Slip critical - USE OF ALL THREAD?

[bobharsh]

Good afternoon - I would like to pick brains about the connection below.

Connection is for a cladding system(non structural precast cladding). The connections original intent is an out of plane connection. The ISSUE now is that this connection is being pretensioned (slip critical) to resist "small" in plane loads (seismic design cat B - resist 5% weight)

Questions
1.) SLIP CRITICAL ON THREADED ASSEMBLY? (assuming faying surfaces are adequate)
- threaded rod is Grade A193 B7 - slip critical design in AISC are A325 and A490 bolts. Has anyone used threaded rods in pretension?
2.) 350 ft-lb on threaded rod?
- Depending on k value of nut - does this look right?
3.) BENDING in threaded rod?
- very little capacity when you add eccentricity to rod - the connection below shows 3"!

 

[BridgeSmith]

Creating a slip-critical connection with threaded rod is theoretically possible, but as you noted, it's outside the scope of the specifications. That being the case, the primary question is: Who will take responsibility for connection design?

The slip resistance would have to be calculated using the tension applied to the threaded rod and the surface condition factor (coefficient of friction) of the faying surfaces.

Article 6.13.2.8 of the AASHTO Bridge design spec has surface condition factors values and descriptions for the surface condition classes. I'm sure other codes & specs have something similar.

Some quick calcs using the min strength of the A325 and A490 bolts from ASTM F3125, the tension values given in the AASHTO Bridge Design spec (Table 6.13.2.8-1), and the net area of the bolts from a bolt supplier's table, indicates the use of a tension equal to 70% of theyield strength on the net area of the bolt cross section. If you allow this approach, I suggest only allowing the use of ASTM F1554, Grade 105 threaded rods, to keep the material properties similar. Edit: I suggested F1554 because we have looked at its ductility, which I do not know about the A193 B7. The strength parameters appear to be the same.

The difficulty I foresee is verifying that the correct tension is achieved. Torque on the nuts is a very poor indicator of the bolt tension, even with proper lubrication of the nut faces and threads (which is also difficult to verify). It's unlikely you'll be able to find tension-indicating washers calibrated for other than A325 or A490 bolts. That leaves turn-of-the-nut as the only marginally accurate method to verify the tension is achieved, and strict oversight of the tensioning process is a must for this method. I'd still require a large reduction from the calculated slip resistance to the required.

 

[BAretired]

I don't think "bending in bolt" is recommended by the powers that be.

BA

 

[Agent666]

If it were me I'd match drill the hole on site, so no need for the slot or tensioning. Seems the allowance for tolerance is admirable, but what if the slot is to high/low to match the thing its screwing into on the left side. What do you do then with the slot? Or does the panel simply end up wherever the tolerance of the adjacent cast in item ends up?

Just to confirm, is it a plan view or looking horizontally, and what direction do the loads act? Are the loads even in the direction of the slot?

If bolt can bend then definitely it needs to be evaluated in conjunction with any shear and tension that might be present.

 

[bobharsh]

Thank you for the responses. my current mind set is if you can get the proper amount of pretension in the THREADED ROD you can achieve slip resistance. However, I am still very green when it comes to slip critical connections.

HOT ROD, NOT I - This is a connection design proposed by a coworker.
- you bring up a good point with the readily availability of the F1554 all thread - one could use a calibrated torque wrench to get the proper tension in the threaded rod.

BAretired - I AGREE bending in a small section is ridiculous! If this was the only in-plane resisting connection - you have no rigidity...

Agent666 - This is a proposed connection on a large scale job - drilling the hole to match will take too much time while the precast is on the crane hook. shown is a section view - The PSA slotted insert in the panel has vertical slip.

 

[BridgeSmith]

"...one could use a calibrated torque wrench to get the proper tension in the threaded rod."

The theoretical relationship between torque and tension is easily quantifiable, but in the real world, the unpredictable nature of friction, etc. makes the margin of error huge. Even with careful lubrication of the threads and nut faces (which is difficult to monitor), the range of tension achieved at a given torque can vary quite a bit. If the margin between the slip resistance required and what can be theoretically achieved is large it may be feasible. I wouldn't consider it unless the margin was at least 2 to 1.

