Hollow concrete block wall 3

[MARDO]

I have the task to design a hollow concrete block wall. The purpose of the wall is a partition wall. The wall will be installed over a concrete floor over ground. The owner of the building indicate verbally that the floor slab is 4” thickness and 3000 psi. The wall will be 10 feet height and will not be anchored on top. The client request the analysis to prevent construct new foundations for the new wall.

Comments request:

I calculate the shear capacity of the floor slab in 2√f’c bd in this case = 2√3000 * 12 * 4 = 5258 pounds per lineal foot (plf) vs weight of wall = 750 lbs plf then ok

How to calculate the seismic lateral reaction on the wall to calculate the anchoring to the floor?

Any other item that should be verify?

Seismic data:

Spectral response acceleration % of g
.2 sec 105
1 sec 34

 

[jayrod12]

In short, don't waste your time. This is not possible given the constraints. There is no way to have a cantilever block wall without a real foundation below it.

You have two options: 1) New foundations, 2) Anchor the wall at the top.

Even the standard 5 PSF of interior wind load you should be designing for will likely induce more moment than that piddly little floor slab can take.

Lastly, something seems off on your shear capacity check. It's one way shear not two way remember. 5 k/ft just seems high in my eyes for a 4" slab.

 

[MotorCity]

In the shear calc above, it needs to be multiplied by 0.75 and "d" is not 4". If your slab is reinforced, its the distance from reinforcing to compression face of the slab. If your slab is unreinforced, you must reduce the thickness to account for uneven ground when the slab is poured (I think its a 2" reduction, but look at the ACI chapter on plain concrete).

 

[jayrod12]

Further to MotorCity's point. It's likely that if your slab is reinforced, it has top steel only. This block wall will produce positive bending in the slab (opposite of what top steel does) so your "d" in your calc would be even less than 2" so you're better off only accounting for about 2" of concrete.

That still doesn't solve your moment issues by not having the wall anchored at the top.

 

[oldestguy]

Can you anchor it at the ends?

 

[MARDO]

Thanks for comments.

I can only anchor the wall at the bottom (over the concrete slab on ground). I'm thinking to anchor reinforcing steel #3 using Hilti Epoxy RE-500 with 2-3/8" min embedment. Gives approx 2000 pounds in tension.

*Revising the shear claculation based on comments.

.75 2√f’c bd in this case = .75 2√3000 * 12 * 2 = 1971.8 plf

The steel should be at the center of the slab.

 

[MotorCity]

Is your 750 plf wall weight factored by 1.2?

If you anchor the wall to the slab w/ rebar, the cells need to be grouted. If the cells are grouted, your wall weight goes up.

Did you calculate the force in the #3 bar based on flexure of the cantilevered wall and compare this to the 2000 lb capacity of the bar? There are also reduction factors in Hilti's catalog that need to be applied that account for concrete thickness, edge distance, anchor spacing, etc.

Even if the slab shear is ok, the flexural capacity of the slab is probably not going to work if the slab is not reinforced. You may need to calculate the flexural capacity of a plain concrete slab (its not very large)

 

[KootK]

In short, don't waste your time. This is not possible given the constraints.

Listen to jayrod... he speaks the truth. What you've got here is a cantilevered retaining wall with wind/seismic load instead of dirt. Even if shear and moment miraculously work, there's no way that the joint between the wall and SOG works. Consider the joint design issues discussed in this thread and then imagine applying the same principles to a 4" SOG: Link.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MARDO]

Spectral response acceleration % of g
.2 sec 105
1 sec 34

I will recommend new foundations however based on the IBC 2009 and the above seismic data how can I calculate the seismic resultant lateral force?

 

[mike20793]

ASCE 7-05 Chapter 13 will give you the seismic load for an architectural component such as this. You will need to calculate a few more seismic parameters to come up with design earthquake spectral response acceleration parameter for short period, or Sds, in Chapter 11 to use the equation in Chapter 13.

 

[spats]

I don't know what flexural shear on one plane has to do with the weight of the wall. As stated by others, you have a cantilevered wall that is laterally loaded by seismic or 5 psf minimum pressure differential if the wall truly encloses a space. It seems to me that you ought to do your own research on how to determine the seismic load (are we being lazy?).

Then there's the matter of anchoring the vertical wall reinforcing at the floor when it's only 4" thick. What makes you think 2000 lbs. of tension capacity is sufficient to anchor a #3 bar? Besides, #3s are like spaghetti. I would never use #3s for vertical reinforcing. It will never wind up in the right place within the wall. I also question if your 2000 lb. calculation is correct. The realities of anchoring the proper reinforcing should kill the idea right there.

Finally, you have a 4" thick footing that you need to make work. Are there any nearby slab joints? You say "the steel should be at the center of the slab". How do you know that? If you're talking about WWR, it's useless anyway, especially at the center of the slab. You have a 4" thick plain concrete footing subjected to bending from both the vertical and lateral loading, and ACI 318 for plain concrete says you have to throw away 2" of that for concrete cast against soil... ridiculous! Don't let the client lead you around by the nose.

 

[spats]

I hadn't seen you latest post "I will recommend new foundations" before I posted above. As for the seismic calcs, again, are you just lazy, or are you not a structural engineer?

 

[oldestguy]

While you are at it add some pilasters.