Collector Force Diagrams

[StrEng007]

I ran a collector force diagram against a line of shear walls/collectors and got the overall diagram to balance out to zero at the end of the resisting line.

However, the connection forces for the collectors do not change signs in some locations (each end has the same positive or negative value).

I'm used to seeing the collector have compression at one end and tension at the other (or the reverse when wind loads are reversed). Does this mean the diagram is wrong even though the total loads balance out to zero? How should I handle a collector with end connection forces in the same direction for each segment?

 

[SteelPE]

Also, your masonry coursing looks all messed up. Do you really expect a mason to build a wall 3'-5 3/4" long?.... or 5'-4 1/2" long?

 

[Celt83]

given the scenario in the original PDF, if there was a splice in the collector at the joint between Collector #4 and Wall #5, would it be in tension or compression? And if it's in tension, please explain why an entire lateral force pushing from left to right creates tension at this joint? Same goes for joint #6 to #7.

From your PDF. Collector 4 is in tension (so really a drag strut). The lateral force is developed linearly through the depth of the diaphragm. The load required to be delivered to Wall #3 based on rigidity exceeds the force that can be developed in the diaphragm up to that point, so beyond wall #3 the collectors need to act in tension, so really they are drag struts, to drag the load back to Wall #3 along the way walls #5 and #7 also need some of the diaphragm force delivered to them which increases the required tension drag of the collectors 6 and 8. The remaining drag tension to satisfy the load delivered to Walls 3,5, and 7 finally comes out in collector 8 and then the remainder of collector 8 acts to push load into wall 9. The load required to be delivered to wall 11 exceeds what can developed over collector 10 alone so a portion of the load for wall 11 is collected in collector 8 which is why from 8 to wall 11 there is net compression. The force required to be delivered to wall 11 exceeds what can be pushed into it by collectors 8 and 10 therefore a portion of collector 12 must act as a drag strut and deliver load to 11 via tension. The remaining length of collector 12 is sufficient to develop the required force to be delivered to wall 13 via compression.

 

[Deker]

I don’t think the point of your post is getting lost. Several people have told you that the sign doesn’t necessarily need to change across the length of the wall. As for whether to design for tension or compression, you would design for both since wind and seismic loads can reverse direction.

I think you may be conflating the physical behavior of the building with the sign of the axial load diagram. It may help to simplify this problem down to two walls—say 20 ft and 5 ft—and play around with different wall arrangements to help illustrate what’s happening in your example. What you’ll find is that the global collector behavior is dominated by the location of the long wall. The short wall will take some load, but it usually won’t be enough to overcome the tension or compression in the collector.

So when you end up with tension on both sides of the short wall, it doesn’t mean that the collector is pulling on both ends of the wall, it means that that the short wall isn’t really impacting the global behavior of the collector.

 

[StrEng007]

Do you really expect a mason to build a wall 3'-5 3/4" long?

In this situation dimensions are dictated by the Architect. Do I expect a mason to cut block to fit this exact shape? No, I don't.

The shell contractor will achieve this with CIP tie-columns that are cast into the wall ends. Similar to a starter column at the end of CMU wall. This allows for as much whole coursing as possible. If block is not to be cut, then I would need better dimensions to work with.

Several people have told you that the sign doesn’t necessarily need to change across the length of the wall.

Yes I know and I'm very grateful for the feedback. This concept (short wall not impacting global behavior) in particular is taking an extra second to click. Your last post in addition to Celt83 help to clarify this. Thank you.