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STR04 (Structural) (OP)
9 Nov 05 18:16
When this type of connection fails, ie. moves but does not shear the bolt off, does my now bearing connection increase in shear capacity (Fv).  Example; a 3/4" A325-SC has the shear capacity of 7.51k (ASD).  When the connection moves and changes to a full bearing condition, does my shear capacity increase to 9.3k for A325-N or am I still held to the SC value?

Now say if a contractor installs A325 TS (twist-off-type control bolt assemblies) bolts (Pre-tension to 70% per Table J3.7 w/ out surface preparation) when I only specified A325-N connections, do I take a shear reduction because of the combined stresses?

TIA
DaveAtkins (Structural)
10 Nov 05 8:49
Isn't it true that the reason SC bolts have a lower shear capacity than N bolts is that the tension you induce in the SC bolt by torquing it reduces the available shear capacity?  Then if it slips into bearing, would it still have that tension?  I guess I would not assume the capacity increases, but I wonder what others think.

DaveAtkins

Helpful Member!  WillisV (Structural)
10 Nov 05 8:55
The reason "slip critical" bolts have a lower shear capacity is that the specified shear capacity is not the actual failure load of the bolt, it is the amount of shear that the torqued bolt can take before the applied load overcomes the clamping force between the bolt and the fraying surface and causes the connection to "slip."  The use of these connections prevents "banging bolt" type failures.  Note that if the connection does slip into bearing then technically it failed the servicability limit state of slipping, but the actual strength of the bolts prior to a bolt failure is the same as that for non-slip critical applications.  So to answer the original question - yes your bearing capacity is greater than the slip critical shear capacity.   
chichuck (Structural)
10 Nov 05 9:00
WIllisV,

A lucid and concise explanation of slip critical connections.  I'll give you a star for this one.


regards,


chichuck
Helpful Member!  JAE (Structural)
10 Nov 05 10:30
I agree with WillisV as well.

To answer the two questions directly:

does my shear capacity increase to 9.3k for A325-N or am I still held to the SC value?


Once it slips, you have exceeded your prescribed limit state of slipping (at 7.5k) but you have not exceeded your shear capacity which is, and always was 9.3k for strength.

....if a contractor installs A325 TS....do I take a shear reduction because of the combined stresses?

No. - you still have the full shear strength of the bolts at 9.3k.
STR04 (Structural) (OP)
10 Nov 05 12:03
Yes, but when the bolt is in tension you do not have the full shear capacity.. v/Va + t/Ta <= 1.0?  Just a simple interaction design...right?  But what I'm hearing is that when the connection slips your bolt looses all of its tensile force and only sees a shear force.  Where in AISC documentation does it say this?
JAE (Structural)
10 Nov 05 12:48
Check out the "Commentary on Specifications for Structural Joints Using ASTM A325 or A490 Bolts" by AISC (I'm looking at the June, 1988 version from the AISC LRFD Manual Second Edition.

In section C4, under "Background for Strength of Bolted Connections", the last sentence in the second paragraph says "Final failure load is independent of the clamping force provided by the bolts".

Also, move ahead a little two pages under the paragraph titled "Design for Shear" and they add a sentence (second to the last sentence) that reads:  "The shear strength of bolts is not affected by the pretension in the fasteners provided the connected material is in contact at teh faying surfaces."
Helpful Member!  aggman (Structural)
10 Nov 05 13:19
First impression:
My professor in college that taught us steel design told us that if you design a connection for slip critical then it will not fail if it slips into bearing.  I don't remember what the reasoning was other than because the connection capacity as a bearing connection is higher but I do remember him saying it.

Second impression:
I think STR04 concern is coming from the interaction of shear and tension.  The interaction would be derived from Table J3.3 (ASD) and would show a reduction in tension (or shear) depending on the shear or tension load.  This may be something to consider but here is also another approach.  In the connections manual pg 1-10 connections are either direct tension, bearing, fully pre-tensioned bearing, or slip critical.  As WillisV pointed out the slip critical failure is related to the clamping force, which is why the pretension is what it is.  But in cases where we are required to have fully pretensioned bolts in bearing connections there is no apparent reduction in shear strength due to this.  So I think you can confortably not take a reduction in strength just because you are preloaded.
One other thought is that have you actually run the numbers using the AISC table (J3.3).  You may be ok, assuming the slip critical bolts are not designed to the end.  For instance;

I will assume that the bolts are at 75% design capacity.

17ksi * .75 = 12.75ksi shear load.

SQRT(44^2 - 4.39*12.75^2) = 34.96 ksi - (Allowable tension stress)

ft = 28 / .7854in^2 = 35.65ksi

Its close but I would certainly be ok with it based on the other criteria given above.  "I would actually be ok with no reduction based on what I said above."
aggman (Structural)
10 Nov 05 13:21
Or what JAE said......
STR04 (Structural) (OP)
10 Nov 05 13:56
Aahhh...proof.  Very nice!  Thank you once again JAE!!!

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