## Approx. 300 sq ft, 19 ft suspended architectural ceiling

## Approx. 300 sq ft, 19 ft suspended architectural ceiling

(OP)

Existing commercial one story building with 18" deep TJI joists

Tenant is proposing a suspended soffit/architectural ceiling 7 psf max weight as of now, 300 sq total across two areas. The ceiling is 8 ft above finish floor, and the highest point of the roof is up to 27 ft at the ridge. So a 19 ft vertical suspension at the high point and 11.5 at the low point.

In the past I have done this with tension ties and all thread rods with enough diagonals to limit compression in the rods, but at much shorter suspension heights. Below is a rough schematic of this situation. Appreciate thoughts, guidance, etc. Do kl/r =200 and buckling apply to the rods in this orientation? It is a bit outside of my wheelhouse and I do have a RISA model but can use some help there.

Tenant is proposing a suspended soffit/architectural ceiling 7 psf max weight as of now, 300 sq total across two areas. The ceiling is 8 ft above finish floor, and the highest point of the roof is up to 27 ft at the ridge. So a 19 ft vertical suspension at the high point and 11.5 at the low point.

In the past I have done this with tension ties and all thread rods with enough diagonals to limit compression in the rods, but at much shorter suspension heights. Below is a rough schematic of this situation. Appreciate thoughts, guidance, etc. Do kl/r =200 and buckling apply to the rods in this orientation? It is a bit outside of my wheelhouse and I do have a RISA model but can use some help there.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

I would consider kL/r=200 as a limit.

Are you able to slip tube over the threaded rod hangers to give it compression support?

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Using kL/r as a limit, starting with 3/4" diameter threaded rods (r=0.1875), I am getting a maximum length of 37.5"

200 = (1.0)(L)/(0.1875)

To get my longest member of 18.5 ft to work, I would need a threaded rod diameter of 4.44" to work...

Something seems off or maybe I am just way off in using threaded rods. How would you detail tube steel over the threaded rod hangers? I was planning on attaching these rods to Simpson HTT/HDU type brackets at the wood framing and cold formed soffit.

Is my idea that I have enough diagonals for them to resist the movement in tension only incorrect?

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Or am I missing something here? This seems a tension only setup.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

It is incorrect if you are expecting truss action, but it should work provided the top joist can sustain the additional ceiling load. In fact, you could eliminate all of the diagonals and use only the verticals. And kl/r for tension members can be 300, not 200.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Risa doesn't appear to check kl/r for tension only members. When they were set to Both Ways, it gave the kl/r failures. Does kl/r apply to tension rods?

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

This is the wrong approach, imo. Better to provide a structure at ceiling level and get rid of a forest of rods.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

I am not at all concerned about the gravity load. I am needing to resist a horizontal nonstructural seismic anchorage load at the soffit level. Without the vertical anchors, there is nothing supporting the gravity weight. Without the diagonals, there is no lateral bracing.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Only green verticals could be in compression and would therefore need to satisfy kL/r<200

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

The former might give enough gravity load in those hangers so the model converges. The latter would be unstable with all "tension only" members.

In RISA you can also assign "euler buckling" behavior to the members so they can take a very small but not zero amount of compression allowing the model to converge.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

-why you are so eager to include compression members

-why you are so eager to have your lateral bracing inefficient and impractical angles

-and why if you do want to inlcude compression member you don't remove half or more of you lateral diagonals

This would seem to be the case. It does seem to be a case of somebody who isn't thinking outside what the computer model says.

So where is your dead load?

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

The diagonals should all be tension only, maybe even with cables such as the Gripple wire rope system.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Assuming yes, I would tend to hang a steel stud or unistrut type structure frame from the structure above and have those 90 degree vertical members act as compression members (chances are if they work for compression at that length, they will be sufficient for tension in the opposite load case). I would then add cable or rod bracing at 45 degree angles each direction for lateral stability. I wouldn't even consider looking at this from a standpoint of a truss.

## RE: Approx. 300 sq ft, 19 ft suspended architectural ceiling

Either way, what's wrong with just suspending the grid (like Aesur suggests) from 12ga wire as pretty much every other suspended ceiling is supported? There are tons and tons of details and references for exactly how much bracing is required per sq. ft. of suspended ceiling for high seismic design cats. Pretty much you just hang everything from wires then provide steel stud bracing every xxx (300?) sq. ft. The steel stud bracing consists of diagonals and compression studs. No need to brace every single vertical.

Otherwise I'd agree with Aesur's suggestion.