Give me your wisdom: how is plasterboard used in your practice? 2

[Greenalleycat]

Hopefully I'm not pushing the boundaries here. I'm writing my Master's project which relates to plasterboard usage in construction (very simplified, it's a part of a much larger project ongoing for my client).

As part of this, I am comparing New Zealand practice to international practice, primarily in Europe & USA (lol we use it as our primary wind and seismic bracing system).

The main uses for plasterboard here are 'standard' (substrate for plaster and paint), wet area, bracing, fire, and noise. For us, bracing is all based on a standardised test, and fire and noise systems are all tested systems with details provided by manufacturers. Standard and wet area boards have a manufacturing standard.

I have dug around on Google to compare, but I know this only ever teaches you so much. So I'm hoping the smart minds on here wouldn't mind sharing their 20s summary of how plasterboard is used in your industry.

 

[JoshPlumSE]

Okay, I'm a little late to this conversation. So, I'm going to some broad over-arching comments:

a) In the US, we do NOT generally consider drywall to be a structural component. It is probably more rigid than many of our wood shear walls, but it isn't very ductile at all. And, OSB or plywood are very effective even when wet. If we started counting on dry wall, then any time there is a significant leak we'd have to re-evaluate if the structure is still laterally safe. Think about hurricanes and such.... Probably not a good idea.

b) Here on the west coast, many (most) of our residential houses have stucco exteriors. This is basically building paper, chicken wire and a stucco. Not all that different from what EN16080 was talking about in NZ. Except that a lot of these walls are also plywood shear walls. Perhaps, I should point out that Stucco is another "alternative" load path for these wood buildings in the US. We don't rely on it, but it does give some redundancy to the structure.

c) Since we're talking about NZ here, I though the NZ codes (especially for concrete) were based on ductility, energy absorption and such. In that sense, relying on extremely brittle materials (like dry wall or stucco) would be something of an anathema.

d) There are cases (usually when trying to retrofit an existing building) where you can rely on drywall for structural support. I've never done it. But, I saw a lecture from an engineer that talked about it. I believe the project was an existing (and very old) building that would never meet current codes. They were retrofitting it to improve it. But, in order to demonstrate that it was acceptable (using ASCE 41 type of 'performance based' design criteria, they had to rely on the drywall strength.

 

[Tomfh]

One thing I'd comment on is how terrible plasterboard performs when wet. Builders in Australia like to line wet areas with because it's cheaper than cement sheeting, but even ‘waterproof’ versions like Aquachek are prone to falling apart. I’ve seen it countless times in bathroom renos, even behind waterproofing. A small leak wrecking the wall lining, leading to a full reline and retile. I’d only ever use cement sheeting eg vilaboard in wet areas—it's way more durable.

 

[Greenalleycat]

Okay, I'm a little late to this conversation. So, I'm going to some broad over-arching comments:

a) In the US, we do NOT generally consider drywall to be a structural component. It is probably more rigid than many of our wood shear walls, but it isn't very ductile at all. And, OSB or plywood are very effective even when wet. If we started counting on dry wall, then any time there is a significant leak we'd have to re-evaluate if the structure is still laterally safe. Think about hurricanes and such.... Probably not a good idea.

b) Here on the west coast, many (most) of our residential houses have stucco exteriors. This is basically building paper, chicken wire and a stucco. Not all that different from what EN16080 was talking about in NZ. Except that a lot of these walls are also plywood shear walls. Perhaps, I should point out that Stucco is another "alternative" load path for these wood buildings in the US. We don't rely on it, but it does give some redundancy to the structure.

c) Since we're talking about NZ here, I though the NZ codes (especially for concrete) were based on ductility, energy absorption and such. In that sense, relying on extremely brittle materials (like dry wall or stucco) would be something of an anathema.

d) There are cases (usually when trying to retrofit an existing building) where you can rely on drywall for structural support. I've never done it. But, I saw a lecture from an engineer that talked about it. I believe the project was an existing (and very old) building that would never meet current codes. They were retrofitting it to improve it. But, in order to demonstrate that it was acceptable (using ASCE 41 type of 'performance based' design criteria, they had to rely on the drywall strength.

Great response, thanks mate.

I can confirm that plasterboard is as ductile as anything you'll find out there. It relies on proprietary testing and following the manufacturers recommendations to achieve, but if done properly you can basically not collapse a plasterboard house. There is testing ongoing at the local uni - I spoke to the PhD guy doing the testing, they're getting ductility 10-13 out of the plasterboard but our designs are limited to 3.5 for seismic load reduction (our equivalent to the US R-value).

