Slip Critical Connection Failure
Slip Critical Connection Failure
(OP)
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
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






RE: Slip Critical Connection Failure
DaveAtkins
RE: Slip Critical Connection Failure
RE: Slip Critical Connection Failure
A lucid and concise explanation of slip critical connections. I'll give you a star for this one.
regards,
chichuck
RE: Slip Critical Connection Failure
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.
RE: Slip Critical Connection Failure
RE: Slip Critical Connection Failure
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."
RE: Slip Critical Connection Failure
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."
RE: Slip Critical Connection Failure
RE: Slip Critical Connection Failure