Integrated Buildings & Structures

Expand all | Collapse all

Safety Factors, Stainless Steel Bolts

  • 1.  Safety Factors, Stainless Steel Bolts

    Posted 12-11-2020 02:11 PM

    Thank you in advance for your time devoted to this request.

     

    I am looking for guidance on where to find the appropriate safety factor for stainless steel bolts in TENSION in ASD design, in a standard document (such as AISC, ASCE,etc).  I have found some somewhat conflicting definitions/direction in the literature;

     

    • A Power point from AISC that says use the same as for carbon steel (LRFD = .75*Fu) (this definition combined with another AISC publication: "Specification for Structural Steel Buildings", that defines ASD safety factor as 1.5/LRFD factor yields 1.5/.75=FS=2...this gives a value that is higher than the yield strength of a SS bolt   (316 SS condition A; Fu = 70 ksi, Fy = 30 ksi This does not make sense to me))
    • The Nickel institute produces a handbook entitled "Stainless Steel Fasteners, a systematic approach to their selection" that states that designers should use Yield strength for design (but that reference does not specify a safety factor, it just says one should be used).  This document does provide a specific safety factor of 3 (Specifically 0.6Fu/3) for shear but does not provide any safety factor for tension.
    • The Steel Construction Institute issues a "Design Manual for Stainless Steel" that provides a "partial factor" for SS bolts of 1.25
    • Online I found a reference that indicated that such a safety factor might be found in "section J4" that did not reference a document
    • Online I found a reference that indicated that such a safety factor might be found in ASCE 8 ... I am willing to purchase this document but would like a comfort factor that it will provide me with the correct Factor of Safety before spending the money.

     

    Can someone point me to the right document?  

     

    Eric



    ------------------------------
    Eric Heiberg P.E., M.ASCE
    Long Valley NJ
    ------------------------------


  • 2.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-12-2020 08:59 AM
    There is a Design guide No.27  of AISC ( American institute of steel construction) for complete design in Stainless steel.

    For Stainless steel bolt property look at ASTM F593.

    ------------------------------
    Jayesh Shah C.Eng, P.E., S.M.ASCE
    Structural Engineer
    Edmonton AB, Canada.
    ------------------------------



  • 3.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-12-2020 05:59 PM
    Eric,
    None of the documents you refer to are applicable to stainless bolts, typically for austenitic stainless steels like 304, 304L and 316 which are commonly available from various sources.  For situations involving vibration, the SAE uses a torque value that results in a thread stress that does not exceed 70-75% of yield strength (YS).  "Turn of the nut", commonly used in carbon steel structural bolts, can result in exceeding yield strength because the difference between tensile and yield strength is substantial for stainless steels.  Moreover, stainless steels often do not have a sharp transition between elastic and plastic behavior in the yield region, so "offset yield strength" is often stated.  If the stainless steel fastener will be subject to a corrosive environment, of which saline water is considered as "corrosive", the stainless fastener can be subject to stress-corrosion cracking.  This stress-corrosion cracking threshold stress differs for austenitic, duplex, martensitic and ferritic stainless steels.  Too many structural engineers stress stainless fasteners to near yield strength, exceeding their stress-corrosion cracking stress limit.  Last, if the stainless steel bolt is subject to shear as in jointed connections, the designer should use the von Mises failure criterion which includes both tensile and shear forces on the fastener to obtain a relative safety factor.

    Christopher Hahin, MetE, CorrE, PE
    States of Illinois & California
    Springfield IL 62704
    217 522-4023 (home)

    ------------------------------
    Christopher Hahin P.E., M.ASCE
    ENG OF STRUCTUR
    Illinois Dept. of Transportation
    Springfield IL
    ------------------------------



  • 4.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-13-2020 09:57 AM
    Christopher,

    Thank you.  My application is in a coastal city, not subject to salt water, (but perhaps subject to salt air?).  I am using austenitic SS.  do you think that this would pose a stress-corrosion threat?  If so,  where do I find data on allowable stresses for stress corrosion ( stress-corrosion cracking threshold stress )?  Is there a different safety factor for stress corrosion cracking (If so, on what value is is placed?  Fu? Fy? Fc (corossion value?))

    Thanks in advance for the benefit of your experience and wisdom.

    Eric

    ------------------------------
    Eric Heiberg P.E., M.ASCE
    Long Valley NJ
    ------------------------------



  • 5.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-14-2020 12:37 PM
    Eric,

    Your exposure could be to road salt or chemicals in a chemical plant, you dont have to be in a coastal region to use stainless bolts.  There are many many stainless steels but the most common are 304 and 316, and those two are very similar for strength.  316 is commonly used in salt water immersion, but there are other stainless steels that may be better for this or any application.  I never tension stainless bolts as you would sometimes do with A325 or A490 bolts.  I am not aware of any corrosion modifiers, as you should not have any measurable corrosion if your material is properly selected.

    ------------------------------
    James Mackenzie S.E., M.ASCE
    Burnaby BC
    ------------------------------



  • 6.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-14-2020 01:44 PM
    James,

    Thank you again.  as I am using 316...I  think I will be OK.

