Load indicating washers are used to indicate the achievement of a specified minimum bolt tension. The ASTM specification covering load indicating washers is F959 and has two different types; 325 (type 1 and type 3) and 490 (type 1 and type 3). When used, the washer is typically placed beneath the head of the bolt. As the bolt is tightened, the protrusions on the washers compress. Once the protrusions have flattened, the installation of the structural bolt and nut assembly has achieved proper tension. Depending on the type of load indicating washer used, the achievement of proper tension is apparent by either the presence of a silicon that “squirts” from the underside of the washer itself, or by the use of a feeler gauge.
A common question that is asked is whether the F959 A325 load indicating washer can be used with an A490 structural bolt. The answer is, no it cannot. The reason for this is because both the types 325 and 490 have different acceptable compression load ranges. According to ASTM F959, the type 325 load indicating washer has a lower acceptable compression load range than the type 490. For example, when a 1” diameter A325 structural bolt is being tensioned with the use of a load indicating washer, the compression load range considered acceptable is between 51 and 61 kips. This would not be an acceptable compression range for a 1” diameter A490, which has a range of 64 to 77 kips. Below is a chart showing the different acceptable compression ranges for F959 type 325 versus 490. As you can see, if a type 325 load indicating washer was used with an A490 structural bolt, the protrusions would compress at a lower range in kips than what is required. It is worth noting that with the new ASTM F3125 specification, which now governs ASTM A325, A325M, A490, A490M, F1852 and F2280, you still need to know which grade of bolt falling under F3125 is being used since that will determine which grade of load indicating washer is needed.
F959 Acceptable Compression Load Range
| Diameter | Type 325 and 325-3, in kips | Type 490 and 490-3, in kips |
|---|---|---|
| 1/2 | 12 – 14 | 15 – 18 |
| 5/8 | 19 – 23 | 24 – 29 |
| 3/4 | 28 – 34 | 35 – 42 |
| 7/8 | 39 – 47 | 49 – 59 |
| 1 | 51 – 61 | 64 – 77 |
| 1-1/8 | 64 – 77 | 80 – 96 |
| 1-1/4 | 81 – 98 | 102 – 122 |
| 1-3/8 | 97 – 117 | 121 – 145 |
| 1-1/2 | 118 – 143 | 148 – 178 |
There are other grades of load indicating washers besides ASTM F959, which fall under the specification F2437. ASTM F2437 load indicating washers indicate a specified bolt tension in cap screws, bolts, anchors, and studs. There are two types of load indicating washers that fall under F2437; type 1, which are available in grades 55 and 105, and type 2, available in grades 5 and grade 8. The recommended grade of bolt to be used with the type 1 grade 55 is ASTM F1554 grade 55 or A307, for type 1 grade 105 the grade of fastener would be ASTM A193/A193M B7 or F1554 grade 105, type 2 grade 5 would be ASTM A449, A354 grade BC, and J429 grade 5, and for type 2 grade 8, the recommended fastener grade would be A354 grade BD or J429 grade 8. These washers are used in the same way as F959 load indicating washers having their own specified compression load range, depending on the grade. Similar to type 325 versus type 490, the grade 55 and grade 5 load indicating washers cannot be used with a grade 105 or grade 8 fastener, due to the acceptable range of compression being much lower than what is required for grade 105 or grade 8.
I have been looking at A490 DTI’s and they are all round until now I am finding some with 4 little ears sticking out what are these?
@Richard- Some A490 DTIs have ears, some have scallops or notches, but they both serve the same function, and that is to show the inspector where to place the feeler gauge in order to verify the compression of the bumps.
1. Project requires 1″ A490 bolts and I am looking at using DTI squirt type washer. Using 1nut/harden washer what will the DTI washer add to the grip?
2. The connecting holes on both parts will be slotted (1.0625″x 1.250″) will this require a additional harden washer under the head of the bolt for the DTI?
@Cecil- The DTI washer will add 0.270″ before compression, and approximately 0.173″ after compression. I was unable to find a publication that specifically talked about DTIs and slotted holes, but the RCSC does require an F436 with any slotted hole, so I would assume that would be true regardless if there is a DTI in use or not.