Identifying existing roofing system

[SteelPE]

I have what appears to be an existing gypsum roof (see attached photo). I am wondering if anyone knows what type of system this is and if anyone also knows what the diaphragm capacity is. I am very interested in the "hooks" from the deck to the TC of the roof joist.

As my client would like to demolish this portion of the structure (and addition to a building), I would like the value to be 0 but I am not sure.

 

[rscassar]

If the value is 0, is their another identifiable load path for the racking forces?

 

[SteelPE]

I would like it to be 0 because of implications with chapter 34 of the building code and the removal of a portion of what appears to be an addition (no continuous diaphragm from one roof to the next with cold joints in the exterior masonry walls).

Basically, I want to remove what appears to be an addition and not have to worry about the "racking"/diaphragm forces of the existing building.

 

[Ron]

Looking at the clips (uplift resistance), it appears that you have one of several panel deck systems. Tectum was one, but there were several others. Ran into the same system shown in the mid-90's in a building on the east coast of Florida. It was a proprietary system, but I don't have the manufacturer's name handy...I'll look to see if I can find.

Bottom line, no diaphragm in this type of system, unless there were bulb tees involved.

 

[msquared48]

Ron:

The no diaphragm action seems funny as I would expect to see horizontal metal X-bracing between the metal trusses if there was no action. I do not see any in the picture.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

A lot of these systems assumed diaphragm action, but nothing formal existed for the justification of such.

 

[Ron]

If the systems had bulb tees, there was pseudo-diaphragm action since the shear could be transferred in two directions. If bulb tees were not present, diaphragm action depends on the integrity of the panels...often suspect and not always fastened sufficiently to keep in-plane.

 

[msquared48]

OK.

Then humor me here... what is keeping them standing?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Trickle theory.

 

[msquared48]

You mean trickle down engineering, as in ecocomics?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

How do we stand as professional engineers if we are required to modify an existing structure that we know is stable because of an assumed diaphragm, but in the modern age of building codes and compliance certificates there is no concrete evidence to say that this structure is 'up to code'.

 

[SteelPE]

I’m not saying I agree with it or don’t agree with it, but in this instance, if I can prove no force transfer from one section of roof to another then that section of the building (what appears to be an addition) can be removed. Then it terms of the code, if I make no other alterations to the other portion of the structure that reduces its lateral load capacity then there is no need to update to today’s standards and the capacity of the existing deck is not my concern.

In this instance, updating the building is not an option and demolition is not as well. Due to some existing setbacks, the building has been grandfathered through the zoning requirements. These setbacks would not be allowed in the community today.

Attached you will find a wall section showing what I believe is currently in place (from a site visit I did yesterday). Unfortunately, I don’t know how the joist is attached to the wall since it is bricked and mortared into the wall. However, I do know the diaphragms do not attach since I can get my hand into a gap between the addition roof and the masonry wall.

 

[msquared48]

With that gap. I assume water can enter the structure at the wall. I would be concerned with deterioration of the truss ends if this is the case.

All I know is that I would not like to be in this structure at all with any lateral force applied. Looks like it is standing due to the lateral resistance of the embedded truss members to transfer shear and moment, and that assumption is ridiculous. There must be some contribution from the gypsum roof, rated or not, for this structure to stand. Nothing else makes any sense to me. I'd hate to think that a tarpaper roofing membrane is the diaphragm.

Unless there is something we are missing here, I feel that buildings of this sort should either be brought up to a minimum code level structurally, or be condemned and demolished, not grandfathered. I guess that's why I'm an engineer and not a politician.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

msquared48,

No, water can not enter the building at the gap. Currently there is a rubber membrane roof over the building (not shown in the detail). Actually, from what I can see, there have only been 2 leaks in the roof... and only over a 1'x1' areas.

As far as lateral forces. There are many masonry walls within building #1 so I'm sure the structure is not relying on lateral resistance of the embedded truss. I just trying to figure out if you can transfer lateral forces through this detail.... and as far as I can see, you can't.

I don't write the codes, I just try to use them the best I can. Demolishing would be at the top of my list but that is not possible in this instance. Unfortunately, according to the code, as long as I am making cosmetic changes to a structure then I don't have to comply with new code standards.

 

[lsmfse]

Attached is some data regarding these roofs. I remember reading something, somewhere about diaphragm capacities(maybe its in the attachment)