Tag structural bolts

The short answer is that A490 heavy hex structural bolts have higher strength requirements than A325 heavy hex structural bolts. A325 bolts have a minimum tensile strength of 120ksi while A490 bolts have a tensile strength range of 150-173ksi.

In addition, there are a few other differences shown below.

Coatings

A325 structural bolts can be hot-dip galvanized and are commonly found with that coating. A325 galvanized bolts are popular due to their corrosion-resistant properties.

While A490 structural bolts are stronger, due to this strength they are unable to be hot-dip galvanized. Due to the high tensile strength of A490 bolts, they run the risk of hydrogen embrittlement by being galvanized. This could cause the bolts to fail prematurely and possibly become structurally unsound.

Configuration

The ASTM F3125 specification that both A325 and A490 bolts fall under is designated for structural bolts. Typically, structural bolts are heavy hex bolts with short bodies, shorter than average threads, and cannot have a reduced body diameter.

There are some exceptions allowed per the specification. Before 2016, ASTM A325 and ASTM A490 were stand-alone specifications. They have since been reclassified as grades within the F3125 specification. Originally, A325 and A490 bolts had to be heavy hex headed and were not allowed to be in another configuration. Also, the short thread length could not be altered.

However, under the new F3125 specification, any head style is allowed and the thread lengths can be altered. Alterations to the typical A325 and A490 configuration are designated by adding an “S” to the permanent grade marking on the head.

Another difference regarding thread lengths is that A325 bolts are mass-produced in a fully threaded version provided they are four diameters in length or less. This type of bolt is typically referred to as A325T.

Testing

A325 galvanized bolts that are being purchased with a nut and hardened washer are required to be rotational capacity tested. A rotational capacity test ensures the bolt assembly is capable of developing the proper clamping force. To pass the test, the assembly must reach a minimum amount of rotations and achieve the required tension before failure which depends on the diameter and length of the galvanized A325 bolt.

All A490 bolts are required to pass a magnetic particle test. This test is used to ensure there are no subsurface flaws or cracks in the steel of the A490 bolt.

Similarities

Besides the differences listed above A325 bolts and A490 bolts are used for the same applications. They are both used as steel connectors. They both will have Unified National Coarse Threads unless otherwise specified. They both have a diameter range from ½” to 1 ½”, however, it is uncommon to find an A490 bolt smaller than 5/8”.

Both A490 and A325 bolt grades are also made from quenched and tempered medium carbon or alloy steel. Both A490 and A325 structural bolts are available in types 1 and 3. Type 1 is the “regular” chemistry, while Type 3 bolts possess weathering characteristics.

Availability

Both A325 and A490 bolts are mass-produced and readily available in the marketplace. A325 bolts can be easily found in diameters ranging from ½” through 1-1/4” diameter and in length up to about 6” or 8”. A490 bolts are readily available in diameter ranges from 5/8” through 1-1/4” in similar lengths. These are some of the most commonly used bolts in the construction industry.

Manufacturing

Portland Bolt provides or manufactures A325 and A490 heavy hex structural bolts in any diameter and length required for the project. We typically produce these structural bolts when they are larger in diameter or longer in length than mass-produced sizes.

We also manufacture these structural bolts when they require extended thread lengths. On the rare occasion when a head style other than heavy hex is required, those would also be manufactured in our 137,0000-square-foot manufacturing and galvanizing facility.

When Portland Bolt manufactures a structural bolt, the first step is to cut the 20’ round bars of medium carbon or alloy steel round bar to length. The end of the round cut round bar is heated and a heavy hex head is forged on the end.

The bolts then undergo a heat-treating process that develops the proper mechanical properties of the given grade. Bolts are then tested to insure specification compliance before being chamfered and threaded. When required, A325 bolts also are galvanized in our in-house hot-dip galvanizing line.

For emergencies, Portland Bolt stocks headed blanks. These are A325 and A490 bolts that are headed, heat treated, and tested to ensure they meet the requirements of the specification.  They are then put in inventory without threads and used when small quantities of custom sizes or short lead times are required.

These headed blanks simply need to be taken from inventory, cut to length, galvanized when necessary, and shipped in only a couple of days.

Portland Bolt is often asked which grade should be used in a structural application. More specifically, when asked what grade is required for a particular fastener that we are receiving an inquiry on, companies will often reply “it’s a structural grade” or “it’s for a structural application”. This usually is the result of a fastener being specified on a drawing, but not having an ASTM grade referenced. Unfortunately this is a little too ambiguous to know what the exact grade of bolt required is. There are two ASTM graded fasteners that are specifically structural bolts, ASTM A325 and ASTM A490. Both of these fasteners typically have a heavy hex head and are used in structural steel connections. The ASTM F3125 specification, which now governs ASTM A325, A325M, A490, A490M, F1852 and F2280, may also be specified, but it is necessary to find out which of the above grades they need under the F3125 specification. If the bolt in question is not being used in this application, then simply calling out a “structural grade” is not enough information to know which grade of fasteners would work best. In this case, the next step would be to contact the engineer involved in the particular project requiring the fasteners and have them specify the appropriate grade.

