Most cost effective materials for new building - Arch has specified wood 1

[mfstructural]

I am working with a new architect client that reached out to me recently. I provided a proposal for the design of a new CMU building with steel floor framing (joists and decking). The building is 100'x60' and is 2 stories. I recently received a preliminary set that shows 2x6 stud walls with wood trusses spanning the short direction, and there is a steel beam/col line down the center of the building. Meaning, the wood trusses will be spanning about 30' and are shown to be bearing on the 2x6 stud walls. There is also a CMU elevator shaft and stair case shaft. I have done design with CMU and wood trusses, but haven't with 2x6s (or I can't remember as it's been a long time). There are a decent amount of windows so shear walls details for wood will be important. The building will serve as a restaurant bar. The wood trusses are obviously pre engineering and will be designed by others but will need to provide snow drift tables. Wood parapet walls will need reinforcing as opposed to CMU that can just continue up.

Just wanted to get any input/advice from others who have designed or have experience with this. I am planning on talking with the client and ask to use masonry, but have not done that yet.

Thanks,

 

[dik]

If it meets fire code, then wood is viable. Check to see that possible added insurance costs don't impact the cost... maybe check with FM Global to see if there are other issues. Wood is good from a carbon footprint, including insulation. I did a large warehouse about 50 years back for Galaxy Glass, a plant, using 2x12 roof joists on steel framing because it was less costly.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jayrod12]

I've had some varying success having the truss supplier frame the parapets right into the roof trusses. I say varying because it often also depends on the building envelope details, you could get a stack effect in the wall area if you run your moisture barrier up and over the parapet wall.

 

[AgMechEngr]

I am involved with a similar project but it's a single story. Wood construction was much cheaper.

 

[Brad805]

This depends on lot on what you have for trades available and the regional costs. Wood used to be quite a bit cheaper, but in our area with the rising cost of labor and materials that is no longer the case. It still tends to be a little cheaper, but it is close enough that one can argue it is a poor investment. It will also depend on what you have to do to that energy leaky masonry wall to comply with energy codes should those exist in your area.

 

[mfstructural]

I just spoke with the architect and he's suggesting that a lightweight concrete topping be placed over the wood trusses. I've seen concrete topping placed on sheathing in the past, however, an EPDM (rubber) membrane needs to be installed. I don't particularly like the idea of installing concrete topping over sheathing/wood trusses. He also suggesting metal decking over the wood trusses, which I also haven't done. It's either been all steel or all wood. Apparently the owner wants to have a concrete floor for the bar/ restaurant on both the first and second story. We are supposed to meet next week to discuss options. The arch specified wood as he and the owner believed it would be cheaper than wood. I think cracking of a 2" concrete slab over trusses that deflect would be a concern. the trusses would have to be designed for minimal deflection. I've seen other posts here, I know placing concrete over wood is not a popular idea.

 

[dik]

Talk to the wood truss supplier to see what problems there can be using concrete topping... long term deflection or whatever... drive nails into the top chord to act as 'headed studs'?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

30' bays, raised concrete floors - yeah, this building wants to be something other than wood.

Try to do a vibration analysis on this and you'll see what I mean. I was in a bar once in college with everyone crammed in as tight as they could...building was built in the late 1700s early 1800s...probably had 12" floor joists spanning 20feet or so...everyone started jumping and dancing together...easily 4" of vertical movement. As an as yet sober senior engineering student, it was a terrifying.

Take a look at a snip from a floor truss span table below. These are essentially SCL top and bottom chord trusses. They don't go to 100psf live load + 30psf dead load floor systems (130psf total load) - only 100psf total. Even at that, you have to get to 22" deep to span 23feet. You need to go 7 more feet and carry 30psf more. How you can accomplish this with anything less than a 30" deep wood floor truss?

 

[phamENG]

Hey...offer up CLT! It's wood, after all...

 

[Brad805]

Toppings over wood are pretty common in hotels and apartments to deal with sound, but if you ever remove the flooring materials you will not find a suitable concrete that one would want to look at. I think your arch needs to go back to the drawing board given some of the parameters.

CLT...hahaha.

 

[mfstructural]

I agree. I don't think this is the right type of construction for the type of building we are talking about. I think if we can add more columns and beams, have the trusses span 20', and install some sort of vinyl/engineered wood floor over the sheathing there's a chance it could work. But not with concrete exposed as they want.

 

[KootK]

Assuming that the project is in the midwest, as you are, I'd think that precast plank might be an attractive option. Either with masonry walls or cold formed steel walls. I work with precast suppliers in the midwest and I don't get the impression that lead times are terrible right now, particular coming from the smaller fabricators.

 

[masonrygeek]

Again, I have a bias, but wood construction is often more expensive than masonry. Here are some studies on multifamily buildings that show that:

https://www.buildingstudies.org/impact_of_construction_type_building_insurance_cost_reports.html

and here are reports with more cities:

https://www.buildingstudies.org/initial_cost_construction_multi-residential_structures_reports.html.

One thing to look at are the insurance costs as dik mentioned. Why all the gymnastics with wood when block and plank do what you need to do?