My company is looking to purchase A325SC, A325N, and A325X heavy hex structural bolts. What is the difference between these three?

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.

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.

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TYPE 2 Withdrawn November 1991.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)
M Metric A325.
S Modified thread length or head dimensions.

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 Type 2 was withdrawn in 1991 and no longer is in use.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.

2015 Update: Under the new ASTM F3125 structural bolting specification that covers A325, A325M, A490, A490M, F1852 and F2280, A325 and A490 bolts can have their head dimensions or thread lengths altered.  If the bolt is altered the head needs a “S” added to the grade marking notifying that the bolt has been altered from the standard dimensional requirements.

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

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59 comments

    Recently I went through one project there it mentioned “Connections with bottles with ASTM A325 high strength bolts N OR F type connections.”. Can anyone please explain what is F type connection?

    @SBK- Type F is the old call out for slip critical connections (SC). The ‘F’ designation hasn’t been used in many years.

    Is there a specific torque value for the A325N and is the torque value different if it were “X” or “SC”?

    @John- The only difference between the N and X designation is whether or not threads are allowed in the shear plane. They are commonly used in shear, and so are only tensioned to snug tight, but it depends on the job. For SC designations, those are more commonly used in tension instead of shear, and these are commonly tensioned to the minimum value recommended by the AISC, which correlates roughly to the torque on our website here: https://www.portlandbolt.com/technical/bolt-torque-chart/#bolt-torque-a325

    What is the meaning of twist off bolt? Where they are used and what are the machines used for this?
    Can you help me to find the answer.

    @Balasubramanian- A twist off bolt is a tension control bolt made per ASTM F3125 grade F1852 or F2280 and used in structural applications. Many places that specialize in selling these bolts are also able to supply or rent the installation guns.

    @PP-The “N” designation has more to do with the joint/connection type than the bolt itself. The ‘N’ means that threads can be included in the shear plane. As for whether or not threads can be included in the shear plane in slip critical connections, we are not certain – you would need to contact a structural engineer or the AISC.

    Is there any difference between A325 SC and A325 TC bolt in terms of tension and shear capacity?

    Thanks in Advance.

    @Kasi- There are three connection types, N, X, and SC. The connection type has nothing to do with the bolt itself, the same bolt is used in all. The difference is how the joint was designed, and whether it was designed to allow threads in the shear plane, exclude threads, or be slip critical. The individual joint design will dictate the connection type.

    @Sridhar- Normally, the shortest length available is about 1.5 times the diameter, so the shortest 1/2″ bolt would be 3/4″ long. For the maximum length, we can manufacture bolts to any length needed, up to 20 foot long and occasionally longer.

    @Sridhar- The available lengths will vary depending on the diameter, and can be as short as 1/2″ and as long as 36″+. Additionally, we are able to manufacture special length bolts if you are unable to find what you are looking for.

    @Sridhar- The available lengths will vary depending on the bolt diameter. Additionally, we are able to manufacture custom lengths on some grades and styles.

    @Balasaheb- Our only guess would be that the designer, after looking at all the information, decided that including the threads in the shear plane was not detrimental to the integrity of the structure.

    Is there installation difference between for X and N? Say if I design it for X, how do it know it is end up N or X?

    @James- The installation method is the same, but for X type connection, the designer needs to make sure that the threads are excluded from the shear plane, so a longer bolt may be needed. For N type connections, threads are allowed in the shear plane, so the only concern is that the bolt is long enough to stick through enough to fully tension. If you design for X, you will need to make sure that the bolts are long enough that only the unthreaded shank is in the shear plane. That can be done by specifying the correct length of bolt, taking into account the thread length. Additional washers can also be added if necessary, in case the longer bolt means that the threads end before the nut is fully tensioned.

    @Karthick- The threads in the shear plane will affect the bolt’s shear capacity since the minor diameter of the threads is smaller than that of the unthreaded shank. The tension capacity is not affected.

    Hi,

    In one project, Engineer’s has recommended using TC Bolts at the connections. But due to insufficient access for Tensioning Gun, can we used the Turn of Nut method on TC Bolts.

    @Jeff- Using a TC bolt without the matching gun is highly unusual and we are not certain if you could achieve the proper tension. We are not in a position to say one way or the other if it will be acceptable in your situation, you would need to pass it by the project engineer.

    I have 2 question..
    1) When are Slip critical bolts mainly used?
    2) Why N type bolts are commonly used and not X bolts since X bolts have higher capacity compared to N bolts?

    @Nomaan- Slip critical bolts are used in slip critical applications and joints. Those are applications where failure of the joint would be catastrophic for the structure. As for A325N or A325X bolts, they are the same bolt, the only difference is in how they are used. A325N joints allow threads in the shear plane, whereas A325X joints do not. If there is a difference in capacity, that is a design choice, not a bolt difference.

    @Harvendra- Yes, A490 TC bolts are available. We believe they are acceptable for use in slip critical connections.

    @Troy- TC bolts are designed to be used with a gun. I am not aware of an approved method of tensioning them manually.

    After completing installation of 1 3/4″ A325 T bolts at 1/3 turn a ten percent check of bolts was completed at 360 Ft. Lbs. arbitration/verification torque established from completed connections which incorporated standard A 325 bolts. The A325 T bolts failed in some locations. Would changing these bolts out to A449 resolve this issue.

    @Mike- First off, A325 bolts are only manufactured up to 1-1/2″ – the ASTM standard does not allow for them to be made larger. If what you have is being sold and certified as A325, I would question the integrity of the supplier. That said, A449 is virtually identical to A325 with the exception that A449 is available up to 3″ and does not have the other dimensional restrictions of A325. A449 is more of a general purpose fastener. Regardless, 360 ft-lbs is not an excessive torque value for bolts this size, so if you are getting failures there must be something else going on. Unfortunately we don’t have any engineers on staff, so we are unable to trouble shoot this issue for you. Apologies.

    @Vijay- A325 is the bolt regardless of how it is used. The letters N SC and X all detail how the bolt is used and the specifics of the application, but the bolt is the same in each of these joint types.

    @Gopal- You are asking about three different items. A325 is a specific kind of structural bolt, grade 5 is a general purpose bolt, and grade 8 is a very high strength general purpose bolt. These are three separate bolts that are each used in their own applications.

    @Gopal- The designer will need to determine if he/she wants the threads in the shear plane of the joint or not. If the threads are to be excluded, then ‘X’ is designated, if the threads are allowable in the shear plane, then “N” is designated. The threads can be included or excluded by adjusting the bolt length or adding/subtracting washers.

    @Gopal- I believe what you are looking for is a F1852 TC bolt that is being used in a slip critical connection with the threads allowed in the shear plane. The N and SC designates how it is used, the bolt itself is not any different. You would need to determine the bolt diameter and length that is appropriate for your needs, and then we or another supplier could supply it.

    Recently in one of my project I came to see a phrase “SPOIL THREAD”. Could anybody can help me on this.?

    @Sreejith- I have not heard that terminology before. My only guess is that they want you to damage one of the threads so that the nut will nut be able to back off.

    Appreciate the A325, Type 2 Structural Bolt, was withdrawn in Nov,1991, was it due to lack of performance, (rebound capability)or was it related to actual bolt failure?

    It is my understanding that galnanized A325 bolts are not to be used to connect weathereing steel components due to the reaction of these materials (coatings). Is this correct!
    With thanks, James M Kett

    @James Kett – There is a possibility of galvanic attack when dissimilar metals come into contact with each other, however it depends a little on the atmospheric (weather) conditions. We do not have any metallurgists on staff, so for your specific application, perhaps one should be contacted to be sure.

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