Bar Joists on CD's by EOR 2

[STR04]

I'm reviewing some shop drawings for a set of bar joists on a 6:12 slope and the mfg'r keeps asking for the axial drag force due to the roof slope. I looked at the 2005 SJI Specification, Section 6.1 and could not find anywhere that this information was required to be specified by the EOR. The joist mfg'r's catalog states this requirement but I'm not sure I agree that it is my responsibility to provide this axial load if the roof loads are only DL+LL, nothing special. Has anyone else come across this issue?

http://www.njb-united.com/ncj/ncj20061.pdf

Section VIII

TIA

 

[miecz]

knelli-

As I said much earlier in this discussion, it's because the axial force is dependant on how the building carries the forces, and only the EOR knows this. Looking at the example in AggieYank's textbook, the joist is fixed at the bottom with a roller at the top. Other buildings have different boundary conditions which will result in different axial forces in the joist.

 

[AggieYank]

jmeic. If there is only a vertical load, there are only vertical reactions. Different boundary conditions won't change that.

I agree with the people who say this should be calculated by the joist designers.

We spell out the loads and geometry (end to end) and they design for any forces. The axial force we are discussing is a direct result of the typical dead and live roof loads.

 

[nutte]

Re: AggieYank's post...

A pin-roller scenario will have no horizontal reaction. I pin-pin reaction will. Different boundary conditions do matter. (This is a pet peeve of mine.)

 

[vincentpa]

nutte,
sovle the problem with a pin-pin condition. There are still no horizontal forces. If you solve it by hand, scan it and post it to the site so you can prove that a pin-pin does have a horizontal reaction. The statics are the statics and the statics dictate that no horizontal forces exist.

 

[nutte]

This is simple statics. A pin-pin has four reactions (a vertical and horizontal at each end). This is statically indeterminate. Three equilibrium equations, four unknowns. When you sum up the forces in the horizontal direction, you get H1+H2=0. You know the magnitudes are equal, but you don't know the value.

A pin-roller only has three reactions, so you can solve it easily.

 

[AggieYank]

The amount of horizontal deflection required for a support to be treated as a roller is extremely small.

While almost all connections in reality are somewhere between pinned and roller, I would never try to design a member as true pinned-pinned.

 

[JLNJ]

The sloped member carries tension (internally) even though there is no horizontal reaction or externally applied force.

 

[Anaximandar]

Vincentpa, no offense, but if you can prove that a sloped member carrying gravity loads will not produce horizontal reactions at its supports - I will tear up my license and eat it Do a simple test: take a plastic ruler, put one end on slippery surface (no horizontal reaction) and elevate the other end at 1:2 slope (or any slope for that matter), take a pencil and use it as a support at other (elevated) end. I publicly announce that I will stop practicing engineering if the ruler doesn't slide towards the lower side!!!
The horizontal part of reaction needs to be accounted for at such a big slope. It needs to be resisted either by the diaphragm (most commonly) or by the biaxial bending of bond beam (if masonry system) or by biaxial bending of steel beam (if steel framing). There will be increase in axial compression of top cord between the puddle welds or fasteners. This can be calculated by obtaining the total axial force that joist will transfer to the diaphragm (assuming one is used) and dividing it by the number of fasteners, or if no diaphragm is used, then entire force is taken by the top cord of the joist. In either case, max. additional axial force taken by any member of the top cord of joist should be reported to manufacturer, since they seem unwilling to run their own calcs (some will do it, some won't, Vulcraft does it, for example).

 

[Anaximandar]

Big appology, Vincentpa, the above comment was addresed to nutte, pin-ruller connection does have horiz reac. The only way to avoid horiz reac. is to provide (as required by SJI) sloped seated connections, where bottom of the seat is horizontal. Sorry for the mix-up. I still stand by what I said, though, drag force occurs either way. Again, sorry Vincentpa, it was directed to nutte.

 

[AggieYank]

Anaximandar, look at the following link.

http://tinypic.com/view/?pic=42i37s7

A pin-roller member with vertical load only, no horizontal reactions.

 

[Anaximandar]

Right, this is equivalent to joist supported by sloped seats, with bottom of seat horizontal. Since the roller provides reaction perpendicular to tangent of contact surface, there will be no horiz reaction as long as joist is supported by sloped seat, like I said earlier. Also, note that your example assumes the point of contact is at the center line of beam. When this is not the case, additional moment is developed. By the same principle, roof deck is supported by the joist, which is sloped. Joist provides reaction perpendicular to the plane of roof deck, so shear between roof deck and joist wiil have to be resisted by pudle welds or fasteners, which will further transfer it to top cord of joist - resulting in compressive stress increase.

 

[haynewp]

There has to be a diaphragm connected back to the LFRS to take the downslope component out. Until then, there is an axial force component in the top chord of the sloped joist as soon as it is installed, participating in support of the construction loads.

There is no horizontal external reaction from vertical loads prior to diaphragm and LFRS connection for a pinned-roller case (assuming horizontal end supports!), unless you are analyzing the joist as pinned-pinned. Then there is still a NET horizontal reaction of zero.

When you place the diaphragm and assume it takes the downslope component along the length of the joist under SUBSEQUENT loads, then it changes the orientation of the external reactions at the top and bottom of the joist under these subsequent loads. At the low end, the horizontal component would actually be directed inward towards the building.

The gist of this whole thread for me is that I am going to specify the downslope component for the joist mfr. and may give a load diagram since I don't know what mfr. will be fabricating the job. And I can't rely on when the diaphragm and LFRS will be installed to assist the joist top chord. That seems to be the conservative way to go with all this.


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