The issue with plasterboard bracing is the method of ductility - it's the screw slotting out the plaster. This occurs from very low drifts (1-2mm, basically once you've broken the glue). This isn't repairable and the damage is very hidden - it's more visible (if visible at all) from the back side of the board, which is facing the wall cavity so you can't inspect it. So, if you get hit by an M8 quake - all good, everyone probably survived. If you get hit by a M5.5....you may find the houses are all a bit sloppier than they used to be (more rattling and movement).

We go around and round on this in the office - in some ways, plasterboard is the perfect structural product. You're going to use it anyway and your houses are basically invincible with it in there (assuming you have a reasonable amount of walls). It's easily workable on site, it's even DIYable, etc. It's cheap. It looks nice. It is just really bad if you have a moderate event rather than a major one.

One thing I'd comment on is how terrible plasterboard performs when wet. Builders in Australia like to line wet areas with because it's cheaper than cement sheeting, but even ‘waterproof’ versions like Aquachek are prone to falling apart. I’ve seen it countless times in bathroom renos, even behind waterproofing. A small leak wrecking the wall lining, leading to a full reline and retile. I’d only ever use cement sheeting eg vilaboard in wet areas—it's way more durable.

Yes, wet plasterboard sucks. We aren't allowed to use plasterboard bracing in wet areas as a result. Leaking won't affect the primary bracing system that way.

 

[Eng16080]

It's certainly interesting to see the differences between NZ and the US. I've always considered sheetrock (plasterboard) to be a rather inferior structural product compared to wood structural panels for shear resistance.

I can confirm that plasterboard is as ductile as anything you'll find out there. It relies on proprietary testing and following the manufacturers recommendations to achieve, but if done properly you can basically not collapse a plasterboard house. There is testing ongoing at the local uni - I spoke to the PhD guy doing the testing, they're getting ductility 10-13 out of the plasterboard but our designs are limited to 3.5 for seismic load reduction (our equivalent to the US R-value).

Are you saying the testing shows ductility better than wood structural panels (plywood/OSB)?

The issue with plasterboard bracing is the method of ductility - it's the screw slotting out the plaster.

That has always been my understanding of the main reason why the ductility is limited. Drywall screws (at least in the US) are also much less ductile than the nails used in a wood shear wall. Although, you mention above a ductility of 10-13 (which I'll assume is good?). Isn't this contradictory?

We go around and round on this in the office - in some ways, plasterboard is the perfect structural product. You're going to use it anyway and your houses are basically invincible with it in there

Based on my understanding, this is giving plasterboard way too much credit. Again, my knowledge is based primarily on the codes. I haven't tested a house to failure with one system versus the other.

I haven't gone through the exercise, but I think it would be quite difficult to get a building to work (by US code) using plasterboard shear walls, for anything beyond a simple one or two story house.

 

[phamENG]

I've stated this opinion several times on the forum, but I'll do it again for good measure here since you asked.

I don't practice in seismic country, but I'm near the east coast of the US and hurricanes are the driver for our lateral design. Hurricanes aren't just wind, though. They're heavy, driving rainstorms accompanied by significant debris and missile damage potential. Whenever we have a decent storm around here, a good friend of mine who runs a tree service is busy for weeks pulling trees and tree limbs off or or out of houses.

Gypsum/plasterboard may work just fine, so long as the building envelope remains intact, but if it doesn't all bets are off. Yes, there are exterior rated products that may hang on long enough to get you through the event (which is all we really care about - whatever repairs are required after a greater-than-serviceability event are what they are), but those aren't used in residential construction. So I'm not going to use a product for my LFRS that may well dissolve during the event it's supposed to resist unless I have no other choice.

I will grant you that I have no empirical evidence for this, but I prefer to let my reasoning lead my conservative decisions and empiricism lead my more....ambitious?...design decisions.

 

[Greenalleycat]

I've stated this opinion several times on the forum, but I'll do it again for good measure here since you asked.

I don't practice in seismic country, but I'm near the east coast of the US and hurricanes are the driver for our lateral design. Hurricanes aren't just wind, though. They're heavy, driving rainstorms accompanied by significant debris and missile damage potential. Whenever we have a decent storm around here, a good friend of mine who runs a tree service is busy for weeks pulling trees and tree limbs off or or out of houses.

Gypsum/plasterboard may work just fine, so long as the building envelope remains intact, but if it doesn't all bets are off. Yes, there are exterior rated products that may hang on long enough to get you through the event (which is all we really care about - whatever repairs are required after a greater-than-serviceability event are what they are), but those aren't used in residential construction. So I'm not going to use a product for my LFRS that may well dissolve during the event it's supposed to resist unless I have no other choice.

