Are washers required to be used with an A325 or A490 structural bolt?

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
  1. This requirement does not apply to round head tension-control bolts that meet the dimensional tolerances of F1852
  2. Multiple stacked washers that add up to a thickness of 5/16" do not meet the requirement
  3. The washer must be structural-grade steel, but does not need to be hardened
  4. A 3/8" thick plate washer and a regular F436 washer can be used instead. The plate washer doesn't need to be hardened
Written April 19, 2011 by
Anthony PorrecoAnthony Porreco

Phone: 800.599.6926


    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 ,

    @S.U.- The two US manufacturers of DTIs both have detailed instructions on their websites. Look at Turnasure or Applied Bolting’s websites.

    @Lakshimi – Washers are not ever required in structural connections, but are usually used. You will need to contact the project engineer to see if washers are required for your job.

    @Rahul – 0.055% Phosphorous is technically over the maximum allowed, so that could be grounds for rejection. However, at the discretion of the project engineer, it could perhaps be accepted on a waiver.

    Though I am a union industrial electrical contractor, my other company is a union millwright company and I provide millwrights to conveyor manufacturers, at O’Hare Airport. We are in the mist of a conveyor installation and the contract drawing details state ASTM bolts and nuts. The bolts and nuts are not ASTM and Portland’s Amanda has been very useful in this research. The material is supplied by the conveyor manufacturer and my millwrights are not very comfortable with using material that is not spec’d
    The question now comes to the washers as the details calls for ASTM A325 washers.
    Question: Are ASTM A325 washers, stamp as such?
    Art / President

    @Art – The washers required for ASTM A325 bolts are made per ASTM F436. The requirement for F436 washers is simply to be marked with the manufacturer’s stamp, but some manufacturers also mark them with “F436”

    @Ali- Plain finish washers for use with A490 bolts have a HRC range of 38-45. Only hot dip galvanized washers have the lower 26-45 HRC range, but they should not be used with A490s because of the hot dip zinc coating.

    Hi, Could you tell me what the Minimum Yield Strength for 2-1/4″ F436 Washers is for use with B7 studs.

    Many thanks

    @Matthew – F436 washers are not tested for tensile or yield (because the material thickness is too thin for machine out test specimens), and are qualified on the basis of hardness only. Hardness range for plain finish washers is 38-45 HRC, and hot dip galvanized 26-45 HRC.

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