Single story commercial 50 x110 rectangle.

[dm567]

What are some creative choices for handling the lateral load in a single story commerical wood frame structure that is 50 by 110 feet? We are considering a clear span using 50' open web trusses every 24 inches. We wish to keep the space as open as possible but some posts in the center are acceptable. We are in San Francisco. The 50' front of the building is a complete glass storefront.

 

[DaveAtkins]

Your diaphragm has a 2:1 aspect ratio, which is not bad. I would be tempted to use a three sided diaphragm, but then, I am not in a seismic zone. Technically speaking, I think the IBC limits a cantilver diaphragm to 25'.

DaveAtkins

 

[JAE]

Is a three sided diaphragm a cantilever diaphragm? I didn't think so.

 

[DaveAtkins]

I guess the proper term is "open front structure" (IBC 2305.2.5). IBC will not allow an open front structure in your case, because the aspect ratio > 1.5 to 1. I would recommend a rigid steel frame at the open end.

DaveAtkins

 

[dm567]

Yes. It will definately require a rigid steel frame in the front along the entire 50 foot frontage.

But because of its length and its location in San Francisco what are the choices for handling the rest of the lateral load economically. The front of the building can be a rigid steel frame and the back of the building can be a sheer wall. Will the length of the building require further lateral support and if so how can it be handled economically. I'm trying to avoid using more rigid steel frames spanning the entire 50 feet.

 

[DaveAtkins]

Can't the diaphragm span 110 feet?

DaveAtkins

 

[dm567]

I'm not a structural engineer but I like to be involved the design of my buildings. Can you briefly explain to me what a diaphragm is?

Note: My architect consulted his strutural engineer and was told there would need to be a rigig frame in the front of the building spaning 50 feet across above the storefronts. He also mentioned that there would need to be additional rigid frames located approximately at 25, 75, and 100 feet along the length of the building to hadle the lateral load. The very back of the building will be a sheer wall and will not need a rigid frame. I'm looking for alternatives to the rigid frames spanning 50 feet at the 25, 50, and 75 foot locations. I've heard of strong walls,for example.

 

[dm567]

OK, I did some reading on the subject.

A diaphram is a flat sturctural unit acting like a deep, thin beam. The term is usually applied to roofs and floors designed to withstand lateral loads. Diaphrmas are commonly created by installing wood structural panels over roof or floor supports.

Can the 50 foot by 110 building be designed with a diaphram so as not to require any additional rigid frames for lateral support? (50 is the store front with a rigid frame, and 110 is the length, and a solid 50 foot rear shear wall)

 

[DaveAtkins]

The diaphragm must be checked to span 110', and the 50' rigid frame must be designed for the lateral load it sees.

DaveAtkins

 

[AggieYank]

dm567, every building has to be designed with a diaphragm, as you've realized. Even if the building has rigid frames at 25, 50, and 75 feet, the diaphragm will have to span between those frames. The question of whether you can remove the rigid frames at the 25, 50, 75 marks will depend on if the diaphragm can span 110 feet, and (the more likely to control scenario in my opinion) if the rigid frame at the end is strong and stiff enough. Strength probably won't be a problem, but it is quite possible the deflection will be higher than acceptable, meaning additional frames at the 50 or 75 mark are required.

A strong wall is another name for a simpson shear wall, which is just a wood/steel shear wall used in wood buildings. If you have an open storefront, this won't work, and so the rigid frame is required.

 

[chichuck]

AggieYank,

I disagree with you. One could design a roof system with in-plane braces to transfer the lateral shear between the rigid frames & to the shear wall. This may or may not be an economical way to resist the lateral loads but it could be done.

Regards,


chichuck

 

[AggieYank]

chichuck, I agree, your option works, but as you said, is rarely cheaper than relying on the diaphragm action of a metal deck if possible, or something similar. I suppose when I originally said "every building has to be designed with a diaphragm", I should have said "every building has to be designed with a diaphragm, or some other means of transferring the lateral loads to the lateral bracing, with a diaphragm action being the best choice". There are several other not very good options in this case as well if we chose to use them.

 

[dm567]

I was under the impression that the rigid frames at 25, 50, and 75 which span the width of the building were handling the lateral load in the width of the building(50 feet) not the length (110 feet. I thought these additional rigid frames were meant to keep the building from tipping over sidways.

Furthermore, since the two 110 foot walls would be sheer walls along the entire length I was under the impression that this would handle the lateral load of the length of the building (110 feet)

 

[DaveAtkins]

Shear walls and rigid frames resist wind loads that are PARALLEL to them. Follow the load path of the wind: Wind load on wall is carried to ground and roof diaphragm by wall; wind load on diaphragm is carried to rigid frames by diaphragm; wind load on rigid frame is carried to ground by rigid frame.

