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 | |
|
||
Is F436 washers still needed for A325 bolts with standard hole only?
@Darrel- I do not believe that a washer is required by the AISC unless the structural detailer has called one out.
For exchanger sliding end where long slotted hole is provided (2 mm gap between saddle surface and bolt nut and no force is carried by connection), is it necessary to provide washer?
@Shilpi- I think with any standard or specification, there is room for interpretation if the given application isn’t text book. I can’t point to anything in the AISC that gives you a specific out, but if you feel like your application does not necessitate the plate washer, I don’t see any reason why you can’t approve a variance.
i can use plate washer for anchor rod A3, example anchor rod 1″
@Isidro- We are not familiar with A3 anchor bolts, so are unsure of the recommended washer.
If using a continuous bar washer in a slip critical connection between a beam and plate, does the surface between the washer and plate need to be Class A or just the surface between the beam and connection plate?
@Adam- Apologies, but we do not know the answer to this question.
do you recommend using lock washers for structural steel connections (column to beam) if not why?
@John- The AISC and RCSC do not mention lock washers one way or the other, so without a specific allowance or prohibition, we’d say it is allowed, but discretionary.