Chord force adjacent to CMU wall 3

[rrmiv]

In reading thread507-104685, it got me wondering about the necessity of diaphragm chords being continuous. Suppose you have a steel deck diaphragm attached to a CMU wall (200' long) via a “continuous” angle bolted to the wall. The wall is designed as a shear wall when the shear force is parallel with it. If the wall has control joints (say every 20') and all horizontal reinforcement (including bond beam) stops at these joints, then the long wall acts as ten individual shear walls. The angle doesn’t even have to be truly continuous.

However when the shear force is in the other direction, the wall itself can act as a chord. It seems to me the same ten individual walls can serve as the chord (again resisting in-plane shear forces), although not continuous. The forces into these walls are not equal - the force increases as you approach the point of maximum moment in the diaphragm.

Is my thinking correct?

 

[haynewp]

You would likely see a more pronounced effect by modeling it as a deep plate with the restraints along the top and bottom as I was saying earlier instead of as a line element with the single line of supports like you describe.


We make a living by what we get, we make a life by what we give.
Sir Winston Churchill

 

[rholder98]

Wouldn't there still be a tension at the discontinuity between walls? I've seen this condition before, in tilt walls. The chord was not continuous, and there was enough deflection to widen the joint between panels and lose the sealant.

 

[JAE]

Yes, even if you consider the CMU walls as elements that take the tension forces out of the diaphragm, there is still some flexibility the CMU and some in the diaphragm such that there is still some lateral flex in the diaphragm.

If there is lateral displacement, that means there is tension in the diaphragm no matter what. The CMU is not infinitely rigid and neither is the diaphragm. Therefore there is, and always will be tensile stress in the diaphragm to some extent.

The difficulty is - how much tension and is it enough to warrant concern over the lack of collector?

 

[miecz]

There would have to be tension taken in the deck, unless each of the individual chord walls deflected laterally the same distance. But that is impossible. Each of the chord walls will deflect laterally a distance which is a fuction of it's stiffness and the force it carries. The stiffness is a function of width, height, and openings for windows and doors.

But it is impossible to determine the force at the top of each wall. Since the chord is broken, Hooke's law doesn't apply. The force in the wall will not be the force that the chord would have taken. In fact, it is impossible to determine the force. Even a finite element analysis is helpless to find the solution, as it is not possible to model the deflection characteristics of a metal deck.

But this much we know. If all the walls deflected the same amount, the building would lean to one side. So, we know that there has to be tension in the metal deck at the wall joints, and we cannot determine the magnitude of the tension. Best solution? Make the chord continuous and go have a beer.

 

[JAE]

jmiec - your last paragraph: I agree - my point exactly.

 

[haynewp]

Word.


We make a living by what we get, we make a life by what we give.
Sir Winston Churchill

 

[archeng59]

I always detail the top bond beam as continuous in a CMU wall, that is, the vertical control joints do not pass thru the bond beam at the top of the wall. When using an angle attached to the wall, I use a "continuous" angle. At locations where the angle must be spliced, the splices must occur between the CMU control joints. However, the ends of the angles are not welded together. I provide a 1/4" gap between the angles for expansion of the steel. I use the angle and bond beam to distribute the chord forces to all of the shear walls.

 

[jike]

archeng59:

It sounds like the bond beam is the diaphragm chord and the angle is the collector in the case you describe.

 

[archeng59]

yessir.