Masonry Handbook Tension Chord Force Problem

[eRok]

So my office recently purchased the 6th edition of the Reinforced Masonry Engineering Handbook (awesome, by the way), and example 4-A illustrates a simple lateral-load-on-diaphragm problem. The author begins by treating the diaphragm like a flexible beam and determines the moment in the diaphragm using M=(w*L^2)/8, then the tension chord force by dividing the moment by the "depth" of the diaphragm-beam. I'm on board so far. Then he designs the steel in the masonry bond beam for this force, and I'm still following. Then, to determine the shear carried by the anchors from the ledger angle into the masonry, the author divides the tension chord force by 0.5*(length of the building). I understand the length of the building, but why half? My brain can't do this so early in the morning.

Thanks in advance for your comments.

 

[woodman88]

If you assume the wall is divided into two equal walls, you would need to connect them together for the tension chord force. So each of the assumed half walls must resist the tension chord force. So the required connection force is the tension chord force divided by one of the half walls.

Garth Dreger PE
AZ Phoenix area

 

[DaveAtkins]

Tension is developed in the chord by the diaphragm shearing along the masonry. The chord tension is zero at each end of the chord, and maximum at the center of the chord, so it takes half the diaphragm length to develop the full tension.

But the example is not quite correct. The shear between the diaphragm and the chord actually varies from zero at the end of the chord to maximum at the center of the chord, so in reality you should take the chord force, divide by one-half of the length of the diaphragm, and multiply by two.

DaveAtkins

 

[eRok]

Dave;

" . . . so in reality you should take the chord force, divide by one-half of the length of the diaphragm, and multiply by two."

If your suggesting that the distributed shear varies linearly from the end of the chord to the center, then shouldn't you take the chord force, divide by one-half the length, then DIVIDE by two?

 

[DaveAtkins]

No, because the shear is zero at the middle, and maximum at each end of the chord.

For example, suppose your chord force is 100K, and the building is 100' long. The shear at each end of the chord, between the diaphragm and the chord, is (100K/50')*2 = 4 KLF. The shear would vary from 4 KLF at each end of the chord down to zero at the middle.

DaveAtkins

 

[OldPaperMaker]

eRok,
I don't know what the example looks like but I think that you need to remember that the load in the diaphram, which is carried by the rebar in the bond beam, is a result of the lateral load in the londitudinal direction. The load in the bolts that attach the ledger (collector or drag strut) to the wall is a result of the lateral load in the other (transverse) direction. You have a tension & compression chord and collectors for the lateral loads in both directions for every diaphragm & shearwall.