Precast Double Tee's on CMU

[ntpe]

I am designing a CMU bearing wall to support precast T's. I have never done this before. I am a little concerned about bearing the stem of the T's on the CMU. Is there anything special that should be considered? I am expecting a point load of about 31k per stem.

 

[DaveAtkins]

Use a bond beam across the top of the CMU wall, to spread the concentrated load. Check the CMU wall per the Code for the axial load and any lateral load it will see. Be careful about any wall openings below.

Consult your precast supplier regarding the connection of the double tees to the top of the CMU wall. They may recommend neoprene bearing pads, but the top of the wall must be stabilized laterally by the double tees.

DaveAtkins

 

[jike]

31 kips seems like a lot of load. If this is a long span, I would be very concerned about end rotation and how that rotation may affect the bending strength of the wall.

I assume that you plan to have a wall with vertical reinforcing in grouted cores. I would consider using a much deeper bond beam than 8", perhaps a poured beam on top of the wall.

 

[UcfSE]

I would definitely consider any eccentricity the tee might have on the wall, including accidental. If the centroid of the bearing area does not coincide with the center of the wal then there is some eccentricity. At a high point load like that it can be significant. Dave has a good point in that you will have to make sure that your connection detail can provide lateral stability to the wall if such is required by your engineering.

 

[ntpe]

Dave, jike,UcfSE, Thanks for the tips. For the wall in question, the bearing height is 9'-9", I have detailed the double T's as bearing on the full width of the wall but I analyzed and designed the wall for e=2". As for the bond beam, I have a bond beam with 2 courses of cmu, all filled with 3000 psi concrete below the stems of the T's. Below the bond beam I have called out 8" CMU w/ #6's vertical @ 24" o/c. The top of the wall will be restrained be the slab pertion of the T's as recommended by the precaster. I have also called for special inspection for all of the masonry. I have not addressed the bearing plates at the stems of the T's yet. Does it sound like I have overlooked anything?

 

[jike]

The current lap (IBC 2000) requirements for #6's in grouted masonry become very large (57") and impractical for bars centered in 8" CMU. We try to use #5's (31") and smaller for reasonable lap lengths.

Please describe the detail you have where the DT flange restrains the wall.

 

[ntpe]

jike,
I actually did use #5's @ 24" O/C. At another location on the building I needed #6's for wind and I had the two confused. About the restraint provided by the T slab; I plan on bringing the cmu up between the stems to the top-of-slab elevation. The slab will be cut back to the face of the cmu. The T slab will have steel embeds. I plan on using a steel plate to tie these embeds back to embeds in my cmu wall. This was recommended by the precaster's engineer.

 

[UcfSE]

I recommend you use the same bar size throughout if at all possible. It's easy to get the two confused or just flat over looked.

 

[archeng59]

one thing to consider, and I might have missed it if it was mentioned already, is expansion/contraction of the double-tees. one end of the double-tee might need to accommodate lateral movement so the CMU is not damaged by the movement.