Be careful when evaluating the shear diaphragm of gypsum decks. Older decks have highly variable properties from two significant issues...roof leaks and post-construction penetrations.
When originally placed, the gypsum deck likely had a compressive strength of somewhere between 500 and 1500 psi. Subsequent roof leaks typically causes leaching of calcium compounds causing strength loss (localized). Further, when modifications are made to the roof such as adding vents or other penetrations, there is rarely any reinforcement provided to restore the diaphragm. Even a penetration as small as 1 sq. ft. can affect the strength of the deck.
All of this should be checked in a careful visual inspection and perhaps even an Infrared Thermographic scan to spot moisture anomalies in the deck.
The connection of the deck to the trusses is also a critical factor if you want to evaluate the shear capacity. After years of buffeting and vibration caused by wind, there may be localized failures around the connectors that could reduce or eliminate diaphragm capacity, Close examination of a few critical connections would be beneficial.
I have not designed a gypsum deck in decades, but my recollection is that the gypsum planks have a tongue-and-groove, sheet metal edge around. The metal edge was fillet-welded to the supporting steel joists or beams at each advancing edge of the plank. Alternatively, they could be welded, overhead, to the edge of the supporting steel.
the owner is proposing an extension and addition of the existing gypsum roof that will eventually get connected to the new roof made from diferent material (e.g. lightweight insulating concrete). and for that purpose part of the existing gypsum roof overhangs will need to be removed and eventually keep the remaining main area of the roof inside the perimeter walls.
does this sound possible? will the remaining existing roof retain its original strength?
I would suggest contacting a contractor that does this type of work. While they may not design the system, their experiance installing systems would definitely give some insight into what works and what doesn't. The only name I have is rather old, but try Anning Johnson Co., they have a Chicago area office 708-681-1300. You might also find some others on the web.
The gypsum roofs I have worked on were poured on steel bulb tees welded at their base flanges to the roof beams or trusses. The bulb tees had gypsum board or plywood board spanning between them and resting on the upper surface of the base flanges. The plastic gypsum was then poured on top of this board to form the roof deck.
The contractor finally removed the ceiling and completly exposd the underside of the roof deck. After a site visit I found a 24" X 24 " opening in the roof that was ued for duct penetration. Now this opening is covered / fixed with some material on top of the original roof. the opening is cmpletly exposed from underneath. and I measured the deck thickness to be about 2.5". aditionally, there was a one very limited and small area of visible moisture spot along the groove and tonge joint. the spot runs about a foot and a half along the joint and about two inches in width.
the big question I have been asked if the roof deck should remain, or be replaced. To answer this question I need to check the shear diaphram. and my question to you is this:
1. what shear (lbs/ft) value do you use for diaphram shear resistance for the deck in a normal condition? (ignoring the opening and moisture presense.)
2. is the opening, as described above, significant enough to warrant the replacement of the roof deck?
3. is the moisture segnificant to order an Infrared Thermographic scan testing?
The one gypsum roof deck I worked on involved spot leak repairs. As I remember it, the proceedure we followed was:
1. We inspected the deck from below to locate suspect areas, as you noted, water damaged areas are generally quite visible.
2. We opened the roof from the topside, working from areas known to be solid. Don't forget your OSHA harnesses and hole protection!
3. We removed the damaged poured gypsum deck and panels (2'x4' if I remember correctly), but we left the bulb tees where we could, since they were still in good condition. However, many had to be cut out along with the deck, so in most instances it is cheeper to install new.
4. We placed new bulb tees, laid gypsum "form panels" in the bulb tees and then poured the new gypsum deck. We used a small grout pump to pump the gypsum to the roof deck.
5. The existing roofing membrane was then completely removed and a new roof membrane installed.
The key, if you do the job yourself, is finding the matching materials. There are specialty contractors that do this work and I'm sure you can find some names on the web.
1. The gypsum plank (you mentioned tongue and groove joints, so I assume this is not a "poured" gyp deck) will contribute only enough resistance as can be supported before buckling the plank (neglecting the T&G effect). You will need to have some testing done on the deck to determine its compressive strength. This can be done by compressive strength testing. You might also consider using a test commonly used in Geotechnical work...the direct shear test. This can be used to develop a direct shear resistance value of the material, just be careful in its application as this is a "micro level" shear of the material versus a "macro level" system resistance desired of the plank/support system.
2. Negate much of the gypsum plank contribution and consider most of your diaphragm resistance comes in the network of the steel framing. Remember, one significant contribution of the deck plane is for lateral bracing of the structural framing members of the roof framing.
3. The opening you described is not sufficient to warrant deck replacement by itself. The penetration can be reinforced to restore diaphragm.
4. An infrared thermographic survey would be good to spot areas of concern, not only for the diaphragm, but also for the roof membrane system.
Thank you for your thoughts. your previous involvement in a similar case is well appreciated. Thank you.
Ron,
thank you for your thoughts. your experience is very helpful.
the deck is actually consisting of poured gypsum over gypsum boards that act as form deck.but thank you for your help.
The question that I still have is what practical shear value could be used to do a quick check on the diaphram capacity in transfering shear to the end walls before ordering any additional expensive remedial or testing work?
Consider ignoring the dubious diaphragm value of the gypsum deck and create a horizontal wind truss in the end bays with bracing at the plane of the bottom chord of the trusses.
v2-if you want to do a quick, conservative check, assume "zero" for shear capacity of the deck. This will tell you how much you need in the struts to pick it up.
I have a 1970 publication from the Gypsum Roof Deck Foundation and the Gypsum Association entitled "Design Data: Poured Gypsum Roof Decks." In it are given shear transfer design details for diaphragm action and the following statement:
"Gypsum roof decks have a high ability to resist lateral forces. Based upon extensive test data, horizontal diaphragm shear values in excess of 1100 pounds per lineal foot are allowed by each of the following regualtory jurisdictions:
ICBO- UBC
City of Los Angeles- General Approvals 22107 and 23267
City of San Francisco-Approval 400G10.1
Departments of the Army, Navy, and the Air Force- "Seismic Design for Buildings"
It does not give diaphram shear tables or assembly/fastener requirements, but refers the reader to the listed approvals for the particulars.
I checked the current military "Seismic Design for Buildings" (TI 809-04), and it does not have design provisions for gypsum deck diaphragms.
Even if you can find these 1970 design criteria, I would be hesitant to use them. You have water damage in parts of the gypsum deck, most likely do not know all the connection details, etc. Also, you would not want a future roof failure to destroy your diaphragm's shear resistance.
I agree with redhead and Ron, assume a zero value and use steel bracing to give your diaphragm its capacity.
