ASTM A325 (and A490) Structural Bolt Connection Types
Question: My company is looking to purchase A325SC, A325N, and A325X heavy hex structural bolts. What is the difference between these three?
Answer: 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||Slip critical connection.|
|N||Bearing type connection with threads included in the shear plane.|
|X||Bearing-type connection with threads excluded from the shear plane.|
“SC”, “N”, and “X” specify solely the type of connections to use the A325 bolt in. 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.
|TYPE 1||Medium carbon, carbon boron, or medium carbon alloy steel.|
|TYPE 2||Withdrawn November 1991.|
|TYPE 3||Weathering steel.|
|T||Fully threaded A325. (Restricted to 4 times the diameter in length)|
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 Type 2 was withdrawn in 1991 and no longer is in use.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.
The connection information provided in this FAQ is applicable to not only A325 heavy hex structural bolts, but to A490 structural bolts as well.