Discussion request:: Use of High Strength Bolts 3

[IJR]

I request a discussion on the use on Proper use of High Strength Bolts with emphasis on when and how they should be employed for Friction or Bearing

My experience and knowledge:

We employ HS bolts often in extended end-plate moment connections mainly for tensile strength and rarely for shear capacity. In our applications, high moments and lower shears are typical. Shear seems to present a small problem.

Reason for end-plate moment connections is "LESS BOLTS LESS HASSLE DURING ERECTION"

We specify a minimum pretension only to ensure plates contact is flush. We dont employ friction grip simply because surface preparation can not usually be guaranteed. We also believe that if pretension is utilized, moment induced bolt tension will cut it down and eliminate any grip. So in end-plate moment connections, there seems to be no use pretensioning, or at least so I believe.

From reports I know that beam splices, the type involving cover plates on flanges and web plates on webs, should be bolted with high strength bolts. Some codes require grip be utilized, some only HS bolts use. Again given poor surface preparation, I specify minimum pretension, but completely ignore it in design, and rely on bearing capacity of the connection.

Again, though hesitatingly, I am often advised to use end-plate kind of beam splices instead of shear based cover plate type, mainly for the same reasion "LESS BOLTS LESS HASSLE", though I force the cover plate type in relatively more serious designs.

I have never before come across a client based spec, which requires a slip critical connection. So far I have not used grip resistance with oversize holes.

It will be nice to learn how others use the bolts effectively and properly.

Any comments welcome

Respectfully
IJR

 

[JAE]

IJR
For end plate moment connections we specify "fully tensioned" but not slip critical. The difference? Only difference is that slip critical requires consideration of the faying surfaces and requires inspection. Fully tensioned does not.

We feel that with an end plate moment connection, where the bolts are put in tension, a pretensioning is required. How much pretensioning? Really up to the engineer, but AISC doesn't really recognize anything between "snug" and "fully tensioned".

Per AISC recommendations, with the fully tensioned bolts, you must use the shear/tension interaction equations of LRFD Table J3.5. With slip critical, you only design for the shear as the tension force in the tension-side bolts is offset by the compression in the compressive-side.

Check out AISC Specification Section J2.11 (requires "fully tensioned" - not slip critical)
Also: AISC LRFD Vol 2, page 10-21 re: end plate connections.
AISC LRFD Specification for Structural Joints using ASTM A325 and A490 Bolts - Section 8.c and commentary Section C8 (page 6-416).

 

[Ron]

IJR...my experience is similar to JAE's.

It doesn't really matter that you achieve slip critical capability (except where needed in design), any friction will help the connection. You mentioned that you can't count on the quality of the surface, then also consider that even with drilled holes, there might be some offset and differential distribution of load on bolts in the connection, thus any surface contact will help to mitigate this condition. The surface contact can best be gained with reasonable fit-up and pre-tensioning. Since as JAE says AISC doesn't recognize anything between snug and fully tensioned, it is reasonable to apply full tensioning and get the benefits of it.

As you know, full tensioning can be checked by a variety of means from indicator systems (bolts or washers), to calibrated wrench, to turn-of-nut method.

 

[Blwtco]

We have been asked by a couple of engineers to put lockwahers on the A-325 bolts we are using along with the flat washers. Is this a common pratice? I was under the assumption that all that is required is the hardened flat washer and the proper torque. before I go and argue my case I want to know how you stand on the subject.


Thank you,
Richard

 

[JAE]

I have never heard of using lock washers on high strength bolts. Is your situation involving a vibrating condition? Sometimes engineers will specify that the threads be damaged just above the nut to avoid vibration induced backing off of the nut.

Anyone else heard of this? (lockwashers)

 

[Ron]

Lockwashers are not appropriate for high strength bolted connections. If appropriately tightened, the lock washer would have no reactive component to assist, can crack under tightening sequence, and provide stress raiser at gap which could impact fatigue resistance.

Thread galling outside the nut is appropriate.

 

[JAE]

There's the term "thread galling" ...I couldn't think of it. I've also seen other terms used (ding, distress, mar, etc.)

 

[Rowland33]

For slip critical connections we have used twist off bolts to ensure proper tensioning. Visual inspection is much simplified with this method. At an early stage in the design of a major bridge, it was also suggested that lock washers be used but this idea was abandonned because we could find no reference to lock wahers in the codes.
Would somebody provide a concise definition of faying surface, I cant find a definition in my dictionaries.
Geoff

 

[ishvaaag]

The contact surface between two adjoining parts.

http://www.purgit.com/dictiona.html

 

[svc430]

According to the AISC "Specifications for Structural Joints Using ASTM A325 or A490 Bolts", (Manual of Steel Construction, ASD, Ninth Edition), bearing type connections may not be considered adequate for some applications, including joints subject to fatigue loading and/or significant load reversal, joints with bolts installed in oversized holes, joints in which bolts and welds share the applied load, joints with bolts installed in slotted holes where the applied load is in a direction not normal to the axis of the slot, or any other joints in which slip would critically affect the performance of the joint or structure (see page 5-270 of the Ninth Edition, which gives some criteria for slip-critical joints).

I would definitely agree that lock washers are not appropriate for installation with high-strength bolts (slip critical connections or otherwise). With a split type lockwasher, the bolts cannot be properly torqued since the washer has a tendancy to "open-up" and spread apart during the installation. A much better solution is to use a locking type nut. Bethlehem makes such a locknut, called ANCO self-locking nuts. These can be purchased in A325 grade (as well as A307) and contain a steel locking pin in the nut which engages with the thread of the bolt as it is tightened. "Stover" locknuts are another option, available in SAE Grade 5 or 8.

 

[fkd]

I just want to say thank you all for this thread, this kind of questions concerning to the use of HS bolts on connections were allways in my mind, a very good and clarifying discussion!


Fred

 

[jrmack]

A325 bolts.
Lock washers, this is the first time I heard of them used with these bolts is on this thread (it must be a joke). If you think you need lock washers, use turn of the nut pretensioning (without lock washers) as per AISC. I have never heard of a lock washer used with a A325 bolt. Is there a specification that allows this? For dyanamically loaded structures, I specify pretensioning with an appropriate faying surface (and of course friction bolts).

For statically loaded connections (normal in my case), what is wrong with specifying pretension without specifying the appropriate faying surface (could be painted with a multicoat of epoxy, and the codes consider this very slippery). This gives you a stiffer structure (there is no slop). This is what I and others do, design for threads included (always even if I have a grip greater than 1/2" or 3/8" depending on the code).

I am not a newcomer to structural steel design.