is 150,000 psi. We have an FAQ that addresses this question here: https://www.portlandbolt.com/technical/faqs/bolt-shear-strength-considerations

Henry:
How are all grade 8 bolts the same tensile strength?

Dane McKinnon:
They are all the same tensile strength per square inch. The larger ones have a larger cross section of material, and so are therefore stronger. To calculate the tensile strength of a particular size, you would multiply 150,000 psi by the tensile stress area, found here: https://www.portlandbolt.com/technical/thread-pitch-chart/

Henry:
Thank you

Henry is now off-line and may not reply. Currently in room: Dane McKinnon.

Dane McKinnon has left the conversation. Currently in room: room is empty.

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35 comments

    @Ozgur Taner: Not true! Lots of bolted connections are designed for shear stresses, especially in designing trusses, mechanical applications such as automotive suspensions, cranes, etc. this is why, single shear and double shear tables are available for bolted connections.

    I would like to make some comments about the shear strength questions asked to Dave, which he is answering very well already, but here it goes anyways:

    When bolt joints are designed, the intention is not to work them under sustained shear stresses.

    It is obviously important to know the shear strength as unintentionally shear stresses may occur due to “slippages” between surfaces may occur, especially when the clamping force weakens, this may be due to many reasons but as an example we can name one: loosened bolts.

    The bolt joint designs typically intend is to keep the two overlapping surfaces together and not allow movements,”slippages”, in the direction that is parallel to the overlap surfaces. This is in fact achieved by the friction force which is related to the force applied in the normal direction and friction coefficient between two surfaces (making 90 deg. to overlap surface). Friction is generated by tightening the bolt. In some cases when joint is under dynamic loads, such as vibration and especially cyclic loads, a pre-tensioning is also done. This is mostly why engineers will give torque specs to be followed during assemblies.

    If the shear stresses on the bolt cannot be avoided bushings should be used to have the tensile stresses carried by the bolts and the shear stresses carried by the bushing.

    Thanks
    Ozgur

    @Adam- We are sorry, but we since we do not have any engineers on staff, we are unable to make application specific recommendations. We are happy to talk you through available materials and sizes, but we can’t tell you which is best for this. Apologies.

    is bolt designed for carrying point load (horizontal load). If yes, then what will be the breaking and bending capacity of bolt and time period of breaking and bending?

    @Kabir- We do not have the thread geometry table needed to properly calculate the shear strength for M16 bolts, but can tell you that shear is approximately 60% of tensile. If you know the grade of bolt you are using, you should be able to look up the tensile requirement and calculate the shear from that.

    Bolting an attachment to 31,000lbs excavator. Lateral down psi on the bolts at the attachments four points are in question. The psi is created with the attachment placed on the ground and using it to push, pull or raise the excavator tracks off the ground to spin and turn. Front of tracks come off the ground 2’ placing the machines weight on the rear of the tracks. Your thoughts ?

    would appreciate it if you can inform me of the breaking load of a grade 8 – 3/8 by 10″ bolt. This bolt will be inserted horizontally one on top of each another spaced 8″ apart. load will be in the center of bolt with 4″ of downward vertical pressure supported by 3″ on each end. Basically bolt is going thru 2- 2 by twelves on each end that equals 3″ on each end. A 2 by 4 -3-1/2″ is in between the 4- 2 by 12s. Sorry for my long question. I guess im asking what would it take to snap the bolt in half. Respectfully Robert

    @Robert- The breaking load of your bolt can be measured in two ways – shear and tensile. For a 3/8 grade 8 bolt, the shear would be 6,975lbs and the tensile would be 11,625lbs. We don’t have a good way to measure the bending point nor can we account for all the variables at play, but the tensile and shear values should help.

    Just yesterday I had a 3/4″ Grade 8 coarse thread bolt snap on me. We have 2 bolts in our distribution hitch for our camper, one is a Grade 5 and the other a grade 8 bolt. I stopped for gas and noticed half of the Grade 8 hanging out of the hitch. We were able to park and pull it all apart. The Grade 5 was bent as well. If the Grade 8 is a 30k lb shear strength, do you have any clue what might have caused this bolt to fail? Our camper is only 9k lbs with only about 1200lbs on the hitch. I can’t imagine anything in that system that would generate close to 30k lbs force.

    @Matt- You are correct that the shear force of that grade 8 bolt is about 30k lbs. However, there are more factor at play than just shear. For example, fatigue, the constant loading and unloading of force, can also cause a fastener to fail, and at a lower load than expected. We can’t be certain that fatigue is what’s causing your issue, but something like that caused it to break. An engineer who specializes in fracture analysis could probably shed more light on the subject.

    I was sent 1″ diameter bolts (8″ long) by a treehouse company. Don’t know the grade, but presume that treehouse companies must have the purpose in mind. Any rough estimate on a shear capacity for such? I want to be safe.

    @Michael- Assuming your bolt is mild steel like A307, and assuming the shear will be in the threads (worst case), Your minimum shear value will be 21,816lbs. (60,000 tensile x 0.60 shear factor x 0.606 root area of 1″ threads).

    My car weighs 5000#. It has four cylindrical jacking points built into the uniframe … on each side of the car, one point behind each front wheel and one point just forward of each rear wheel.

    When performing work on the car’s brakes, I commonly jack the car from axial center-line jacking points with a floor jack and insert a 10″ Grade 8 3/4″-10 bolt into each jacking point, then set the car atop 4 jack stands, each supporting a 10″ Grade 8 bolt.

    Is there any realistic risk that one or more of the bolts might fail … bend or snap in two … under these facts?

    @Arlen- The minimum shear strength of one 3/4-10 grade 8 bolt is approximately 30,060lbs, so we don’t see a scenario where your 5,000lb car should bend or break any of the bolts. Jack stands are typically rated from 3 to 12 tons, so we’d think that the jack stand would fail before the bolt does.

    @Davie- It would be the same as usual, 2000lbs equals 1 ton. For example if your bolt can withstand 4,000lbs of shear, that is two tons.

    @Steve- SAE J429 gr.8 bolts are not ‘rated’ for shear, however their approximate shear strength is 60% of tensile. Grade 8 tensile is 150,000psi, therefore shear is 90,000psi.

    @Edward- We are sorry, but we are not familiar with what a dyna bolt is, so we are unable to help. Apologies.

    Is there a strength requirement for 1/8″ Grade 8 Bolts? Every standard that I have seen only includes 1/4″ to 1 1/2″ sized bolts.

    @Clayton – No, 1/8″ is too small to be covered under most SAE or ASTM fastener standards. The only standard I am aware of that covers small screws is ASTM A574 for socket head cap screws. In that case the tensile is 180ksi minimum and the proofload is 140ksi.

    what is the shear and tension capacity of the steel anchor bolt ASTM 1554 grade 105 1.5 inches and how is it found?

    thank you

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