I will grant you that I have no empirical evidence for this, but I prefer to let my reasoning lead my conservative decisions and empiricism lead my more....ambitious?...design decisions.

This is a very good point that I haven't considered. Flooding is a hazard here but we don't really consider lateral systems in that context. Hurricanes etc are very rare so not a typical design case for us. I think your reasoning is solid. I imagine if you're using non-gypsum bracing then it will be external? i.e. sheathing to the outside of the studs? In my head that's a lot more resilient than internal linings...I can just imagine the wind going into the wall cavity and blowing gypsum off the studs by putting tension into the screws and pulling them through the board. Sheathing will be much stronger for those kinds of loads.

 

[phamENG]

Yep. Normal around here for wood framed buildings is plywood or OSB sheathing on the exterior. Multifamily (occasionally) and nearly all commercial use exterior rated gypsum panels for fire ratings over metal studs. I'm okay with that stuff. It's not as strong as wood panel sheathing, but it's at least designed to hold up to the elements long enough to do the job when it matters.

 

[Greenalleycat]

It's certainly interesting to see the differences between NZ and the US. I've always considered sheetrock (plasterboard) to be a rather inferior structural product compared to wood structural panels for shear resistance.


Are you saying the testing shows ductility better than wood structural panels (plywood/OSB)?


That has always been my understanding of the main reason why the ductility is limited. Drywall screws (at least in the US) are also much less ductile than the nails used in a wood shear wall. Although, you mention above a ductility of 10-13 (which I'll assume is good?). Isn't this contradictory?

Based on my understanding, this is giving plasterboard way too much credit. Again, my knowledge is based primarily on the codes. I haven't tested a house to failure with one system versus the other.

I haven't gone through the exercise, but I think it would be quite difficult to get a building to work (by US code) using plasterboard shear walls, for anything beyond a simple one or two story house.

Yes, effectively our plasterboard systems are more ductile than wood panels (or at least, comparably as good) - Ductility 10-13 is very good. The hysteresis loops are pretty bad, but these things are going 5%+ drift in the testing that's underway currently regardless. So the life safety performance is excellent; the serviceability performance has other challenges.

I think you're crossing your wires on the screws there: the mechanism for energy dissipation is the plaster slotting out around the screw. Plasterboard only achieves ~0.3kN of shear capacity at each screw, so the screws themselves remain elastic and undamaged by a wide margin.

These systems are exentensively used here (read: 99.9% of houses) so the performance is known to be good, especially having had the earthquakes in 2010/11 to test them. They are only intended for 1-2 story houses though - we aren't bracing high rises with these things! In practice, people push the boat and some 1-2 storey commercial buildings will use plasterboard as well (basically for commercial that looks exactly like a house) and people push them to 3 storey houses too. I've never seen 3 story commercial or 4 story houses solely braced with plasterboard. You'd be into engineered timber, steel, concrete, or masonry systems at that scale.

 

[NorthCivil]

@Greenalleycat

im sorry mate
you are talking about how "highly ductile" plasterboard/gib is
but then in all your descriptions of it, you are describing how poor the ductility is?

• the plasterboard fails via brittle failure of the board material (the crumbling of the plaster), immediately adjacent to the screw fixing points.
  thats a brittle material failure by definition.
• this failure happens with very low deflections (1-2mm)

what you are describing, is very brittle behaviour.

yes it does seem to work. as evidenced by CHCH, wellington quakes etc. but that doesnt mean gib bracing is elastic...

as you mentioned, its also not inspectable post quake, as the damage presents on the back side of the board.

we are getting away with doing it down here, but its not really a great ductile system. i doubt it would hold up in places like japan that get serious quakes annually.

The GIB cartel pour a lot of money into engineering data and swaying code committees so that its allowable.

As other posters have mentioned, its quite stark the difference between some of the cutting edge seismic design you see on big buildings in CHCH and wellington, compared to how close we fly to the sun with our 2 level timber frame lateral designs.

 

[Eng16080]

Yes, effectively our plasterboard systems are more ductile than wood panels (or at least, comparably as good)

The US codes have it wrong if this is the case.

 

[Greenalleycat]

@Greenalleycat

im sorry mate
you are talking about how "highly ductile" plasterboard/gib is
but then in all your descriptions of it, you are describing how poor the ductility is?

• the plasterboard fails via brittle failure of the board material (the crumbling of the plaster), immediately adjacent to the screw fixing points.
  thats a brittle material failure by definition.
• this failure happens with very low deflections (1-2mm)

what you are describing, is very brittle behaviour.

yes it does seem to work. as evidenced by CHCH, wellington quakes etc. but that doesnt mean gib bracing is elastic...

as you mentioned, its also not inspectable post quake, as the damage presents on the back side of the board.

we are getting away with doing it down here, but its not really a great ductile system. i doubt it would hold up in places like japan that get serious quakes annually.