According to ASTM A490-11, the answer is no. The A490 specification only covers diameters from 1/2″ to 1-1/2″, inclusive. An A490 bolt cannot be made above 1-1/2″ diameter; ASTM does not allow it. Since there are no chemical requirements, hardness requirements, or strength requirements for A490 fasteners above 1-1/2″ diameter, no manufacturer would be able to certify a bolt above 1-1/2″ to ASTM A490. An alternative grade to consider in diameters above 1-1/2″ would be an ASTM A354 Grade BD. This grade of fastener can be manufactured in diameters from 1/4″ to 4″, inclusive, and is virtually identical to A490 in strength and chemistry through 1-1/2″ diameter. However, there is no way to compare the two grades above 1-1/2″ diameter, due to the fact that A490 does not have any strength requirements in diameters larger than 1-1/2″. Additionally, A354 Grade BD bolts do not require a Magnetic Particle Inspection, which is a requirement for ASTM A490 structural bolts. A490 bolts are only available in a heavy hex head configuration, while A354 Grade BD bolts are not restricted to a specific configuration. Because of these configuration restrictions, A490 bolts often readily available as stock items in certain diameter/length combinations, but A354 Grade BD bolts typically need to be made per order.

If the situations comes up where an A490 bolt is specified in a size larger than 1½” diameter, it is advisable to have the engineer of record specify a grade of fastener that is available in that diameter.

There is no difference between these bolts. The “X”, “SC”, and “N” simply identify the type of connection the bolts are used in. “X” and “N” are bearing type connections, where the bolts are being used in shear.

“X” means these particular A325 heavy hex structural bolts will be used in a bearing type connection where the threads will be excluded from the shear plane, whereas “N” means the threads are included in the shear plane. “SC” signifies a slip-critical connection where the bolts are not being used in shear, but instead, the tension from the connection resists the shearing force. You simply require the same standard A325 heavy hex structural bolt but will be using them in three different connection types.

“SC”, “N”, and “X” specify solely the type of connections to use the A325 bolt in.

According to Note 2 in Section 3.8 (Ordering Information) of the new F3125 specification covering high strength structural bolts, “Bolts are sometimes detailed with names such as A325 HS, A325 SC, A325 x or A490 N. These names relate to connection design and bolt installation, but do not change the manufacturing requirements and are preferably not shown on bolt orders.”

There are other types of A325 bolts that do affect the type of bolt that needs to be purchased. For these, see the following chart.

The majority of A325 bolts being made in the market are A325 Type 1 and are available both plain and hot-dipped galvanized. Type 2 was withdrawn in 1991 and no longer is in use. Type 3 is a naturally corrosion-resistant weathering steel that typically is used in a plain finish (no finish). Availability for the steel can be limited and standard, mass-produced bolts start at 5/8″ diameter. Below 5/8″ diameter, heat-treatable weathering steel is not commonly available.

A325T bolts (covered under supplementary requirement (S1) of the A325 specification signifies that the A325 bolt must be completely threaded, but is limited to 4 times the diameter in length. Fully threaded A325 bolts longer than 4 times the diameter do not comply with the specification, will not be available in the marketplace, and technically cannot be manufactured. ASTM A449 should be considered in lieu of A325 bolts with extended threads that don’t meet the requirements of A325T. However, ASTM 3125, the new combined high strength structural bolting specification, added a new type S that allows for an A325 bolt to have altered thread length and head dimensions. Previously, altered thread lengths meant switching grades or accepting a bolt that didn’t technically conform to the A325 specification, but now marking the bolt head with an S allows for extended threads or using different head dimensions.

The connection information provided in this FAQ is applicable to not only A325 heavy hex structural bolts, but to A490 structural bolts as well.

According to section 9 of the ASME B18.2.6 specification, which covers dimensional tolerances for hex bolts, A325 and A490 structural bolts can actually be produced between .12 to .25 of inch less than the published length and still meet the specification.

This small difference isn’t much, but mass bolt producers save a significant amount of money in the long run by producing all of their sizes slightly shorter than the advertised length. It doesn’t make sense but that’s the way it is. The chart below outlines the allowable tolerances under ASME B18.2.6.

Bolt Length Tolerances

Structural Bolt Length Tolerance

When considering bolt lengths for specific applications keep these length tolerances in mind. Also, remember RCSC Specification Section 2.3.2 recommends to ensure proper thread engagement the end of the bolt should be at least flush with the nut. These bolt tolerances are especially important when working with short bolt lengths. AISC offers a formula for calculating correct lengths to order bolts under Table 7-15 (7-80).