DaveAtkins

 

[dm567]

I should have become an engineer. You have an interesting profession. Thanks for the lessons.

Let me try to understand your last posting.

If the building is a rectangle 50x110 with the 50 being the front of the building and the front is facing north and there is a westerly wind, then the wind hits the 110 foot long sheer wall and the wind load is carried to the ground and roof diaphragm by the wall.

Now, it seems intuitive to me that the wind is trying to knock the wall down and the rigid frame on the front (50")and the sheer wall on the rear (50") and the diaphragm between them are what will stop the forces of the wind.

With the diaphram on the roof and sheer walls on 3 sides and a rigid frame on the fourth side we have a very strong box. I still don't intuitively see why additional rigid frames are needed. The diaphragm on the roof seems like a sheer wall built horizontally and it doesn't seem like it would flex much. It would remain a rectangle whether there are westerly winds or north winds. If there are westerly winds the front rigid frame and the rear sheer wall should stop the 110 foot wall from being pushed, raised, or toppled over. Its hard to break a box.

What am I missing? I'd like to understand for purely scientific interest. Obviously, an engeener will have to design the final structure according to local codes.

 

[UcfSE]

You seem to have a qualitative understanding of what's going on. What you would do from there if you were a structural engineer would be to quantify all of that information. A diaphragm is pretty stiff and strong, but how stiff and how strong, and is it enough.

Let's say you come up with a given required diaphragm strength to support your building and stabilize it. If the material you want to use will not be able to supply that strength, you will have two choices: change to a stronger material or reduce the demand on the diaphragm. Adding additional walls or frames in the interior region of the building would serve to reduce the diaphragm's internal forces and potentially make a material work that wouldn't otherwise. The other option would be to switch to a stronger diaphragm material, such as a steel deck in stead of plywood, or to use a stronger connection, such as having blocking under all the plywood joints and lots and lots of extra nails.

Basically, wind load will hit a wall and travel to the roof diaphragm for the upper part of the wall, and to the slab or foundation directly for the lower portion of the wall. All of this load will then travel through the diaphragm to either a shear wall or a frame. Those in turn will carry the load to the ground. That's not to say there aren't alternate methods of designing your load path but it's probably cheaper than most if not all to build. Besides finding all of these forces and strengths, you also have to check how all of these components will cooperate with each other, and if they will actually behave as assumed. It's fine to assume, for instance, that a frame will share load evenly with a shear wall, but will it actually do so? These are some of the things we check, or should check when necessary.

 

[dm567]

What does a steel deck look like? How is it configured? How is it fastened? How think is it typically?I have not seen one.

Do you roof over a steel deck just like a plywood one? What is the preferred or typical roofing material over a steel deck? Can modified bitumen membrane be used?

Can a steel deck be installed over open wood trusses set at 24 inch intervals?

Can 1 1/4" plywood decking with plenty of nails and blocks ever be as strong as a steel deck?

 

[UcfSE]

Steel deck is corrugated sheet steel. It is normally used over a steel roof, whether that is steel beams, joists or steel trusses. I have never seen one used for wood trusses and I don't know of any values. Steel deck may range from 28-gauge to 16-gauge and range in depth from 9/16" to 3"+. The following is a link to Vulcraft's steel deck catalog. Vulcraft is a major manufacturer of steel deck and their catalog is widely used. I'm including this just for your information and reading pleasure.

Vulcraft Steel Deck Catalog (pdf file)

I also don't recall ever seeing any values for 1.25" plywood deck. Almost all the diaphragm strength values we use are based on testing. What that means is you get the most bang for your buck so to speak since with testing you know for sure just how much a material and fastener configuration can handle. What it also means is that if what you want to do isn't tested you have to go down to the next lowest size that is tested.

There is, however, some over lap in the strengths of steel and wood deck. The best wood deck will not be as strong as the best wood deck, but if you have enough money you could probably get a wood deck to achieve strength comparable to a steel deck in terms of diaphragm shear values. Now that doesn't mean you need to worry about having to have a steel deck. My post above was intended to give very general information about how some simple buildings may work. Whether you can use wood deck will be governed by your actual building and location and will be up to your professional engineer.

These questions would be best directed toward the professional structural engineer whom you hire in the end, but I'm sure most people here don't mind helping someone learn a bit.

 

[dm567]

Thanks to all who have helped me to a better understanding of what is involved in the structural design of a simple single story building. I won't try to learn what is involved in designing a sky scraper just yet. Thanks again.

dominic