Question on installing washers on structural bolts?

[alanganes]

This is possibly one of those "can of worms" discussions, but here goes:

I was on the fringes of a discussion the other day concerning bolting together some structural steel plates. Imagine three plates, each 1/2" thick. There are holes through all three plates that closely fit appropriately sized structural bolts. The bolts are such that the unthreaded shank of the bolt passes through all three plates.

Now imagine that the shank is long enough so that it will take two thick structural washers and the appropriate nut. The question at hand is this: Does it matter if there is one washer on each side of the plates (one under the bolt head and one under the nut) or if the bolt head has no washer and there are two washers under the nut? If we assume that in both cases the bolt is tightened to the same appropriate amount, and that in both cases the only the shank of the bolt is in the shear area of the joint, is one configuration better that the other strength-wise?

One of the people involved was saying a bolt supplier they deal with (a large industrial bolt specialty pace) says that washers should go only on the nut side and none under the bolt head. This was a largely hypothetical discussion, but it got me curious. I don't think I can see any difference but I am no structural engineer either.

What does the HSM brain trust say?

[Joel]

Visualize your assembly, except make the plates out of wood, and crank down the bolt.
A washer on both sides is superior. A washer spreads the load from the smaller bolt head (dramatically so with a SHCS), reduces damage from torquing, allows better control of torquing, and as Carroll Smith says "if nothing else, it just shows you care".


Added on edit: The above is the short version. Entire books have been written about bolted assemblies, so if you would like more/specific details, just ask and many here can provide it.

[Rustybolt]

Depends on the bolt and material. When bolting box sections I always used a grade 8 large washer under the bolt head. I did that because as sure as god made little green apples some ham handed fool would tighten up hard enough to dimple the steel.

[wierdscience]

This is something I deal with on a semi-regular basis. The answer is it all depends.

https://www.boltcouncil.org/files/2004RCSCSpecification.pdf

RESEARCH COUNCIL ON STRUCTURAL CONNECTIONS SECTION 6. USE OF WASHERS

6.1. Snug-Tightened Joints Washers are not required in snug-tightened joints, except as required in Sections 6.1.1 and 6.1.2.

6.1.1. Sloping Surfaces: When the outer face of the joint has a slope that is greater than 1:20 with respect to a plane that is normal to the bolt axis, an ASTM F436 beveled washer shall be used to compensate for the lack of parallelism.

6.1.2. Slotted Hole: When a slotted hole occurs in an outer ply, an ASTM F436 washer or 5/16 in. thick common plate washer shall be used to cover the hole.

6.2 Pretensioned Joints and Slip-Critical Joints Washers are not required in pretensioned joints and slip-critical joints, except as required in Sections 6.1.1, 6.1.2, 6.2.1, 6.2.2, 6.2.3, 6.2.4 and 6.2.5.

6.2.1. Specified Minimum Yield Strength of Connected Material Less Than 40 ksi: When ASTM A490 bolts are pretensioned in connected material of specified minimum yield strength less than 40 ksi, ASTM F436 washers shall be used under both the bolt head and nut, except that a washer is not needed under the head of an A490-strength round head twist-off bolt that meets the minimum bearing circle diameter requirements of ASTM F1852.

6.2.2. Calibrated Wrench Pretensioning: When the calibrated wrench pretensioning method is used, an ASTM F436 washer shall be used under the turned element.

6.2.3. Twist-Off-Type Tension-Control Bolt Pretensioning: When the twist-off-type tension-control bolt pretensioning method is used, an ASTM F436 washer shall be used under the nut as part of the fastener assembly.

6.2.4. Direct-Tension-Indicator Pretensioning: When the direct-tension-indicator pretensioning method is used, an ASTM F436 washer shall be used as follows: (1) When the nut is turned and the direct tension indicator is located under the bolt head, an ASTM F436 washer shall be used under the nut; (2) When the nut is turned and the direct tension indicator is located under the nut, an ASTM F436 washer shall be used between the nut and the direct tension indicator; (3) When the bolt head is turned and the direct tension indicator is located under the nut, an ASTM F436 washer shall be used under the bolt head; and, (4) When the bolt head is turned and the direct tension indicator is located under the bolt head, an ASTM F436 washer shall be used between the bolt head and the direct tension indicator. 6.2.5. Oversized or Slotted Hole: When an oversized or slotted hole occurs in an outer ply, the washer requirements shall be as given in Table 6.1. The washer used shall be of sufficient size to completely cover the hole.

