L Shaped Building Expansion Joint

[SteelPE]

I was taught to avoid L shaped structures..... meaning they need to have an expansion joint near the vertex to avoid odd seismic behavior.

Last week I had a client send me over a new building.

Code = IBC 2015
Location = North East USA
Single Story (approximately 29' to top of steel)
Material = Steel framed with metal mixture of CMU and metal studs

After a bit of work I was able to get a ton of bracing into the building (see attached).... maybe too much and I may be able to take some out upon further analysis. The issue I have with splitting the building is that I will have to add even more bracing into the building.

With this much bracing, do you really think I need to add an expansion joint to it (considering I have the vertex braced pretty well)? What do other thing of this issue?

 

[KootK]

In a low seismic region, and with a building where you're just starting to get into the "need a joint" range, I'd be inclined to omit the joint. It's also worth noting that the seismic issues are predominantly an issue in buildings where the diaphragm is rigid. At best, a bare steel deck roof is going to be semi-rigid and, in times gone by, would have been classified as flexible. I'd run some diaphragm collectors from the re-entrant corner into the knuckle for good measure / good story telling but that usually tends to fall into place naturally on buildings like this anyhow.

 

[HTURKAK]

I would prefer a seperation joint along axis (F). and i think the green dashed lines are the proposed vertical bracings..

If so, the bldg restrained along axis (1)

moreover, i did not see VBs along axis 4 and F1

You may post the proposed structural plan and roof bracings to get better responds..

 

[rowingengineer]

I would add a joint on grid 4 based on assuming your using the CMU as bracing/part of the structural system.

 

[SteelPE]

HTURKAK,

Yes, the dashed lines are the vertical bracing locations. We have bracing against forces in the E/W directions at lines 1 5 and 8.... and the N/S direction at A, F and I.

Currently there is no roof bracing. We are planning on using the steel decking as a diaphragm to distribute the lateral loads to the vertical bracing location. Currently we have 3" 18ga N decking. We have calculated the shear capacity of this type of deck and found it to be with allowable limits.

rowingengineer,

Masonry is only 12'-0" high at the loading dock locations. Overall building is 29' tall. So metal studs and EFIS will be used to fill in this remaining distance.

 

[HTURKAK]

The roof cladding may be designed in the form of stressed-skin diaphragm and provide lateral stability for the structure. I think the span of the frame 48 ft and there are columns at axis 7, G, G2 .

For this plan, my personnel choose would be PC columns with flat steel truss roof so the VBs could be totally deleted.

 

[SteelPE]

PC columns... as in precast columns? Yeah, that's not how it's done here.

Also, thanks to Amazon a steel truss roof is out too.... unless you want to wait a year for your steel trusses (aka steel joists). Client it requesting a steel beam roof, hence the 3" 18ga N deck.

 

[rowingengineer]

I would go without a joint if the masonry isn't part the the system, as I wouldn't for a 100mx100m building of steel.

How does the roof sheet load transfer go at the hip or valley at the turn in the building. In Aust we don't have the higher level roof sheeting bracing system so our roofs are limited in capacity and details, so we would provide a braced load path at this location.

 

[SteelPE]

This isn't a gable system but rather a flat roof with internal drains. So there is no hip/valley condition. So the complexity is a big deal.

 

[rowingengineer]

Not the done thing in my area so never heard of a flat roof, internal drains and steel sheet. But then again my dad found it interesting that when sailing the Nile roofs for a building were a choice not a requirement.

 

[m_struct]

"L" shapes => reentrant corners

The code has detailing requirements (collectors & connections) for reentrant corner irregularity.

It is more of a multi story, rigid diaphragm, in high seismic region issue. With the proper detailing and layout of the seismic elements, it can be designed around.

Good to see BF at the end of the building. Are their collectors that deliver the lateral load from line 4 into the BF along 5? Can you add a BF alone 4?

From my understanding, your building is single story building in a low seismic region, with a light weight roof (likely a flexible diaphragm) - is it wind governed or seismic governed?

 

[hokie66]

An expansion joint in this type structure would mainly affect the roof (diaphragm and roofing). The block walls should have control joints which should suffice.

But with 12' high block and metal studs above that, what gives the wall its strength and stability normal to the wall?

 

[SteelPE]

hokie66

Not really looking to get into a debate on how we are going to get things to work. I was just mostly interested in whether or not we needed an expansion joint.

To answer your question, we will need to add a structural channel at the top of the wall and introduce wind columns at locations with this channel to support the channel (approximately 20' o.c.).