Lateral Analysis With Raised Floor 1

[SliceCity]

Hello All,

I am working on a lateral analysis for a 1 story residential remodel with a raised floor system. Typical floor is raised about 18" above grade and in some sections about 2'6" with a cripple wall. When determining the seismic mass, do I consider the weight of the floor? What is typically done for a raised floor?

Based on the vertical distribution of the seismic forces and the difference in height between the floor and roof above grade, the majority of the seismic weight is moving up to the roof. In a sense I am dumping the total weight of the walls plus the weight of the roof into the roof diaphragm. Is there justification to neglect the floor from the seismic mass and consider this as a single story analysis? My gut is telling that the floor should be considered.

 

[phamENG]

How would you handle it for a two story slab on grade? That's essentially what you have here, but the first story walls are very, very short. A slab on grade usually isn't used as a diaphragm and is ignored for the LFRS, so it might as well be dirt.

 

[SliceCity]

For a slab on grade, yes I would not consider it as part of the seismic system. So I should consider this as you mentioned, a two story structure where the raised floor is a diaphragm of its own?

 

[phamENG]

Yep. What's the difference between this raised floor, and the raised floor of a second story that would be a diaphragm? I would guess nothing. (At least, around here they're built the same.)

 

[SliceCity]

My guess is the height above grade. Based on Eq 12.8-11, this is causing the entire load of the structure to more to the roof diaphragm. I'm wondering if there is justification to assume that the raised floor is basically at grade and it can be treated similar to a slab on grade. If something is elevated 2ft above grade, will it really cause that much influence to the seismic mass? Is this typical for a raised floor to design it as a two story system? I am just wondering if I can justify it as a 1 story and not design my shear walls to take that much load.

 

[SWComposites]

Short framed stub walls between a basement foundation and the first floor diaphragm are notorious for failing in a seismic event. And also notorious for sliding off the foundations if not well anchored. Look for photos from Northridge, San Francisco and Seattle area earthquakes. So the first floor mass should likely be included in the seismic analysis. Retrofits that anchor the walls to foundations and provide shear resistance to the walls are required. (I personally installed a bunch of anchor bolts and OSB sheathing all around the stub walls in the basement of the 1914 house I used to own in Seattle).

 

[driftLimiter]

I want to clarify something with OP and for others. Raised floor is a specific term in ASCE 7 regarding an architectural assembly. These floors are normally inside the building envelope (i.e. the studwalls are continuous as they go up from the slab to the roof. Raised floors are generally self supporting for seismic and should be designed for site specific hazards.

Hard to tell if your scenario is like I have described above, or if you have perimeter cripple walls and the studwalls are sitting on top of the (raised floor).

If you consider it as a structural level for the vertical distribution of base shear I am curious what type of Cvx factors your getting at the roof and at the floor.
In any case a floor down that low is most likely going to be controlled by the Fp seismic load to the diaphragm not the Fx load.

I think if it were me I would treat it as a floor diaphragm and use the Fp level diaphragm loading on it for design for in-plane shear and boundary connections, and chords.

If the existing floor is inadequate for that loading you could look to provide interior support, or find a way to strengthen the diaphragm.

At a minimum it is important to provide sufficient connection at the boundaries to adequate vertical lateral resisting elements (shear walls or stem walls).

 

[phamENG]

driftLimiter - thank you for pointing that out. I've dealt with those in data centers where they needed to route cooling and cable trays under the floors, but never in a house. This sounds (reads?) like a crawl space.

 

[driftLimiter]

Agreed seems like a crawl space. I would contend that if the lower floor diaphragm is anchored (for shear) into the stemwall then its not a bona fide floor and its seismic mass does not make its way up to the roof. If instead we have a stacked wall configuration or a 'raised floor' type diaphragm that is anchored for shear into the sides of studwalls then the mass does need to be included.