[chipmaker4130]

. . .There are holes through all three plates that closely fit appropriately sized structural bolts. . .

How close is 'closely'? I personally think washer placement, steel-on-steel, is largely irrelevant. BUT, if the hypothetical close-fitting holes have sharp corners, a properly sized washer under the head is a must. No sharp corners are allowed to impinge on the bolt head to shank radius.

[Paul Alciatore]

Wow, thanks for that. Seems like washers are "required" when the hole is irregular (angled or slotted) or any kind of tension/torsion operation is to take place.

I could have missed something, but it does not seem to say anything about the strength of the joint. At least, not directly. I believe the strength of the joint is more related to the size and grade of the fastener.

But, IMHO washers can improve the durability of a joint that may be of sufficient strength originally. By spreading the force applied by torquing a bolt, there can be less distortion of the metal being born upon, both immediately and with time and cycles of loading. Joel's wood example is a good one to consider.

Perhaps the document being referenced does not exhaust the subject. Perhaps it only addresses some aspects of it.

This is something I deal with on a semi-regular basis. The answer is it all depends.

https://www.boltcouncil.org/files/2004RCSCSpecification.pdf

RESEARCH COUNCIL ON STRUCTURAL CONNECTIONS SECTION 6. USE OF WASHERS

6.1. Snug-Tightened Joints Washers are not required in snug-tightened joints, except as required in Sections 6.1.1 and 6.1.2.

6.1.1. Sloping Surfaces: When the outer face of the joint has a slope that is greater than 1:20 with respect to a plane that is normal to the bolt axis, an ASTM F436 beveled washer shall be used to compensate for the lack of parallelism.

6.1.2. Slotted Hole: When a slotted hole occurs in an outer ply, an ASTM F436 washer or 5/16 in. thick common plate washer shall be used to cover the hole.

6.2 Pretensioned Joints and Slip-Critical Joints Washers are not required in pretensioned joints and slip-critical joints, except as required in Sections 6.1.1, 6.1.2, 6.2.1, 6.2.2, 6.2.3, 6.2.4 and 6.2.5.

6.2.1. Specified Minimum Yield Strength of Connected Material Less Than 40 ksi: When ASTM A490 bolts are pretensioned in connected material of specified minimum yield strength less than 40 ksi, ASTM F436 washers shall be used under both the bolt head and nut, except that a washer is not needed under the head of an A490-strength round head twist-off bolt that meets the minimum bearing circle diameter requirements of ASTM F1852.

6.2.2. Calibrated Wrench Pretensioning: When the calibrated wrench pretensioning method is used, an ASTM F436 washer shall be used under the turned element.

6.2.3. Twist-Off-Type Tension-Control Bolt Pretensioning: When the twist-off-type tension-control bolt pretensioning method is used, an ASTM F436 washer shall be used under the nut as part of the fastener assembly.

6.2.4. Direct-Tension-Indicator Pretensioning: When the direct-tension-indicator pretensioning method is used, an ASTM F436 washer shall be used as follows: (1) When the nut is turned and the direct tension indicator is located under the bolt head, an ASTM F436 washer shall be used under the nut; (2) When the nut is turned and the direct tension indicator is located under the nut, an ASTM F436 washer shall be used between the nut and the direct tension indicator; (3) When the bolt head is turned and the direct tension indicator is located under the nut, an ASTM F436 washer shall be used under the bolt head; and, (4) When the bolt head is turned and the direct tension indicator is located under the bolt head, an ASTM F436 washer shall be used between the bolt head and the direct tension indicator. 6.2.5. Oversized or Slotted Hole: When an oversized or slotted hole occurs in an outer ply, the washer requirements shall be as given in Table 6.1. The washer used shall be of sufficient size to completely cover the hole.

[darryl]

I will often use a size down washer under a bolt head if it fits. The rounded side of a washer is up, in other words under the bolt head. If the washer cannot seat to the bolt head, then the next larger size, the one called whatever the bolt diameter is, is used. I want to see the whole contact area touching with no gap, and I also want to see the washer centered. Little bit of ocd here-

I can see that if the bolt centers the washer the way it wants, that could be better because it should mean that there is equal pressure all around the bolt head. I don't know if it works this way in practice, but it sounds good. But I'm kind of thinking that the larger hole in the washer is just so that it will always fit and be easier to put on the bolt.

In this example, bolting 1/2 inch plate, the washer adds nothing to the integrity of the join unless it adds a squish factor to equalize bolt head pressure. Putting it under the nut likewise does nothing except add a squish factor (if that is even active) and provide a surface for the nut to grind on as it's being tightened.

