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:


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
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
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    @Senthilkumar- It depends on the application. Sometimes there are no washers used, sometimes washers are required. You’ll need to consult the AISC/RCSC to see when washers are required.

    @Narendra- We see two immediate disadvantages. First is cost. Washers may only be a few cents per unit, but over several thousand connections that can add up. Second is thread length. A325 and A490 bolts have short thread lengths to help control threads in the shear plane, so depending on how your joint was designed, it is possible that you may not have enough thread stick out to accommodate a washer.

    Please let me know

    1.What’s the standard thickness of F436 washer for the A325 bolt for which 3/4″ and 1″ Dia.

    2.What’s the difference between A325 and F1852 structural bolts vice Versa in A490 and F2256

    @Veeramani- The thickness for a 3/4″ F436 washer is 0.122″ to 0.177″.For a 1″ washer, the thickness is 0.136″ to 0.177″. As for your second question, the difference is bolt style. A325 and F1852 are mechanically and chemically identical, but A325 covers heavy hex head bolts, whereas F1852 covers tension control/twist-off bolts. The difference is the same for A490 (heavy hex head) and F2280 (twist-off/TC).

    hi I wanted to know about slotted bolted connection. Please let me know about in slotted connection the bolt nut with washer will come on the face of slotted side or on the either side. Please do your valuable advise.

    @Sonu- The thick washer need to go up against the slotted hole. If there are slotted holes on both faces, then two washers would be used, one under the head, one under the nut.

    How about a straight answer.
    Can we stack multiple washers on too long a bolt for a non-slip critical connection. If not why not?
    No ‘bad practice’ comments or quoting ‘Codes’ without solid reasoning please?

    @Peter- Yes, you can stack washers in order to make up for a too long bolt. The only prohibition is when a 5/16″ thick washer is required, you cannot use two thin washers to satisfy that requirement.

    How about for 1 1/8″Ø bolts with standard hole only? Is 5/16″ thick ASTM F436 still needed or it is okay to use standard washer (ASTM F436)

    @Darrel- A490 bolts with a standard sized hole can use a standard F436 washer. The 5/16″ thick washers are only required for oversized or slotted holes.

    How many f436 washers are allow for a pretensioned joint with A325 hex bolt in a slotted hole? The use of more than one F436 washer is not documented neither on AISC 13th Edition nor on 14th Edition.
    Please advice.

    @Miguel- I do not believe there is a prohibition on the amount of washers. The RCSC says you are able to use as many as you like.

    @Russell – You’ll need to be more specific as to the information you are looking for. “General Requirements” is too broad.

    hi , i would like to know how many washers required for each bolt of pipe rack structure steel connection joints in combined cycle power plant .

    with regards

    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?

    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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