    Eric

    ------------------------------
    Eric Heiberg P.E., M.ASCE
    Long Valley NJ
    ------------------------------



  • 7.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-14-2020 01:45 PM
    Eric,
    Since you have mentioned that your application is in a coastal city, salt fall will be involved.  Also, since I don't know what structure or product you are designing, I don't exactly know what preloads you need for your bolting.  In general, I would recommend 2205 stainless steel bolting, which is a duplex steel that is resistant to chloride stress-corrosion cracking, unlike many of the austenitic grades which have lower threshold stresses.  The threshold stresses for 304 or 316 stainless vary with temperature, so that is a factor.  Previous research for 2205 has shown good resistance to cracking in chlorides up to 70% of yield, and they are more readily available than other duplex steels for general bolting.

    Christopher Hahin, MetE, CorrE, PE
    States of Illinois & California
    Springfield IL 62704

    ------------------------------
    Christopher Hahin P.E., M.ASCE
    ENG OF STRUCTUR
    Illinois Dept. of Transportation
    Springfield IL
    ------------------------------



  • 8.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-12-2020 06:00 PM
    AISC Design Guide 27 is what you should be using for Stainless Steel (2013 is what I have, and that is probably latest).  It is similar to the AISC 360 code for carbon steel.  Chapter 9.  Formula J3-1:  Rn=FnAb phi=0.75 (LRFD) omega=2.00 (ASD) Fnt=0.75Fu Fnv=0.45Fu Ab is nominal (not net) area.  Some properties are given in Chapter 2 and that is a range of 75 to 100 for Condition A in Table 2-4, so your 75 is good.

    ASCE8 also has information on bolts.  The last code I used ASD in was the AISC in 1989, and I dont see ASD in the ASCE8-02.  Pn=AbF phi=0.75 (same as AISC).  Fnt is given in Table E6 and yes (same as AISC) there is 56 ksi for 304/316 in footnote b Condition A in ASTM A276-85a, hot finished and Class1 (solution-treated) in ASTM A193/A193M-86.  Note that Fnt is valid for ranges of diameters of bolts, but in this case it is all.  For shear you would have to divide the area by 1.33.  Also for diameters less than 1/2" use a factor of 0.90 footnote g (I dont think AISC says anything, but the line in the sand there is 1/2")..

    ------------------------------
    James Mackenzie S.E., M.ASCE
    Burnaby BC
    ------------------------------



  • 9.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-13-2020 09:57 AM
    James, Jayesh,

    Thank you!  I have purchased Design Guide 27.  James, in your text, you mention Fnt as 56 ksi...I assume that 56ksi=.75*75ksi (for SS type 31600) and therefore the allowable tension in the bolt would be FntAb/omega or .75*Fu(Ab)/omega= .75*Fu/2 = Fu/2.67 for ASD (26250psi times bolt area for 70 ksi 31603 and 28125 times bolt area for 75 ksi 31600.  Obviously this load is higher for stronger batches with certificates of compliance).

    You mentioned that the bolt area is the area from the nominal diameter...I am assuming that I use that value only for shear loads going through the shank and that for tensile strength I would use tensile area and for shear through thread plane that I would use the minor diameter area.

    Does that sound right?

    Eric

    ------------------------------
    Eric Heiberg P.E., M.ASCE
    Long Valley NJ
    ------------------------------



  • 10.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-14-2020 12:36 PM
    Eric,

    The result is the same in LRFD (strength design) for both ASCE8-01 and AISC 360.  Note that there is a newer ASCE8 that I dont have, but I dont expect much change.  If you are using sheet steel, less than 3/16" material, you should have ASCE8 as well as Design Guide 27.

    As for the use of omega, if you just divide the ultimate tensile capacity by omega to get the allowable tensile capacity.  LRFD capacity = 1.5 * ASD capacity.  The 1.5 is considered the average load factor so the result should be same in most cases.  You can look in the AISC 360 and both ASD and LRFD capacities are given in some tables, and the difference is always this 1.5 factor.  If the actual LRFD load factor is not exactly 1.5, so the ASD and LRFD results will never be exactly the same, but close enough.  I think it was the 13th edition (2006) of AISC 360 that the ASD was put back in (since the 9th (1989)) to accommodate the working stress design holdouts.  Although I came from the working stress design era, I have moved on to LRFD.  The 10th, 11th and 12th were LRFD only, but did not sell too well because of all the holdouts.

    For shear it is the nominal area if the threads are excluded.  Bolts A325 or A490 will have threads excluded is there if the outer layer is more than 1/2" with proper stick out.  For stainless bolts or say SAE bolts you would normally assume the threads are included, unless you know for sure the shank is in the shear plane (unlikely, so not conservative).  If the threads are included in the shear plane, the nominal area should be reduced by a factor of 0.75.  This is approximately the net area/nominal area.  The shear strength is 0.60 times the tensile strength otherwise (threads excluded), so for threads included it is even less (0.60 times 0.75).

    If you are a member of AISC, you are entitled to all the Design Guides in *.pdf free.  There is a substantial reduced fee on hard copies such as the manuals.  There is a reduced fee on courses.

    ------------------------------
    James Mackenzie S.E., M.ASCE
    Burnaby BC
    ------------------------------



  • 11.  RE: Safety Factors, Stainless Steel Bolts

    Posted 12-15-2020 09:11 AM
    Hello Eric,

    Note also that AISC has just finished their second public review cycle on a new draft standard for stainless steel structural design, AISC 370:
    https://www.aisc.org/pressreleases/press-releases/new-structural-stainless-steel-standard-available-for-public-review2/

    Unfortunately, since the review period has closed, AISC is no longer hosting a PDF for download.  When released, this will presumably supersede DG27.

    ------------------------------
    Christian Parker EIT, A.M.ASCE
    Structural Project Engineer
    Washington DC
    ------------------------------