 

[SliceCity]

DriftLimiter- I have two conditions going on in this project. The whole house is raised floor in which the joists bear directly on the footing pedestal. The existing living room floor is at a lower elevation compared to other sections of the house. We are raising that floor so the whole floor is now leveled. In doing so, we are installing crippled walls, about 1'-6" high that are anchored to the footing pedestal and the floor joists sit above. The stud walls will then continue to the roof above the floor joists. The sheathing for the shear wall is continuous from the roof level top plate to the bottom of the cripple wall or the floor joists to the sil.

By considering the raised floor as its own level and including it as part of the seismic mass, I get a Cvx of 0.92 for the roof and 0.08 for the floor. This indicates that yes, the floor diaphragm, Fp will govern the design on the floor diaphragm, chord, & collector.

When designing my shear walls, since I am considering the raised floor as a level, the seismic mass is the total weight of the floors, walls, and roof. If I dont consider the floor as a level, the seismic mass will be half the wall weight and the roof only. In addition, since my Cvx is 0.92, I am basically taking the whole weight of my structure as my Fx load at the roof. This is drastically increasing my floor shear.

My main question is that since my floor is raised only 1'-6" off the ground, will it really influence the design of my shear walls that much? Will it cause all the weight of my walls to be included into the seismic mass as if I was designing a two story house? Its just hard to by that raising my floor that little off the ground will cause such a drastic influence on my seismic design.

 

[driftLimiter]

If its tied into the foundation on the high side then I wouldn't treat it as part of the seismic mass. Not the floor joists, the floor diaphragm. If you have some ledger or mudsill that you get boundary nails from the diaphragm anchored into the foundation system.

 

[SliceCity]

Im not following what you are implying here?

 

[driftLimiter]

Well Im still trying to guess at the details of the house. If you can take the seismic shear of the lower floor level directly into the foundation system then its not going to contribute to seismic force at the roof level.

 

[SliceCity]

This is the First Detail for my typical raised footing. I dont have one yet for the addition of the cripple wall. But just picture that it the joists will bear ontop of the cripple wall that will then be anchored to the foundation in a similar manner.

 

[SliceCity]

For the Section on cripple wall image the stud wall is just a cripple wall here and the sheathing is continuous and cripple wall is on a sil with anchor bolts. The max height of the cripple wall is like 1'-3" above the footing shown in the detail above.

 

[driftLimiter]

Okay based on your detail and my last post, I would feel comfortable saying the seismic mass of the floor does not make its way up to the roof.

But I would still design the floor as a diaphragm for Fpx. And I would strive to ignore the cripple walls for shear, consequently I would need to ensure there are enough E.N. and Sillplate anchor bolts over the concrete wall to transfer the Fpx level reaction force directly into the concrete. The other consequence is that your rim joist becomes a collector so you may need to amplify the load accordingly, and will want to ensure rim joist continuity or use tension ties.

**EDIT**

You also have to decide if you want to take the shear from the walls above through this load path or if you want to continue the shear walls down the cripple wall area. This would greatly impact the level of force in the Rim.

 

[SliceCity]

So you would basically design the shear walls to support only the roof weight and half the wall weights when designing the sheathing and the hold downs, but you would use the floor and roof diagrams forces to design the anchorage and splice locations?

 

[skeletron]

In general, I take it into account. At the very least, I do a little back-and-forth about what it's overall effect. If the case is a cripple wall (which we tend to avoid), then definitely take into account and make a loadpath.

 

[SWComposites]

you should also use the floor and roof diagrams forces to design the crippling walls as shear walls. see my post above. see also

https://www.earthquakecountry.org/step4/cripplewalls/

and

https://www.earthquakecountry.org/step4/cripplewalls/

 

[driftLimiter]

@SliceCity

Really this one comes down to the details and the load path. If you have enough concrete stemwall and footing to resist all the loads generated at the lower floor and you can drag them back to it, then you could justify not including the mass in the vertical distribution. If that load path is in sufficient then look towards using the cripple walls as shearwalls and distribute the mass vertically with Cvx.

I don't want to further confuse you by speculating about the specifics.