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Steel Joist Seat Shear Transfer 2

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NewbieStruct

NewbieStruct

Structural
May 31, 2011
101
Is a joist seat capable of transferring shear perpendicular to the span of the joist on to a wall it bears on.

I have a condition where I have a joist bearing on top of masonry wall with an extended top chord. In between, the joists I put in an inverted U shaped plate (or a HSS tube) to transfer the diaphragm shears into the wall (masonry) below. The steel deck will be welded to the this plate or tube here. Another plate runs across the wall below the tube (or U plate) that is attached to the masonry with embed anchors.

Another engineer, in the office is telling me that I can rely just on the joist seat to transfer these shears. I do not need to add anything in between the joists.

I just wanted to know if my approach was incorrect.

I am in 90 mph wind zone with almost no seismic.
 
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I have seen engineers put on their plans a note that reads:

JOIST MANUFACTURER SHALL DESIGN JOIST SEATS TO RESIST X KIPS OF LATERAL WIND LOAD, PERPENDICULAR TO EACH SEAT

I have never done this as I always use a steel tube between joists and inter-connect the edge angle collector with the beam below.

 
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I also meant to add that you should contact your local joist supplier and ask them if there is a maximum shear that they can even design for. Might be interesting what you find out.
 
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In my previous firm we used to do that all the time, put a note on the drawings to indicate the amount of shear force (p.l.f) to be transferred to the wall (usually precast). The joist manufacturer would figure out the shear/seat and put it on the shops for us to verify. I think there is a maximum limit of shear you can transfer thru a typical K joist seat, I think its like 250# (but don't quote me on that).

All these projects were in Midwest, so no seismic issue with 90 mph wind.

But I can see JAE's logic and I think I would consider it if the shear was too high.
 
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Also, if you put that note on the drawings, the jst mnfr is going to ask you to confirm that his calculated lateral deflection at the top of joist seat is acceptable.
 
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I talked to a joist engineer that my firm regularly contacts and asked him the same question.

He told me that they use to it a few years ago but it lead to intense warping when actually designed. He recommends the use of blocking in between.

Thank you guys.
 
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You are asking about rollover capacity.

I don't recall where I read the information, so use it with a grain of salt, but I have written in my "Designing with Vulcraft.. " manual that the standard rollover capacity of a 2 1/2" seat is 1000 lbs. and for a 5" seat it's 500 lb.
 
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Good point frv - it's not just the lateral capacity of the bolts at the seat, it's also the capacity of the joist connection to the seat considering rollover, and that would induce a torsional force, in addition to the shear, to the seat.

So then the seat would see gravity, lateral shear, and induced torsion. Interesting.

I'll go as I always have with the edge angles too for the diaphragm shear.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
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There is good discussion of rollover capacity of joist seats in the "Designing with Vulcraft" book on Page 126.

 
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Designing with Steel Joists, Joist Girders and Steel Deck written by Fisher and West, they have an example They give the calculations for a given seat and arrive at a ultimate strength of 7,500#. After applying FOS and warping limits they arrive at an allowable number of 1650#.

They also give the results from some testing they performed on actual seats (getting failure greater than 8,000#).

If in doubt, indicate it on your drawings and then have the Joist Company design appropriately.
 
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Seems to me that this figure would depend on the size of the angle for the joist seat and the height which will vary here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
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msquared,

They performed 4 tests on 8H3 joists (8940#), 4 tests on 24H6 joists (13,500#) and 2 tests on 26H8 joists (15,360#). The values given are the average failure load. Not a large variety of tests but tests on multiple joist seats of varying size none the less.
 
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With such large allowable rollover forces I would think that the limiting conponent would be the welds to the deck and not the seat itself.
 
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JLNJ

The tests were done on the seats only not considering the weld of the deck to the joist seat. Also, as stated above, they reduced the allowable capacity considerably due to serviceability concerns.
 
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Along with the deck connection, the connection at the joist bearing plate to the wall below wouldn't that be critical too?

Wouldn't the edge distances for the anchors also govern the total force that can be allowed into the wall below?
 
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I'm in a high wind area. Almost always give wall reactions parallel to joist. If I think it is a problem perpendicular to the joist I wouldn't hesitate to give that load also.

"Designing with Vulcraft" as mentioned gives some guidance perpendicular to the joist. The load gets to the joist thru the angle used as a chord for the diaphragm not deck welds to the joist only.
 
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Interesting discussion, something that could be easily overlooked. Based on Steel PE's numbers I would say you will have connection problems (you may have combined uplift, rollover, or out-of-plane lateral forces on that connection), before you have joist rollover problems. Also probably not one documented failure of this type on record.

Angles and bolts aren't expensive, which is why I always use them. Then the joist seats are my redundancy.

Unrelated but related- I have found this is often overlooked in wood trusses, ie, blocking vs no blocking.

 
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a2mfk

Although I have framed a few houses during my college summer vacations I have not really designed many wood structures. The one garage I did I used a Simpson truss connector that had a given lateral capacity our to the plane of the truss. The contractor b*****d about it as he had never had to use the connectors before in the 500 houses they had built prior.

I imagine that using blocking to transfer the lateral forces would interfere with the venting of the roof and therefore undesirable.
 
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