 

[dik]

A really good place to use indicating washers... there's no reason I can think of why the connection cannot function as a slip critical one. You only need the pre-load and that can likely be provided by washers. I've used them on many critical connections, including the local floodway.

Dik

 

[BridgeSmith]

Dik, are indicating washers available calibrated to the lower load that would be required for a 105ksi threaded rod instead of a 120ksi A325 bolt? Or can the regular indicating washers be used to measure lower loading?

 

[dik]

FYI

Load Indicating Washers (DTI) - Portland Bolt

Load indicating washers (LIW) are used to develop the correct tension when installing A325 or A490 structural bolts.

www.portlandbolt.com

We had them for A490 bolts... A long while back, I posted a failure of one of the bolts (intentional) and Ron picked up on the unusual failure... high shear load and high tensile load.

They work by the tension in the rod... and 'squish' accordingly. With a longer rod, it may take a turn or two before they flatten to the proper amount, indicating that the tension has been met. For other strengths, you may have to do some testing to determine how much they should 'squish'.

Dik

 

[BridgeSmith]

"For other strengths, you may have to do some testing to determine how much they should 'squish'."

That makes sense. I thought maybe there were published values, and if not it could be done as you say. I just didn't know if it would be consistent enough at the lower load values. Good to know, thanks.

 

[dik]

I've used DTIs for numerous projects and my experience is that they are very good and reliable. It would be very easy to set up a test rig to measure the deformation. They are quite reliable (and results repeatable) as far as being a measuring device and if you have an unusual 'grip' the washers should accommodate that.

If you are looking at 'slip critical' you are in the upper ranges of the fastener capability.

Dik

 

[BridgeSmith]

Dik, I knew DTIs were very reliable for the A325 & A490 fasteners they are calibrated for. I just didn't know if results would be as reliable and repeatable for a Grade 105 threaded rod, which would, by necessity, have a lower tension force. If I understand correctly, you're saying they will work as well at the lower tension levels. That's good enough for me and good to know in case we ever have a need to tension threaded rods. (Haven't yet, but we might have a need at some point)

Now that we've discussed how to make the threaded rods work, I'll ask the question maybe I should have asked at the beginning. Why are you not just using high strength bolts?

 

[dik]

Why not? and the test results based on other fasteners are very reliable... I used them for the main hydraulic ram connections for the Floodway in Manitoba, and, did a bunch of testing, first. Some very serious loads... and a high reliability required.

Dik

 

[BridgeSmith]

"...test results based on other fasteners are very reliable..."

Ok, you've convinced me. (Actually I was convinced a couple of posts ago) I just hadn't seen any test results or done any hand-on work with DTIs. Thanks for the clarification.

 

[bobharsh]

Very good information here - Thank you for the responses!

I have an additional question I would like to pose. What is the effect of applying additional tension to a pretensioned bolt? For instance if an eccentric force is applied to slip critical connection(see connection below)

 

[SwinnyGG]

You're standing on the edge of a deep, dark hole. But an interesting one.

'Additional tension' means nothing, if the additional tension does not exceed the preload.

The arrangement you're showing, however, introduces prying into that single fastener.

Fastener prying is the deep dark hole- and it does very bad things to fasteners of all kinds.

 

[bobharsh]

HA - VERY Deep AND Dark is right! Due to fire watch requirements and codes in certain areas of the country welding is becoming increasingly expensive and/or not allowed. My company has several jobs in New York that are ALL BOLTED. Slip critical and serrated bolted connections are being used to resist forces acting in-plane while allowing for construction tolerances.

I am having a hard time grasping the concept that additional tension on a preloaded bolt will not cause detrimental effect. Say if you stress an A325 bolt to its max capacity of 34kips. What happens when an additional 10kips of straight tension is applied to the bolt?

 

[klaus.]

Nothing happens to the bolt.....

 

[jayrod12]

Agree with klaus.

Draw your FBD. The 10 kips tension applies load to the members, the bolt doesn't see anything.

 

[BAretired]

When the bolt is stressed to 34 kips, it is putting the plates in compression. When a 10 kip tension is applied to the bolt, it is relieving some of the compression in the plates.

BA