Deep Beam Detail - Reduce Section Beyond Cantilever Support 1

[puszka]

Hi everyone -- new (3-months) EIT here.

I am working on my first detail where I will be specifying a modification to a beam profile to meet an architectural requirement.

My main roof beam has a large span ~50' so will be fairly deep (not yet analyzed but W24 section perhaps). One end extends about 5' past its support post to pick up some structural fascia for an overhang detail. The architect wants the visible roof width to be much thinner than the depth of the roof beam required at the interior section, so I am proposing to cut out the lower section of the roof beam past the post to meet these requirements.

I'm wondering if anyone had done a detail like this and how the analysis works -- I understand how the moment is operating in this area but I never enjoy having an abrupt change of geometry and the rules of analysis aren't clear cut (that I know of).

I was thinking to match the flange on the top side by welding a similar plate to the cut edge, and round the 90 degree angle to prevent a buildup of stress at the point where the two sections transition. I'm also going to put in some web stiffening plates at the point of transition to keep everything stable.

Beyond these details, is there anything else to consider which I am overlooking that anyone can see? I posted an image for clarity.

Would you consider a detail where the beam stops at the post and I sister a member onto it for the overhang a better pursuit?

Thanks in advance for the opinions/advice

 

[KootK]

Your instincts are pretty good on this I'd say. This is what I'd consider to be the classic, heavy duty solution to this situation. The new bottom flange on the overhang probably does not need to be as beefy as the top flange of the original beam. If the numbers panned out, one could even eliminate the bottom flange on the overhang altogether and let that just be a WT section.

 

[KootK]

Would you consider a detail where the beam stops at the post and I sister a member onto it for the overhang a better pursuit?

I would. There might be economic advantages to setting it up as I've shown below as an alternate. Maybe you need/want that horizontal stiffener, and maybe you don't.

 

[dik]

Koot's second one is the approach I would use; it's likely less costly. A couple of minor concerns... depending on your climate, you may experience cold transfer through the joint. I usually use a Fabreeka product for a thermal break... The reason Koot's second approach is better. Another issue is that the 50' span may deflect and lift the end of your cantilever.

I assume you are using the W24s to pick up purlins or joists on the interior... if not, have you looked at using long span joists?

Good luck... as Koot noted, you're on the correct path.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EdStainless]

You can't sell him on a taper?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[puszka]

@KootK

Ah, yes, I see how the horizontal stiffener into the flange is a better detail to transfer the compression into the main beam.

And secondly, it makes sense to save a lot of steel to rig up a detail where I don't have to waste 50% of the material from 5' of a huge beam. I'll look into #2 as the option to pursue.

Thanks!

 

[puszka]

@EdStainless

Unfortunately, no. I can't mess up the look of planes and right angles. The AESTHETICS, man! The AESTHETICS!!

 

[puszka]

@dik

This is just a boathouse, so no need to worry about a thermal break. It's *three* season but effectively just for between May24 and labour day.

The W24 (or similar) is the main roof beam, and spans from opposite corners of rectangular footprint. So the roofline is skewed and looks kindof like a butterfly -- two low corners and two high corners.

 

[dik]

Thanks... It's nice to see Architects doing boathouses... kinda warms the cockles...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik