Townhouse Wind Loading - parallel to front wall

[NateB]

Wasn't sure where to post this (here or Codes).

For 4- and 3-story townhouses separated by firewalls, resulting in structurel isolation of each unit:
Does the IBC/IRC require (or do you personally consider) the lateral design of an individual townhouse unit to include wind pressures resulting from the accidental or decided removal of one or more adjacent townhouses (i.e., windward and leeward pressures on the full vertical projection of each townhouse)?

Clearly the most conservative approach is to design as a stand-alone unit, but this may result (depending on available walls at the lower levels in particuarl) in significant cost to address this unlikely condition.

Please leave seismic considerations out of this discussion. Your input is appreciated.

 

[msquared48]

What happens if one of the end units burns down, if that was the primary resisting element in the long direction?

Chances are that the roof diaphragm will be continuous even if the floor diaphragms are not, so all the units will see the top wind load distributed over all the units. That will help with shear and overturning, but I still would design the others to resist the remainder of the wind.

Mike McCann
MMC Engineering

 

[ofthesea]

In my experience I would not design the walls for the full exterior wind load since the approved architectural does not include them as stand alone. If they decide to isolate a portion in the future, it would be considered an alteration and would fall under the EIBC (existing building IBC), and retrofit would be required.
I would inform the owner about that possibility and design accordingly.

 

[racookpe1978]

Only so much you can logically design for: If the first and third units burned down, will the middle unit see a full-design-pressure wind load? That is not a reasonable design problem for a code. Smoke (fumes) damage alone would be serious. Firewalls are for occupant safety as well.

 

[a2mfk]

I have never seen anything in code requiring you to do one thing or the other. You are the structural engineer, design it as economically as you see fit. These types of structures do not get modified like a one story single family house might. If there is a fire that causes structural damage, then some other structural engineer will be doing repair designs at that time. Note the shear walls (especially if you use interior walls) on your drawings and be done with it.

I am not sure I agree with your assumption of a cost increase for stand alone design. If you have these perfectly good shear walls between each unit, and all you need are a few more connectors in terms of tie downs, then I am not sure that won't be just as economical. You made find shear load path easiest to use interior party walls.

It has been way too long since I had to deal with them, but there may be some type of requirement for redundancy or independent bearing walls or something like that in relation to progressive collapse due to fire??

 

[kipfoot]

Give some thought to the IRC section R302.2.4, which generally covers 3 story single family homes in the US:

"Each individual townhouse shall be structurally independent."

For me, this means that each unit gets its own mwfrs.

 

[Ron]

There is a discontinuity of the roof diaphragm at the fire walls, since for a townhouse, the firewalls are required to extend above the roof surface.

 

[a2mfk]

Ron- if detailed correctly, you could get the shear from one roof, through the wall, and into the other roof. But once you do that, how are you not putting the shear into the wall? Stiffest element gets the shear, in a wood-framed rectangular townhome building, with walls between each unit, they are going to attract the shear anyway.

kipfoot- I don't know that section. Does it explicitly state MWFRS or gravity systems? I wonder if that is not getting back to my other question about independent bearing walls for fire reasons?

 

[DST148]

@NateB>>> See if post by Steve1415 at the very end of the thread507-309907 would help.

 

[kipfoot]

dst148: I don't think that the interpretation in the post you linked is correct. Here's the provision:

"R302.2.4 Structural independence. Each individual townhouse shall be structurally independent.
Exceptions:
1. Foundations supporting exterior walls or common walls.
2. Structural roof and wall sheathing from each unit may fasten to the common wall framing.
3. Nonstructural wall and roof coverings.
4. Flashing at termination of roof covering over common wall.
5. Townhouses separated by a common 1-hour fire-resistance-rated wall as provided in Section R302.2."

In my opinion, each exception is an example where a lack of independence is permitted, but they are not conditions which negate the requirement for overall structural independence.

 

[NateB]

Thank you all for your responses. Some of the responses appear to be addressing wind load in the "front-to-back" direction. While I see the opportunity to share load through the roof diaphragm (sheathing can be, and often is, continuous over the party/fire walls per exception 2, 706.6), I would typically use full wind load at all levels for the design of each unit in this direction. As a2mfk alludes to, the walls are already there, why not use them.

As for the "side-to-side" direction (wind parallel to front wall), racookpe's thinking is in line with my own (be logical). However, if the second unit in a row is removed, the end unit now has both leeward and windward pressures. Maybe the question boils down to how long until the removed house is replaced?... is it even required to be replaced (i.e., permanently removed)? Both of which are no longer engineering questions. Nevertheless, if the condition is temporary, a wind pressure could be developed to reflect the anticipated length of time for reconstruction of the adjacent unit.

It seems unreasonable to design all units for a condition that has a very low probabilty of occurrence.

 

[NateB]

kipfoot,
The IRC commentary on this section seems to indicate that structural independence refers to the abilty of one building to be removed (fire is mentioned) while the adjacent building remains. There is no mention of design lateral loads for the remaining structure, just that it wont collpase with the failed structure. However, I am not suggesting that they aren't required to be considered. Just my take on what that section is requiring.

 

[a2mfk]

Nate- I agree with your last post, that the structural independence discussed and required in these sections of code is related to fire and progressive collapse.

I do not think it is the intent of this section of the code that you design each townhome unit as a stand-alone structure which can resist wind from all four directions, then take these individual units, and shove them into one big building. This would require a large and unnecessary overdesign. When a townhome, apartment, or condo is damaged from wind or fire, they get rebuilt or repaired to code. This is what insurance is for. I have done plenty of this type of work. Nobody is expecting engineers to anticipate every possible renovation, remodel, addition or subtraction to a building.

My question, since it has been a while since I designed multi-family with wood frame, is how to you use the common fire wall for bearing or shear and still maintain the one hour rating? Blocking and/or fireproofing spray foam? You have to have lateral stability in the short (front-back) direction which includes tying your floor diaphragm into a shear wall...

 

[NateB]

a2mfk- The structures I am used to dealing with have a double wood stud wall that serves as the unit separation, each wall is bearing. Between the structural walls there is 2 layers of 1" gyp... the system provides a 2-hour rating. The walls continue up to the underside of roof sheathing. I beleive there are excpetions that allow a common 1-hour wall, but many developers are opting for the double wall... I beleive since it has built in STC.

 

[dcarr82775]

I wouldn't design an interior unit for the side to side wind load. But I have seen a couple of lawsuits where the local ambulance chaser engineer claims it is necessary to do so.

 

[racookpe1978]

Advantage of the double-stud-wall, double-gypsum-with-space design is "2x4 nailed construction (ie, cheap labor)" and the quieter neighbor-to-neighbor noise blocks.

 

[msquared48]

The issue here would not be the shear on the double party walls as that would be wind normal to the long face of the structure.

The issue is the wind against the end units in the transverse or long direction of the townhouse complex - the short direction of each townhouse.

The post by kipfoot should settle matters here...

Mike McCann
MMC Engineering

 

[JLNJ]

I agree that it can be designed as one unit, but I would still provide at least enough of a lateral force resisting system to make each unit stable under a nominal load. Just in case.