Wood structures: story shear & unit shear question

In summary, Motorbiker calculated the story shear on shear wall between roof and second-floor level along short walls using the equation Fx(ROOF)=Fxcoeff*height. He also calculated the unit shear in roof diaphragm along short walls using the equation Fx(2nd)=Fxcoeff*height. Cyclovenom calculated the weight of roof, tributary weight of roof, and weight of 2nd floor using the equation Wupr=(0.115)*2920=335.8 lb/ft, Vur=(Wupr*L)/2=(335.8*100)/2=16790 lbs, and vur=Vur/b
  • #1
mrmotobiker
48
0

Homework Statement



A rectangular two-story building with a wood roof and floor system and masonry walls is analyzed to determine seismic forces.

Given: I=1, roof dead load D=15psf, floor load=25psf including partition load. Exterior wall dead load D=80 psf along both long and short walls. Ta=0.189seconds. SDS=0.4g and SD1=0.145. Redundancy factor is 1.0. R=5 for a bearing wall system with special reinforced masonry shearwalls.

The story height is 12 ft each floor. Additional 2ft of parapet is above the roof. Assume the masonry walls are along both long and short direction. The story dimension is 100'x40'

FIND:
a.) Determine the story shear on shear wall between roof and second-floor level along short walls.
b.) Determine the unit shear in roof diaphragm along short walls.

Homework Equations



Cs=SDSI/R=0.08g (given)
k=1 because 0.189s < 0.5s

Fx Coefficient=(0.08*608)wi*hi=0.00478

The Attempt at a Solution



a.)

Weight of Roof:
1ft*40'*15psf+(2'+12'/2)*80psf*2=1880plf
Tributary Weight of Roof: 1880plf*100'+(12'/2+2')(80psf)(40')*2=239.2kips

Weight of 2nd Floor:
1ft*25psf*40'+2*(12'/2+12'/2)*80psf=2920plf
Tributary Weight of 2nd floor: 2920plf*100'+2(12'/2+12'/2)*40'*80psf=368.8kips

Fx(ROOF)=Fxcoeff*height=0.00478*24'=0.115
Fx(2nd)=Fxcoeff*height=0.00478*12;=0.057

Roof Reaction:
1/2*0.115*1880plf*100'=10.81kips
2nd Floor Reaction:
1/2*0.057*2920pcf*100=8.322kips

Mid-story shear=10.81+(0.115*80*(12;/2+2)*40)=13.754kipsb.)

Weight of Roof=2920plf
Wupr=(0.115)*2920=335.8 lb/ft
Vur=(Wupr*L)/2=(335.8*100)/2=16790 lbs
vur=Vur/b=16790/40=419.75 lb/ft

Could anyone please check if I'm doing this correctly? I'm very uneasy about this answer, not because it doesn't sound right, but I'd really like to have this correct or find out what I'm doing wrong. Thanks ahead of time!
 
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  • #2
bump. this is done using ASD in california.
 
  • #3
Hey mrmotobiker,

My wood structure design is a little rusty, but i'll give it a try.

So far your calculations look correct. I might that for the tributary second floor loas a partition load is usually added about 10 psf for seismic design force, but maybe you don't have to add it.

So far your a) part looks ok. Another method for obtaining the shear at midheight of second-story walls only for a simple rectangular building with two equal transverse shearwalls is by dividing the total roof shear by 2.

Now part b) looks weird to me.
 
  • #4
thanks. i got part a right, used the wrong weight for part b.
 
  • #5
hey cyclovenom, i realized we're the same age (your profile) lol. what school do you go to? I'm really looking into grad school for structural as well.
 
  • #6
Hey motorbiker, I'm dominican and i studied (graduated already) at http://www.unibe.edu.do" [Broken] . I'm planning on going to GATech on 2008 summer.
 
Last edited by a moderator:

What is story shear in wood structures?

Story shear refers to the lateral force acting on a single floor level of a wood structure. It is caused by wind or seismic loads and is transferred through the walls and floors of the building. It is an important consideration in the design and construction of wood structures to ensure structural stability.

What is unit shear in wood structures?

Unit shear is the shear force per unit length of a wood member, usually expressed in pounds per linear foot (plf). It is a measure of the stress on a specific section of the wood member and is important in determining the structural integrity of the entire wood structure.

How is story shear calculated in wood structures?

Story shear is calculated by multiplying the design lateral force by the height of the story being considered. This calculation is based on the assumption that the lateral force is uniformly distributed across the height of the structure.

What factors can affect story shear in wood structures?

The main factors that can affect story shear in wood structures include the height and shape of the building, the type of lateral load (wind or seismic), the location of the building (e.g. near a fault line), and the construction materials and methods used.

How is unit shear determined for a wood member?

Unit shear is determined by dividing the total shear force acting on the wood member by its length. This calculation is important in determining the appropriate size and strength of the wood member needed to withstand the forces acting on it.

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