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

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

Answer: 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

Posted by Anthony Porreco

Phone: (800) 599-6926 Email: anthony@portlandbolt.com
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