Slotted HSS End Connections - eccentricity 8

[dik]

How do other engineers deal with the slight eccentricity caused by the slotted end connection to the gusset plate due to the thickness of the connecting plates. I generally offset the slotted plate and the gusset plate so that the face of the plates falls on the centreline of the HSS to minimise the moment. I neglect this when I design the connections. What do other engineers do? Are there any references?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

No problem with tension members, but there have been some studies which promulgated rules about this for compression members. In highly loaded struts, this can be overcome by transverse stiffeners. This has been discussed here before. I will try to find it.

 

[hokie66]

thread507-247873

The link which rowingengineer gave in this thread is from New Zealand and a few years old, but it addresses the issue in principle. As to whether this has been codified anywhere, I don't know.

 

[hokie66]

http://ascjournal.com/down/vol6no2/vol6no2_2.pdf

Another reference.

 

[dik]

My response just disappeared... thanks Hokie, I have a couple of papers for eccentric compresson including the ascjournal you note... have to find it... Is there a chance you can dig up a copy of Australian paper... it's a dead link. I have to find if the EOR will accept eccentric connections just letting them yield the first time in tension of if he wants a portion of the bracing loads in compression.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

dik,

Yes, the links in my first post are dead, but the paper in rowingengineer's link is alive.

Advanced Steel Construction is an international journal, and the last one I linked was an Australian paper. Based on the earlier HERA work, with a lot of experimental work to verify.

http://ascjournal.com/index.php?option=com_djcatalog2&view=item&id=23:vol6no2&cid=0:all&Itemid=559

 

[dik]

Sorry Hokie, it was the other one noted in the earlier thread...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

Sorry, I don't have that one.

 

[dik]

Thanks, Hokie... I owe you a whole bunch of beer!

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

You are welcome, dik. If I ever get the chance to visit your part of the world again, will take you up on it. Meanwhile, a virtual one will have to do.

FYI, I remember that the 2006 link was just to an advisory note from ASI, warning that there had been some problems with that type of eccentric compression connection.

 

[human909]

Yes. Australia and New Zealand had a wake up call on the design of slotted hollow section cleat plates and the significant loss in capacity due to eccentricity. That paper, a few others and ASI advance tried to fix that. However I still see many cleat plates in structures that are of insufficient thickness to compensate for the eccentricity.

I generally offset the slotted plate and the gusset plate so that the face of the plates falls on the centreline of the HSS to minimise the moment.

Unless I'm sorely mistaken that doesn't gain you anything. You still have the same effect eccentricity across the connection.

How do other engineers deal with the slight eccentricity caused by the slotted end connection to the gusset plate due to the thickness of the connecting plates.

I use the recipe provided by ASI and blindly follow it.** I end up with some pretty thick cleats, but it allows me to sleep at night.

**Actually I choose one step easier and let software apply and cook up the recipe for me. I generally make a few manual edits and provide a few 2-6 suitable connection examples per job for my hollow section struts.

 

[human909]

Sorry Hokie, it was the other one noted in the earlier thread...

Sorry, I don't have that one.

Here is the link for the advisory note:

https://files.engineering.com/getfi...file=ASI_Advisory_Note_eccent_cleats_2006.pdf

 

[hokie66]

Another solution, my preferred one, is to use a stiffener on one of the plates to prevent sidesway.

 

[human909]

That is less cost effective unless welding labour is very cheap or when we are talking very large structs. The cost difference between say a 10mm plate and a stiffener and a single 16mm plate is neglible a few dollars, maybe up to $5-8. The cost of handling and getting somebody to weld it up with stiffener is greater. The structural elegance and efficiency of a thick plate may not be as appealing by it is generally cheaper.

For very large loads my alternative is sandwich plate either on the strut or as 2 loose additional plates. This avoids eccentricity, avoid extra welding plus you get bonus double shear in your bolts.


Anyway now I've started to sound like a unwanted preacher at your door. [flush] There are many approaches here, good to hear other approaches.

 

[DrZoidberWoop]

Over here (Northeastern USA), as delegated connection designers, we have run into EORs who reject the eccentric slotted/T-stem HSS connections entirely, others who are more lenient, and some who want an alternate calc method used. We generally tend to take a lazy approach by thickening the HSS brace connection plate to handle the full flexure/axial interaction (AISC H1-1a/b) and keeping the support/gusset plate centered on the workline, with no eccentricity considered. Sometimes we get away with the connection and gusset plates having equal thickness, sometimes we have a suspiciously thick plate at the brace, in which case we consider moving to a concentric double-plated HSS connection or other connection concept.

I think our method is a bit old-school and rudimentary in light of the many international studies/warnings presented above, but AISC can be slow to adapt or take a hardline stance. I'm not aware of a generally accepted method on the topic. For me, the most logical approach is probably to use some type of flexural distribution to each plate, based on each of their relative stiffness, and then check flexural/axial interactions independently.

 

[dik]

Thanks for the note...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[dik]

I have some of the connections where the compression load is in the magnitude of 150K... and will use 'sandwich plates' for these. Just for the lesser ones.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[dik]

Can you elaborate on that? Do they have something special? or is it P/A and M/Z...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[GC_Hopi]

AISC discourages the use of plates and give some recommendations; mostly they are saying to use channels or angles to sandwich the connections dik stated. Link

For light bracing loads the old school method DrAoidberWoop mentions seems reasonable and I have used it in the past.

 

[80PercentTruth]

I generally consider the weak axis moment induced if the axial loads are high...even a small eccentricity (0.5"-1") can put a big out of plane moment in your plate and the out of plane bending check is easy to run. I've run this consideration many times with beam shear tab connections using thick plates with high axial demands and horizontal shear. We use 3/8" minimum thickness for shear tabs so unless the out of plane force is greater than 0.5k and the axial is less than 5k, I'll tend to ignore the effect.