5 on 1 wood contruction

[dakota99]

I am working on a project that is going to have 4 floors of wood and a wood roof on top of a level of precast. I have just started to lay out the shear wall plan. My initial thought is to treat the diaphragm as rigid and push all of the shear into the center corridor walls and partition walls and not use the exterior walls for resisting shear since there is so many openings. The diaphragm will be ¾” T&G plywood on top of wood floor trusses at 24” o.c.. There will be ¾” of gypcrete topping and 2 layers of 5/8” gyp on the underside. The roof diaphragm will just be 5/8” OSB on wood trusses at 24” o.c. The lateral system will be controlled by wind.

I would like some input from those of you that have done this type of construction before. Am I correct in assuming a rigid diaphragm? Are the exterior walls typically ignored for shear on this type of construction? What hold down system works good for the amount of shrinkage to be expected between floors? I have attached a typical floor that shows the walls I am anticipating using for shear.

 

[msquared48]

I would go rigid for forces normal to the long dimension and flexible for forces normal to the short.

I have to assume you are in a low seismic region, for with the gypcrete I would expect seismic to control for forces parallel to the long dimension.

Since this is an "L" shaped building, watch out for drag links here. May have to use a few exterior walls... depending.

As for holddowns - Simpson DTS system is good for shrinkage and a=]has auto takeups too if I remember correctly.

Use TJI joists and SPF studs, glulam beams, parallams and microlams, no solid sticks as that is where you will get the shrinkage (in addition to the plates, but can't help that.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Teguci]

Why is there a double gridline at E?

 

[dakota99]

Thanks for the response Mike

I have broken the diaphragms into sub diaphragms that go from party wall to party wall and from corridor wall to exterior wall. being that the party walls on either side of the corridors do not line up. When the wind force acts perpendicular to grid 'A'. Am i correct in assuming that the shear wall on the opposite of the corridor can not be used to resist the windward force acting perpendicular to grid A since there is no drag linking the two? Trusses span from party wall to party wall which makes it difficult to get a continuous drag. i am analyzing the the diaphragm as flexible in this direction and rigid parallel to grid A. This sounds opposite from what you are recommending.

The two grids at E are from offset columns below.

 

[Teguci]

Oh well, hoping you had an expansion joint there.

Just because you are subdividing the diaphragm doesn't mean it will behave like that. If wind acts normal to grid A, both sides of the corridor will resist that load unless you have an expansion joint separating the two sides (you don't). The question is whether the wind distribution to each shear wall is based on tributary area (flexible diaphragm) or relative stiffness across the entire length (rigid diaphragm). For wind normal to grid A, you will get some flexibility. For the other direction, the diaphragm will be very stiff (excluding the leg of the L).

 

[Steve1415]

I think it would be difficult to justify treating a wood floor or roof system as rigid diaphragm. When considering a flexible diaphragm, you aren't allowed to cantilever the diaphragm out past shear walls. It seems to me that you'll need to use as many exterior walls as you can. The interior walls will help strengthen the system as a whole, but you can't use them to avoid using the exterior walls.

 

[woodman88]

Based on the information I seen the plywood diaphragm acts as a semi-rigid diaphragm. So in a building where you have exterior shearwalls and an interior shearwall in the center, the shear load is closer to 1/6 at each exterior shear line and 2/3 at the interior shear line. Than the flexible diaphragm loading of ¼ at the exteriors and ½ at the interior.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dakota99]

Thank you for your input. I ended having more shear walls than I originally thought. I made all bearing walls shear walls. By doing this I am able to just use gyp sheathing throughout the entire structure down to the precast level. Even though the numbers work out, it makes me nervous just using all gyp for the shear walls. Does anyone know of studies that have been done on mid rise wood construction using all gyp sheathing for the lateral system? My fear is that over time the nails will wear on the gyp creating large deflections. Is there a guide that discusses deflection of gyp shear walls?

 

[BAretired]

What is the detail at party walls? Some architects use double stud walls with staggered studs and cut the floor deck between the studs to prevent sound transmission. This means that the floor deck is not continuous through party walls.

Our code does not permit 100% of the shear to be taken by Gypsum Wallboard. For a four story building, the maximum percentage of total shear force resisted by gypsum wallboard in a story is 80, 60, 40 and 40 for 4th, 3rd, 2nd and 1st story respectively.

BA

 

[msquared48]

I never use GWB or GSB for shearwalls anymore. Since seismic usually contgrols for these structures in my region, any GSB or GWB wall w2ould immediately be restricted to half stresses, rendering them relatively useless, except in the very upper levels. I have switched to plywood everywhere, interior and exterior.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dakota99]

The floor deck will be continuous across the corridor and party walls. We are using a double stud wall with a separation between. However I have put a continuous top plate that ties the two walls together.

This design is controlled by wind. I am guessing that the 80, 60 40 rule applies to seismic controlled designs. I could not find anything in the 2006 IBC that would restrict gypsum shear walls for wind controlled design

 

[msquared48]

There isn't anything regarding restrictions for wind and gyp.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

In CSA 086-01, the restriction applies to shear in a story. There is no differentiation between wind and seismic.

See Article 10.2.3.3 attached.

BA