huangyhg

grade 8 vs. a325
we are provider of packaged modular energy solutions and we use a325 bolts for skid steel framing (one skid package has about 15% a325 bolts and 85% grade 8 used for piping and mechanical equipment). in order to simplify purchasing, handling and storage of bolts we would like to use grade 8 bolts instead of a325.
guidance and counsel would be greatly appreciated.
thanks,
ibro v.
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is there any particular code or contractual requirement that you use one bolt or the other? or is there any reason that you're using a325 in the first place?
could you use a325 all around, also?
i think that a grade 8 bolt is equivalent to an a490 in strength, but i don't have these specs at home today. another difference is that stuctural bolts have a larger head size so the a325 would be bigger than the grade 8. also thread length can be different which could result in threads being in the shear plane.
if there is no specification or contractual requirements and the physical differences are not a factor, the substitution could make sense. i would still be interested why they were doing it the present way, however.
regards,
-mike
this modern steel construction quiz has some useful information...
the "machinery handbook" is a great reference for bolts, and provides all the mechanical properties of both a325 and grade 8 bolts.
addressing the question of why there would be two different bolt specifications for the same skid: i have seen this many times. i attribute it to the lack of three c's; coordination, communication, and control. many times project documents are put together with the only oversight consisting of a review to make sure all the documents are there without any review of their contents. project managers never seem to delve into issues such as this until after the fact.
in addition each discipline (mechanical and structural in this case) is more comfortable with specifications they are familiar with. most likely either bolt spec, a325 or grade 8, would work for the entire project. which enineer should have to go out of their way to learn the others spec?
in my opinion the grade 8 bolts should be spec'd for the skid. their mechanical properties are greater than or equal to a325. more importantly, in a manufacturing environment, use of a single bolt spec will save money while making the qa&qc requirements easier to meet (less specs = less chance of error).
the above notwithstanding i would go with an all welded tubular skid frame as opposed to a bolted arrangement using wf and c sections, clip angled together. the tube steel has much greater torsional strength and overall stiffness with very little cost differance.
i understand there are differences in testing or proof testing of bolts, not just necessarily differences in mechanical properties. where you are working to a specific building code that requires a certain bolt, it makes sense to use it. but if, as sounds likely, you're just using the a325 "because that's what we've always used on structural steel", then it could certainly be worth a look the alternatives.
is the skid something temporary for shipping or is it a permanent part of a structural support? if it's structural i would tend to stay with the a325 bolts. it could however, all be considered a mechanical component and argued that it is exempt from building codes. i have been involved in the a325 versus grade 5 issue myself and then caught in a situation when purchasing started buying bolts threaded to the head, which was not per the design. but the extra strength of the grade 8 could cover some of these concerns.
-mike
here's a blatant cut and paste. keep in mind that this is from a building structural steel perspective. grade 8 may be just fine for your application - your call.
from aisc’s publication, a guide to engineering and quality criteria: common questions answered:
question 6.2.5: is it acceptable to substitute sae j429 grades 5 and 8 bolts for astm a325 and a490 bolts, respectively?
answer: no. the strength properties of sae j429 grade 5 bolts and astm a325 bolts are identical; likewise, sae j429 grade 8 bolts are the strength equivalent of astm a490 bolts. these material specifications differ, however, in that astm a325 and a490 specify thread length and head size, whereas sae j429 does not. additionally, quality assurance and inspection requirements for astm a325 and a490 bolts are more stringent.
since aisc and rcsc specifications call for pretensioned installation for your application (which involves fatigue), i would question whether the sae fasteners that were installed were properly installed. also, i’d question if the head size, threading and quality assurance were adequate for the application. usually, the request to substitute an sae fastener for a325 or a490 comes because sae’s are cheaper. perhaps that gives you the answer right there.
charles j. carter
american institute of steel construction
chicago, il
hope this helps.

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