Many washers show a burr from the stamping, usually on one side of the hole. If the goal was to equalize clamping pressure around the head, then this burr has to be crushed first before the rest of the contact is made. This is an offset pressure to begin with, so I'm not really seeing the advantage of a washer if the materials are thick enough in the first place.

Keeping wrench marks and socket marks off the main material is a function that a washer will provide.

Where your clamped material is relatively thin, washers provide some load spreading capability. And if you're bolting through square tubing, washers help increase clamping strength with less crushing of the tubing.

I don't know what the real rules are, but those are mine.

[elf]

How do you pretension a bolt?

[epicfail48]

Ive always assumed that the rule was if the hole is a slightly sloppy fit on the fastened and needs a slight gap to be bridged, or the head on a side rotates, there needs to be a washer under it, either to spread the load out or to allow for the bolt head/washer to rotate without gouging into the part. No idea if thats the actual rule, thats just what i use

[wierdscience]

How do you pretension a bolt?

https://engineeringcivil.org/article...0the%20preload.

[RMinMN]

I was on the fringes of a discussion the other day concerning bolting together some structural steel plates. Imagine three plates, each 1/2" thick. There are holes through all three plates that closely fit appropriately sized structural bolts. The bolts are such that the unthreaded shank of the bolt passes through all three plates.

This isn't quite clear but....the holes are a close fit to the shank of the bolt. There are no threads there but the bolt has threads beyond the steel plates to fasten them together. You want a tight joint so you tighten the nut down...and the threads run out before the nut is tight enough against the plates. In that case you effectively extend the thickness of the plates to allow the nut to completely tighten by putting in the appropriate number (usually one) of washers. The washers have no other function in this case except to allow the nut to tighten fully.

[alanganes]

Thanks, interesting input. No doubt this is an "it depends" sort of question, there can be lots of variables.

To answer what a few have asked, "close fitting hole" in this case means that there is 0.025" clearance between the shank of the bolt and the hole. The bolts that were being discussed were 20MM, 10.9 grade with proper grade nuts and thick structural washers. No slope to the material faces, they are all flat.

The discussion came up due to something like what RMinMN speaks to. When a bolt was put through the 3 plates and a washer was placed on the nut side, the threads came through the washer such that the last few threads were within the thickness of the washer. There were a couple of bolts where the threaded section was a few turns short, such it looked like you could potentially run out of threads before the bolt was properly tightened, though it could look and feel tight. The discussion came up about adding another washer which then kicked off a minor debate about where the most appropriate place for the second washer was. In the end they just avoided using the couple of bolts with the short threads. But the discussion got me curious about if there was a right or wrong way to do this or if it even mattered.

[Richard P Wilson]

If it was a job of mine, I'd put a washer under the bolt head and a washer under the nut. Double washers under a nut always looks like a lash up to me.
Of course, if its a slotted or oversize hole, you must put large thick washers under head and nut.
The other alternative, if its only a few bolts, is to run a die down the thread for a couple of extra turns.

[Mcgyver]

Ive always assumed that the rule was if the hole is a slightly sloppy fit on the fastened and needs a slight gap to be bridged, or the head on a side rotates, there needs to be a washer under it, either to spread the load out or to allow for the bolt head/washer to rotate without gouging into the part. No idea if thats the actual rule, thats just what i use

Along those lines, on installations, its often necessary to move a hole via torch in say a base plate to accommodate anchors whose location is a bit off. SOP is to use a plate, as thick and large as the situation requires (it'll be spec'd), as a washer that is then welded to the base plate after things are bolted down.

[J Tiers]

In aircraft construction, there are various standard bolt sizes and lengths. The rule has been that threads are never allowed within the bolted materials, so if they would be, the next longer bolt is used, and washers make up the difference to allow tightening the nut without running out of threads.

Info is from a manual I have that was used at (IIRC) Lockheed.

As far as I know, plain washers are used to spread the load, to provide a good surface for the bolt head or nut, to avoid a rough or sharp edge, to modify the friction and allow more consistent torque, to fill up space (as with the aircraft example) to convert an angled surface to a perpendicular one, etc. If something like that does not apply, then they may not be needed.

Lots of bolts (usually hex, less often square) have a surface under the head that is finished flat, and provides some of the features that a washer might. That might change the need for a washer.

As for which side, washers typically lower friction, and if the tightening can only be done on the nut side, then the head directly on the surface may be better. Otherwise my default would be to put one on each side rather than two on the nut.