Shear Flow vs Direct Moment Transfer 1

[David Deck Guy]

I'm having a difficult time wrapping my head around this.

We often have aluminum tubes that connect to steel posts for our pergolas. Given the welding properties (or lack thereof) of aluminum, its been really hard to develop a satisfactory moment connection. One idea recently floated by our construction guy is to run an angle underneath the aluminum tubes and place the moment connection at the post-angle interface.

If I model the tubes as taking the entirety of the moment in RISA, is getting the moment into the posts really as simple as:
-check shear flow between tubes and angle (its low and can easily be taken care of with some tek screws)
-design moment connection between angle and post

I don't need to design a direct moment connection between the tubes and the angle?

My intuition is telling me there is something I am missing. Any confirmation of my (in)sanity on this problem would be greatly appreciated.

 

[jayrod12]

Can your angle handle that level of moment? That would be where my question comes in.

 

[David Deck Guy]

That's a good question. In retrospect, I'll need to beef up the thickness of the angle. In looking at the angle, I am considering ϕMy with 36ksi steel. If I were to consider the angle as the primary load carrying member, discount the tubes, and considering buckling, then the angle would not work.

 

[BAretired]

I don't need to design a direct moment connection between the tubes and the angle?

You certainly do! Otherwise the HSS beam is not moment connected to anything.

Why not use a steel HSS beam and skip the angle?

 

[canwesteng]

You'll have some shear lag effect in order to get the moment from the angle into the HSS as well. I'd probably design as though you had two interted L

 

[JAE]

If the upper aluminum tubes are imparting moment to the vertical tube via the angles, the screws will take direct tension (depending on the direction of the moment).

 

[TLHS]

My honest question is whether using aluminum is actually saving you anything here? If you're using steel posts and are open to steel angles, it's presumably not a corrosion issue.

When I've tried to do fixed/continuous style construction with aluminum it's generally given annoying detailing issues to the point where the benefits of using aluminum are not justifiable unless it's very very repeatable work or very very low loads. Straightforward detailing for fixed connections generally requires some amount of welding for either plates or direct connection between elements. This puts a weld at a peak moment location, which tanks your capacity pretty significantly depending on the grade of aluminum being used. It's never going to be great.

With things like what you are proposing, there's going to be issues with bending in the wall of the HSS. Trying to connect any significant forces at right angles to the thin wall of the aluminum tube is going to be an adventure.

 

[Eng16080]

I agree with jayrod12. If the moment is being transferred between only the angle and the column, then the angle will need to be able to resist that moment. Considering that, I suspect this would be similar to designing the angle and the HSS pieces on top as separate members. I'm not sure what you gain by transferring shear flow (to make them act compositely) when the moment at the ends cannot be shared.

In summary, I think I would design the angle to take the entire load if I were to do what you're intending. I would, however, look for a different type of connection first, which can transfer moment between the aluminum HSS pieces and the column.

 

[TLHS]

You might be able to do something with side plates welded to the steel posts and through bolts into the aluminum. Might be pretty high deflection for a moment connection, though.

 

[David Deck Guy]

Eng16080, I think you just made things click for me. With the shear connection between the tubes and the angle, moment is shared. Shared, meaning there is still moment in the tubes that I need to transfer into the post.

Thank you everyone else for the input as well.

 

[human909]

Eng16080, I think you just made things click for me. With the shear connection between the tubes and the angle, moment is shared. Shared, meaning there is still moment in the tubes that I need to transfer into the post.

Thank you everyone else for the input as well.

You are close, but still not there yet. Your statement that there is still moment in the tube is incorrect and misunderstands the behaviour. Reread Eng16080's post. "the moment at the ends cannot be shared".

With that connection there is no moment in the tube at the end as there is no reaction. All the moment is transferred to the angle and the angle to post connection sees the full moment.

 

[David Deck Guy]

I think I am there, but just may have been imprecise with my language.

If I use the tube-angle built up section to carry the moment, I need the moment connection to connect to both the tubes and the angle. If I want the moment connection to only be between the post and the angle, then the angle needs to carry the full moment across the member length.

Right now I am leaning towards the latter and planning on increasing the depth of the angle. The aluminum tubes will just kind of....sit there. The aluminum tubes are necessary for all the other pergola attachments, such as gutters and louvers.

As for another commenter's question about why have aluminum tubes in the first place, our construction director does not like to use steel tubes because water can get trapped in there and corrode the tubes.

 

[BAretired]

Sounds like a poor reason to choose such an inefficient combination of materials. Using end plates, a steel HSS can't take on water.

 

[David Deck Guy]

BAretired, I don't like using these aluminum tubes, but they are perforated all over the place for gutter and louver connections. This is one of the manufacture's details.

 

[BAretired]

Okay...didn't know that. I don't have any experience with aluminum. Carry on.

 

[Eng16080]

It's perhaps not relevant to the original question, but concerning the detail above (per some recent research I'd done) I'm not sure it's a good idea to have stainless steel and aluminum in contact. (I assume #10 SS TEK refers to stainless steel.) My research on this was limited and might not apply to this particular condition.