WOODEN LIGHT FRAME WALLS SYSTEM FOR 4 STOREY BLDG

[rook1]

Greetings...
Is Wooden light framed walls with gypsum or plywood sheathing,in non - seismic region allowed for four storey bldg?
If not, what are the alternative design using wood structure.
Thanks...

 

[uleaum]

I have done several of these types of structures. Expect to use 2x6 stud walls (with several floors with double studs). I have never found code preventing any height of wood structure with the proper engineering(other than fire code restrictions). One of the Structures I did was a four story building with a mezzanine (making it a 4 1/2 story building) on top of a parking garage.

You are required to do a shrinkage calc. I am using the UBC 97 and the Code doesn't require you do anything with it(other than be aware of it in your design), but I would suggest sending the floor to floor and total drift to anyone doing mechanical.

Also make sure your architect knows that brick veneer will require steel lintels and the proper interval (I think it's 35 feet but I'm not sure of the top of my head) unless they use "lick and stick" brick. If you do use brick remeber it will grow over time. So if you are in a specially dry region the differentail can be large. You have to a lot calculations you can usually just assume in shorte would structures. As a rule of thumb, if you think you can analyze it, then do it.

I normally don't go below 4" spacing on plywood assemblies just to be safe about wood splitting. If you use double studs at all floors(K.I.S.S.), I will go all the way down to 2" spacing(you'll need it to take up the wind forces). I also detail the lap conditions for plywood shear walls (you don't want framers messing this up with tight spacings).

Also, don't let the architect give you too many diaphragm irregularities, like dropped corridors. Your going to need every advantage you can get.

Given the Size of the these buildings, They are usually type III construction, so do what you can to stay out of steel, especially in the walls(fire detailing steel in wood walls is a nightmare). 2x6 walls aren't to bad, but depending on how things stack you may not get a 2x6 wall all the time. I have found that using beam grade PSLs can be analyzed to provide equivelant values to steel.

Don't forget that Siesmic forces can control in the long direction of buildings even in lower siesmic Zones. Expect to do a complete diaphragm analysis too.

 

[rook1]

thanks uleaum for the very helpfull tips.
Actually, we already have a preliminary design of the building. We came out with 2 - 2"x6" Southern Pine No 2 dense studs for the first floor with shrinkage & windload consideration. But one of our problem is the wall layout for the first floor, because some the shearwall have spacing greater than 25 ft. With our computations, shear in this walls can be satisfied by plywood sheathing on both sides with 2" nail spacing(with appropriate tie down anchors,anchor bolts, strap & end post sizes ). Based on the UBC, when the basic wind speed exceeds 80 mph, the spacing of the exterior and the interior braced wall lines shall not exceed 25 ft on center on both longitudinal and transverse direction in each storey. Were thinking of providing steel frames on the first floor to satisfy the shear and code reqt. but this is a little bit costly..

 

[uleaum]

It sounds like your mixing methods. The 25' on center spacing requirement is for brace wall design. Brace wall design utilizes the "Conventional Light Frame Construction" methods from division IV of Chapter 23 of the UBC. This project definitally sound like it falls outside the scope of this exemption(limited to 3 story buildings). Check out 2320.1 of the 97 UBC for the limitations. Although 3 story buildings can be analyzed, we normally perform a full a lateral analysis on Apartment Buildings(wall spacing usually prevents the brace wall method, but not always). Under the IBC single family, two family, and townhomes are exempt, but every thing else has to have a full lateral analysis.

As for the wall spacing requirements see table 23-II-G on page 2-286 of the 97 UBC. You will find that the aspect ratio of a horizontal diaphragm is 4:1. Say your diaphram was 25' deep. You could space your shear walls at 100'. The trick is finding wood walls long enough to take these kinds of loads. Don't forget to check your diaphragm deflections and story drift. (In case your wondering no code specifies story drift requirements(d/h), but 0.0015 to 0.0035 is the range and 0.0025 is the recomded values for wind; Siesmic drift requirements are in UBC 97 1630.10.2 but should be 0.025)

If you are going to use moment frames keep in mind the 25% rule for siesmic. (UBC 1629.6.5 item 2)