drag force on rigid diaphragm

[tcaoz]

I am working on a single story masonry building with concrete roof(see attached sketch) . I am wondering do I need to consider drag force when design the roof (e.g. for wall 3).

 

[kslee1000]

You have nothing to drag. Distribute the horizontal load and corresponding torsion on the bearing walls (ignore partitions) per wall rigidity. Do it for each direction.

 

[JAE]

Yes, but if the interior walls are similar in construction to the exterior walls, and they are also attached to the roof, then they will collect lateral forces.

If the force transferred from the diaphragm to the Wall 3 can be connected within the length of Wall 3, then technically a collector/drag strut isn't needed. I might add additional reinforcing in the roof diaphragm along wall 3 just in case.

 

[kslee1000]

The reason to ignore the partition walls is because there lies high possibility the interior space would be altered in the future, thus create difficulty by then. Yes, the ignored walls inevitably will share the loads, but the method is conservative, shouldn't have much impact on their's strength and performance.

 

[slickdeals]

"Yes, the ignored walls inevitably will share the loads"

It is true and must be accounted for in design unless a means to prevent force transfer to these walls is made.

 

[kslee1000]

"It is true and must be accounted for in design unless a means to prevent force transfer to these walls is made."

If you don't make such provision (for partitions - non bearing) in your design, the trouble is down the road. The wall colud be removed entirely without replacement for space recongiguration/re-utilization purposes . Now which member is to carry the add'l load after the removal? How you do retrofit under concrete roof (not impossible, but costly)?

Distingusish the terms - BEARING vs PARTITION (non-bearing, intend for removal).

 

[kslee1000]

One more noe:

Not all interior walls are automatically labelled "partitions", it is up to your design. So, it's good practice to label the bearing walls on the drawings.

 

[tcaoz]

the reason I use wall 3 and 4 as shear wall is because wall 2 and wall 6 have too much openings and wall 3 and wall 4 are solid.

JAE, if I add additional rebars for drag force, where should I add them, just the stripe of the roof aligned with wall 3 or 4 with the width of the wall? Do I need add stir up since the drag force could be compression?

 

[msquared48]

I would add them to either side of the opening and extend them parallel to wall 3 for at least half the wall length. No need to curve them into wall 3 itself - just get them into the area of the diaphragm to either side of wall 3. The slab will do the rest.

Mike McCann
MMC Engineering

 

[JAE]

What Mike said....