Lateral support for basement wall

[LOKSTR]

Hi All,

I have a single story building with basement. The basement walls(18'high)are all concrete walls(18" thick) upto grade and then 10" CMU walls (15' high) from grade to roof. The roof is steel joists and metal deck. The base slab is mat foundation. There is a slab at the grade level which provide lateral support to top of basement walls.

For the design of basement stair well walls, can I assume that CMU wall provide lateral support for the top of basement wall? ( There is no slab at grade level and the stairs are metal stairs)

I am using PCA tables for wall moment design and need to decide if I use base fix-top hinge case or Base fix-top free case for the coeff.

Thanks.

 

[DaveAtkins]

No, the CMU wall does not brace the top of the basement wall. In fact, it will put wind or seismic load INTO the top of the basement wall.

In this situation, I design the top two or three feet of the basement wall as a horizontal concrete beam, spanning across the stair opening.

DaveAtkins

 

[LOKSTR]

Dave,

Thanks for reply.
Could you please explain how this beam will work in tranferring lateral force to at grade slab.

The basement with slabon top is about 60'x60' and the star well is 12'x20' attached to one side.

Thanks

 

[hokie66]

Is the stairwell inside or outside the basement footprint? A plan at the ground floor level would help in understanding your problem.

 

[RFreund]

Wouldn't you just design the outside stairwell wall to span horizontally between the 2 end walls. Use a strip say 2/3 the way down to the bottom of the wall.

Wsoil=2/3*h*Ka*gamma
Wsurcharg=100psf*ka

L=20ft

If the masonry bears on this wall check the top 3' for soil plus wind load on the masonry wall.

possibly?

EIT

 

[LOKSTR]

hokie66,

The stairwell is outside. See the attached sketch.

 

[hokie66]

A few ways you could do it. You could span the walls horizontally, or if you want 2-way action in the long wall, you could cast a horizontally spanning beam outside the top of the wall.

Why are the stairs spanning in that direction? Most of the width will be used up in landings. If you spanned the stairs parallel to the long wall, you should be able to extend part of the floor slab to brace the outside wall and still maintain headroom in the stair. Or cast the horizontally spanning beam referred to above on the inside rather than outside.

 

[apsix]

Alternatively, if the stair span is kept as shown, the 2/3 landing could be full length and used as lateral restraint to the wall.

 

[DaveAtkins]

The horizontal "beam", which is really the top of the wall, will span between the two perpendicular walls. The reaction at each perpendicular wall can transfer into the main floor slab.

DaveAtkins