Chord force adjacent to CMU wall
Chord force adjacent to CMU wall
(OP)
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?
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?






RE: Chord force adjacent to CMU wall
Think Hooke's Law. When the diaphragm resists lateral forces, it deflects not only through shear deflection, but also in flexural deflection. So the outer "midspan" edge of the diaphragm has stretched because the diaphragm has deflected in a large, sweeping, but shallow, "U" shape. This stretching (via Hooke's Law) means that the deck, or sheathing, is being placed in tension. With deep diaphragms the tension isn't all that large, but sometimes it can be with shallow diaphragms. Thus, the need for a chord. The presence of the wall and connection to it doesn't change this tension condition in the diaphragm.
RE: Chord force adjacent to CMU wall
I suppose for a small enough building, it might even be a portion of the metal deck spanning between joists immediately adjacent to the wall. Obviously, this needs to be in the strong direction of the deck.
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
JAE, interesting comment about Hooke’s Law – I’m not sure. But I believe the force is taken by the walls and is not allowed to accumulate in the decking.
RE: Chord force adjacent to CMU wall
If your wall is continuous, with a bond beam, then the diaphragm now looks more like a dumbell cross section (thin web = deck and fat flanges = bond beams) and the moment of inertia goes up, so the lateral "bending" of the diaphragm is significantly reduced - so the stress in the thin web (deck) is reduced at the edge.
If you don't have a continuous wall, as you described above, there are abrupt areas where the flange (the bond beam) is cut (per UcfSE's analogy) and the web is exposed to a concentrated stress at the cut. With the cuts (the control/expansion joints) there would be less stiffness in the dumbell "beam" and more deflection in the diaphragm. Thus more strain = more stress in the decking at concentrated locations....thus the concern over tearing of the sheathing/decking.
It may not be a significant amount of stress, but it is a concern non-the-less.
So with no collector, you have to at least consider/worry about:
1. Tearing stress in the deck at locations of masonry control joints.
2. Higher flexibility of the diaphragm and thus more lateral sway in your building.
RE: Chord force adjacent to CMU wall
I agree with you and disagree with everyone else in this thread. If you have no continuous chord, the chord force can be resisted by the series of shear walls. The shear walls near the ends of the diaphragm will take the most force, while the ones near the middle of the diaphragm will take the least force.
But I think you may be worrying about nothing. Most of the time, in my experience, the control joint does not extend through the bond beam at the top of the wall.
DaveAtkins
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
You could make a finite element model of the roof with element nodes restrained along the top and bottom of the diaphragm preventing curvature, and you would see reactions at each shear wall pointing left or right toward the middle of the beam. But personally, I would suggest leaving the chords continuous since it shouldn't be that much more work, and everyone else is doing it. (ie. Don't have to worry about proving your new theory in court).
Insanity in individuals is something rare - but in groups, parties, nations and epochs, it is the rule.
-Friedrich Nietzsche
RE: Chord force adjacent to CMU wall
DaveAtkins
RE: Chord force adjacent to CMU wall
Insanity in individuals is something rare - but in groups, parties, nations and epochs, it is the rule.
-Friedrich Nietzsche
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
I guess I can see this sort of thing working if you have a series of cut up shearwalls serving to take out the build-up in chord forces along the diaphragm tension edge, but I guess I'd ask....why would you?
A continuous angle is, or should be, used along this tension edge anyway to assist in connecting the exterior wall to the diaphragm for normal forces anyway. So if you have some sort of element there, why not utilize it as a simple chord collector? Why go the complicated route?
RE: Chord force adjacent to CMU wall
The point is that he and his friends probably designed hundreds of these buildings without considering chord forces and the reason they work is because of this parallel shearwall reaction effect. So I guess there are people like him that think designing chords is overkill?? But I believe that codes require a continuous chord in seismic areas anyway.
Insanity in individuals is something rare - but in groups, parties, nations and epochs, it is the rule.
-Friedrich Nietzsche
RE: Chord force adjacent to CMU wall
I also began my career under the wngs of some "older" engineers (mine started their careers in 1927, 1946 and 1947). After working with them for my first few years, I finally asked one of them why they never performed lateral analyses of some of their buildings, primarily for a grocery store client for which they designed dozens of structures.
The answer was sort of like...."ya, we probably should do that, but we never have in the past."
I just believe that as smart as some "older" engineers are, there were many who really didn't thoroughly analyze and design their buildings for the REAL loads, but were fortunate over time that many of the older construction techniques involved built-in redundancy that took care of their lack of concern.
RE: Chord force adjacent to CMU wall
Insanity in individuals is something rare - but in groups, parties, nations and epochs, it is the rule.
-Friedrich Nietzsche
RE: Chord force adjacent to CMU wall
One thing I haven't seen much on other engineers' drawings is a splice for the angle at the edge of the diaphragm, if it is a chord. I use a plate to splice angle to angle, because if you don't, the chord is discontinuous. This detail is almost always needed, because the angle will not be one continous piece the whole length of the diaphragm.
DaveAtkins
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
We make a living by what we get, we make a life by what we give.
Sir Winston Churchill
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
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?
RE: Chord force adjacent to CMU wall
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.
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
We make a living by what we get, we make a life by what we give.
Sir Winston Churchill
RE: Chord force adjacent to CMU wall
RE: Chord force adjacent to CMU wall
It sounds like the bond beam is the diaphragm chord and the angle is the collector in the case you describe.
RE: Chord force adjacent to CMU wall