Structural Robustness/Disproportional Collapse in a Terraced Development 1

[ooox]

Hi all,

I'm in the early stages of a concept design for a residential project which involves a 3 storey terraced building. The individual apartments are approximately 50ft x 15ft with about 20 in total, all adjoined. The client is pushing timber framed construction, however, from initial review I have concerns that the lack of internal walls and small area of walls to the front and rear of the apartments will make the lateral design difficult to justify. The only way I can justify the lateral design is to assume the total frontage of all the apartments will contribute towards the stability. However, in the event of a fire in one apartment (worst case scenario would be 2nd from the end), the standalone apartment at the end would effectively be structurally inadequate which leads me to think that each apartment should be designed as a independent structure.

Any thoughts/opinions would be welcome.

 

[msquared48]

I just used the same approach for a series of townhouse buildings I just finished with anywhere from three to seven units per building. Just made sense to me, not only for the reason you state, but also considering the fact that the Architect was probably going to cut the floor diaphragms between the units for sound mitigation. If he ended up cutting the diaphragm, my design would be OK, but there would be a problem if I relied on continuity between the units. These buildings also had "green" roofs, driving the seismic forces through the green roof. Gee, but how I hate that concept!

Mike McCann
MMC Engineering

 

[asixth]

The building code I work under (building code of australia) the individual terrace houses need to be completely separated with a fire batt between houses. This requires each house to have it's own lateral stability.

 

[AELLC]

It is really hard to tell a client that wood construction won't work, because construction cost trumps everything. If a shear wall is really heavily loaded, you need to look at APA technical papers and consider 2-ply shear walls, staples instead of nailing.

 

[Archie264]

Agree with what's posted above.

Msquared,

I share your opinions of "green" roofs. If there's a silver lining to that dark cloud it's that when (not if) the roof eventually leaks the services of an engineer might be required. That gives me more confidence to take out a loan for that bass boat I've always wanted. The most delicious part of it, though, is that the boat motor is not very "green".

The sad part, of course, is that in reality this is the flawed "broken window" theory of economics. When a window is broken the services of a window repairer are required, therefore his wealth increases. So, to follow that to it's "logical" conclusion break all windows (and everything else) everywhere and we'll all be wealthy. Except that we won't. Broken, destroyed, and wasted assets represent a net decrease in wealth. In other words, if the window hadn't been broken in the first place the home owner would have had more money to spend on a new asset, whereby overall wealth actually would have increased. Ditto "green" roofs. They are a waste of resource and in so being we are the poorer for it. Off topic, perhaps, but then, we do work in the industry in question...

 

[XR250]

Geez Archie264 - watch Fox News much :>

Any roof is going to leak at some point. Do you have any evidence that a properly constructed green roof will leak more quickly than a standard roof?
What about the benefits of lower heating and cooling costs? How is that a waste of resources?

 

[Archie264]

Excel,

Just the facts, please; where I might get my news has no bearing on the issue. The fact is that "green" roofs add substantial weight, which must then be accounted for in the design. They also require maintenance and upkeep, which also costs money. So, is the added insulation they provide worth the more expensive upfront construction costs and the further maintenance costs? If so, why couldn't the same be accomplished with additional (much lighter) inert insulation of some sort, which doesn't require irrigation and upkeep? If it can, then I suggest that "green" roofs aren't so much about good engineering as they are about an ideological agenda. If it can't...well, then these "green" roofs must have grown from magic beans with special insulation properties.

The green roofs I know of where eventually abandoned when the owners got tired of the soil, plant, and animal matter clogging their roof drains. Putting 120 psf soil on a roof is no trivial matter. I understand there are now lighter ways to do it but it still involves putting biological materials one the roof, which can be problematic. And then there's the issue of roots attacking roof membranes...

You speak of the benefit of "green" roofs, well, what about their downside? And what benefit do they offer that can't be accomplished more efficiently with inert matter?

 

[JLNJ]

Rooftop greenery gets you one of those gold or platinum stickers from the self-appointed LEED gods to put on your hard hat. Plus bragging rights. And extra letters to put after your name in your email signature (especially if PE and SE are beyond your reach).

The value of these kinds of things can't be measured in dollars alone!

 

[XR250]

@Archie;

There are plenty of lightweight roofs out there in the 30 psf saturated weight. Reportedly, the roof membrane actually lasts longer as it is protected from UV etc.
Whether it makes sense economically is another question when you factor in the maintenance etc. However, as, JLNJ said, there are many benefits that cannot be measured in dollars. There are plenty of them in my area and the business owners seem to think it draws more customers. I live in a pretty liberal area, however.