The GIB cartel pour a lot of money into engineering data and swaying code committees so that its allowable.

As other posters have mentioned, its quite stark the difference between some of the cutting edge seismic design you see on big buildings in CHCH and wellington, compared to how close we fly to the sun with our 2 level timber frame lateral designs.

I'm not saying I like the way we do things here, but you've also got to acknowledge the reality of what we do and how it performs.

Ductility in an engineering sense is the ratio of ultimate to yield. Plasterboard achieves a very high ductility ratio as you can just keep pushing it and pushing it.
The actual mechanism for the ductility, sure, its' "brittle" in the sense that there is no ability for the plasterboard to recover itself or be repaired post-event
However, it is NOT brittle in the traditional engineering sense of 'brittle failure' i.e., suddenly snapping and killing a bunch of people

As I said - you can keep pushing it and pushing it and you basically cannot kill the occupants, which is a great outcome for the life safety focus of engineering design
The plaster starts slotting out at 1-2mm (i.e. after you fail the glue) but it is not a failure - that's just how it performs
If we're not happy with the performance....well, this is a big problem for insurers and regulators to consider

And believe me - we deal with more than our share of insurance/post-EQ work, so I'm very well aware of how difficult it is to deal with this performance in an insurance context

 

[Greenalleycat]

The US codes have it wrong if this is the case.

I don't think so haha. If you read between the lines of what I've been saying (or just read @NorthCivil 's comment) plasterboard has amazing performance in ULS but terrible performance in SLS
We figured out post EQ that relying on insurance to pick up the slack for life-safety focused designs (rather than low damage or easy repair) a) in the very best case is still a very very difficult and disruptive job b) is not guaranteed forever...now that they know what they're on the hook for, premiums are up, cover is down, and some insurers have exited the market

 

[Brad805]

We tried using drywall as part of our lateral system years ago. Nobody I know uses staples, but maybe the modular trailer companies do. The problem we had since this is not standard practice is nobody informed us of when the drywall work was being done. In our case we had shown extra studs at drywall joints, and none of the framers added that. It was our last venture into this conept. The discusion is interesting.

 

[Greenalleycat]

Why would they not do studs at the joints? Even ignoring structural aspects, that's just asking for a callback due to damage at the junction, surely
Drywall here is typically installed horizontly and fixed with screws
There are lots of screws round the outside and as few as possible within the body of the board to minimise risk of popping (= callbacks)

 

[JoshPlumSE]

north civil: what you are describing, is very brittle behaviour.

I think there is some miscommunication happening here. My guess is that GreenAlleyCat is saying is that their testing shows good energy absorption even if there is a lot of inelasticity and stiffness degradation.

If you look at "performance based" design criteria, we would be talking about a system with lots of deflection and damage, but you easily pass "life safety" requirements. You can expect repairs after even a relatively minor event, but no collapse.

 

[Greenalleycat]

I think there is some miscommunication happening here. My guess is that GreenAlleyCat is saying is that their testing shows good energy absorption even if there is a lot of inelasticity and stiffness degradation.

If you look at "performance based" design criteria, we would be talking about a system with lots of deflection and damage, but you easily pass "life safety" requirements. You can expect repairs after even a relatively minor event, but no collapse.

This is well summed up

 

[Brad805]

The normal drywall practice is to have joints at the centerline of a single stud. Our code at the time suggested a second stud in cases where the drywall was being used as as a shear element. Having hung drywall I know how easy it is to have your drywall only bearing on a stud 3/8" or so. They did not put in the second stud.

 

[Eng16080]

After reading this, I'm afraid I have no idea what ductility means.

Is the TLDR of this just that: the NZ testing suggests that plasterboard is simply more ductile (performs better in an EQ) than what the codes would lead us to believe?

 

[Greenalleycat]

Amazing. Vertical installation is rarely done here now as it's less aesthetic than horizontal typically. But only requires one stud regardless of what you're doing

 

[Brad805]

Less astethic? You do mud your joints? The joints here are mudded in almost all homes except for modular homes. In modular homes they have plastic pieces that cover the joints. In schools they use vinyl covered drywall, but that is for durability and has nothing to do with shear loading.

The drywall trade does a board count and will hang sheets vertical or horizontal to keep the number of joints to a minimum.

I had to go searching. It seems you may have some products we do not have. I am not sure the difference. It even comes with its own Tech Book. Egad. I will stick with my plywood.