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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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.
What are the general requirements when using Gr 8.8, 10.9, etc bolts and washers?
@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
ali
@Ali- Apologies, but we do not have that information. You’ll have to talk to the project engineer.