Warehouse Foyer Appendage - ASCE 7-16 Seismic Considerations

[Futzin]

I submit for discussion a large warehouse with a relatively small foyer area, half the height of the warehouse. The warehouse is a steel braced-frame structure. In the interest of avoiding an additional row of columns (i.e., design the foyer as a structurally independent "addition"), I'm considering the foyer essentially as a "lean-to", supported by the warehouse columns on one side. See the below plan, detail, and section sketches.

The foyer will feature a concrete roof diaphragm (architecture-driven). With reference to plan north, N-S lateral loads acting on the foyer will be resisted by moment frames along the west column line (architectural constraint, cannot use braces) and by the warehouse columns in flexure along the east column line. I'm conceptualizing these warehouse columns as simply-supported beam-columns spanning between the column base and the warehouse roof diaphragm, acted upon by a mid-span point load from the foyer lateral reaction (obviously, the baseplate connection is more rigid than the warehouse diaphragm so not truly simply-supported). Similarly, for E-W lateral loads acting the on foyer, I'm considering the warehouse columns to again act roughly as a simply-supported beam, distributing the mid-span reaction from the foyer to the ground and the warehouse roof diaphragm (I acknowledge that I'm showing in the sketches that this reaction would be resisted in weak-axis bending, does look problematic).

Theoretically, I think this works. My FE model is statically determinate and the trial column sections are passing code checks. ASCE 7 code space is where I am unsure. The structure is SDC C, so am using R=3, no AISC 341 special system. It seems clear the the foyer, being half the height and using a combination of moment frames and hanging onto the warehouse, will have completely different dynamic characteristic than the braced-frame warehouse. It seems appropriate to consider a different C[sub]s[/sub] factor to calculate the seismic base shear of the foyer per ASCE 7-16 Section 12.8 ELF procedure, effectively treating it as a separate structure sharing structural elements with the warehouse. Applying the ELF seismic loads concurrently on both sub-structures is probably conservative since I would not expect the foyer and warehouse to have the same natural frequency. Is there anything I'm missing, though? Do the warehouse columns I'm attaching to effectively become "collector elements" requiring the foyer reactions acting upon them to be amplified by an over-strength factor per Section 12.10.2.1? Any other practical considerations? Easier just to add the double column line and design the foyer as a independent structures, moment frames in both directions (architectural constraints)?

TL;DR - What to do when a building appendage is too much of a "real" structure to be considered under Ch. 13 of ASCE 7?

 

[dik]

If an addition, unless you want some significant foundation costs to the existing, it would be good to look at minimising the weight added. This would be achieved by steel, not concrete. Just a thought... You also have a significant snow accumulation, if you are lucky enough to live in such an area.

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

 

[Futzin]

@dik - Glass half full, I like it.

Not an addition. New construction.

 

[phamENG]

If it's one structure, analyze it as one. Otherwise, you'll need to physically separate them and that'll get messy.

Have you looked at 12.2.3.3 R, C[sub]d[/sub], and Omega[sub]0[/sub] Values for Horizontal Combinations?

 

[driftLimiter]

TL;DR - What to do when a building appendage is too much of a "real" structure to be considered under Ch. 13 of ASCE 7?

Consider going to ASCE 7 Ch 15? Or like phamENG says consider it as part of the main building.

 

[Futzin]

@phamENG - Agreed that separating it is not preferable. The concern is that the foyer portion will act as a completely different structure that the remainder of the structure (considering it's half the the warehouse's height, utilizing a different LFRS) despite sharing structural elements, calling into question whether analyzing it as one is valid, short of performing a response spectrum analysis. Trying to stay in ELF space.

 

[phamENG]

Why not design them to use the same LFRS? That would be my preferred option.

 

[Futzin]

@phamENG - Architectural constraints make that a bit tricky. Even if I was able to get a brace or two on the west side of the foyer, I'd still be counting on the flexural resistance of the warehouse columns on the east to transfer the foyer's lateral reactions either directly to the ground or into the warehouse braces. That is unless I stack two 20' braces on top of one another where the foyer meets the warehouse. Doesn't resolve the issue for the E-W lateral reactions on the warehouse columns though.

 

[phamENG]

You can mix and match and still have the same system (see my code reference above on horizontal combinations). If it's a different/separate system, you have to have structural separation to keep the independent structural systems from slamming into each other.

So I'd stack braces along with warehouse column line, use moment frames if needed on the other long side, and then either rotate the warehouse columns to take the reaction on the strong axis to carry the load up into the diaphragm or add bracing in that direction.

 

[skeletron]

Option 1: Model/analyze as one structure.
Option 2: Model the warehouse as the warehouse (plus some additional mass to account for the weight of the foyer).
Option 3: Model the foyer as it's own structure (plus some additional weight from the main the building that would be transferred in the region of the foyer)

Design all components to be within the bracket of above. Then check the foyer roof similar to a non-structural component category (exterior appendage, horizontally cantilevered floor, etc.). If all that works, you're at least in the right ballpark after 4 checks and an eng-tips thread. Finally, are there exits that need to be protected? Or glass/walls that need to not move in order to provide egress? If so, then spend time detailing those items.

What is the % weight of the foyer vs. the 400x400 warehouse? 5-10%?

Probably easiest is to design the foyer independently of the warehouse. That's usually the preferred method in warehouse mezzanines, which is kind of analogous.

 

[dik]

If the existing is in Concrete, I'd likely do the additon in steel... it's just a matter of 'gluing' them together. If a seismic area, it could get a little interesting. Rotate your outer columns by 90deg for a start. Do you need clear space? or can you put x-bracing at 80' and at the ends? Building envelope issues will be the most fun.

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

 

[HTURKAK]

This is a large warehouse having dimensions 400' X 400 ' and H=40 ft. Just curious ; are there EJ'S ?.
Your post implies the warehouse is one str.and without EJ's. What about the diaphragm flexibility when you check as per 12.3.1 ?
Can you post the locations of vertical bracings and roof level horizontal bracings?

I screened the previous posts and my opinion is ( ofcourse if it can be an option) , to raise the roof of the foyer to 40' and Model/analyze as one structure. I would prefer the foyer columns as gravity columns ( nominally pinned ).

If the foyer roof shall be at level 20' , i would do two analysis ;

1- Analyze as one structure ( still the foyer columns gravity ) and foyer girders to warehouse columns conn. would be moment conn.

2- Assume the foyer is non-strutural component and analyze as per Chapter 13

...and take the more stringent one .