It is known that the required grade of washer to be used with an A325 or A490 structural bolt is an ASTM F436 hardened washer, however whether or not a washer is required to be used in a given application is often brought into question. The purpose of using a flat washer with a bolt is to provide a smooth bearing surface under the head of the bolt and / or nut. There are different grades and styles of washers, which may differ in their thickness and outside diameter, providing a larger or smaller bearing surface. Per the ASTM A325 and A490 specification, they are specific to the grade of washer that is recommended, however it is not stated that a washer is required.
Although the ASTM A325 and A490 specifications do not require the use of a washer, there are a few circumstances where we know for sure that F436 hardened washers are required. When using A490 structural bolts, the strength of the material being connected can determine whether or not a washer should be used. Specifically, if the material being connected has specified minimum yield strength of less than 40 ksi, a washer should be used. According to the AISC 13th Edition, page 16.2-40:
“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 the nut”
It is worth noting that an A490 strength, round head tension control bolt who’s head meets the bearing surface diameter requirements of ASTM F1852, does not require a flat washer under the head. The only other circumstance where a washer is required is if the hole in which the bolt is being inserted is oversize or slotted. The washer requirements for this situation are found in Table 6.1 in the AISC 13th Edition, page 16.2-41, which is summarized below.
| ASTM Grade and Diameter | Hole Type in Outer Ply | Required Washer |
|---|---|---|
| A325 or F1852 (½" - 1½") | Oversized or Short-Slotted | ASTM F4361 |
| Long-Slotted | 5/16" thick plate washer or continuous bar2, 3 | |
| A490 (≤ 1") | Oversized or Short-Slotted | ASTM F4361 |
| Long-Slotted | 5/16" thick plate washer or continuous bar2, 3 | |
| A490 (> 1") | Oversized or Short-Slotted | 5/16" thick ASTM F4362, 4 |
| Long-Slotted | ASTM F436 with a 3/8" thick structural grade plate washer or continuous bar2 | |
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brief explanation please as to why two stacked washers are not as strong as a single washer twice as thick. Many thanks
@Duncan- There will be some slip between two or more washers, much like a stack of papers is pliable and fiberboard of the same thickness is much less so. The AISC and RCSC is very specific that the substitution of stacked washers for one thick washer is prohibited.
is it has any limitation to use how many washer? in our project due to construction issues it need to use 3 washers? is it true?
@Hamid- We are not aware of any limitations on the number of washers allowed.
Hi,Please advice me if we can use plain washer for anchor bolt or not
@Hany- If your anchor bolt is plain finish, then a plain finish washer is what we would provide.
Thank you guys for useful information, I’d like to ask, For step bolt with 2 nuts, one from inside and the other outside, do we need two washers or one inside enough (where we will apply the torque)?
*step bolt used as steps actually and not for connecting parts.
Thanks in advance.
@Ahmed- I am sorry, but we are not sure if the second washer is required. You might need to contact your project engineer.
If we are using DTI wahers for A325 M bolts , how to tighten the bolt assembly ?
Can we use spanners and check the specified gap while tightening the bolt assembly ?
Or what is the method please ?
Thanks and Best Regards ,
vaswani
@S.U.- The two US manufacturers of DTIs both have detailed instructions on their websites. Look at Turnasure or Applied Bolting’s websites.