Reading truss reactions 2

[JStructsteel]

Got a cut sheet for a girder truss from a truss mfg. Its simple span, but they have a group of reactions. Anyone know why, or how to read the reactions?

Obviously I can figure out the reaction if need be, just want to know what they are trying to report.

Their drawings suck, so it makes it that much harder to read too.

 

[HouseBoy]

The trusses don't seem impossible, especially with some refinements as KK mentioned. Still, bottom chord nailing seems like a CR$P TON of nailing.
Other strategies might work also. All that load has to go somewhere!!

Celt83, for 41K and 7.5" wide (5 ply), you prob can spec southern pine or doug fir and get something close to 600 psi for bearing and have 10" or so of contact.

JS - Is it realistic that the interior walls can take those loads? From your response, I assume that might be difficult. How else would they support the load/did you indicate another location for the load to be supported? Were you expecting to use something clear span? Others (not you I don't think) didn't seem to like the truss supplier to "dictate the structural system" but.... what was the request that was made?

Ron - I agree with you but.... What's a truss designer supposed to use? (I assume you are not disparaging them for that, just stating the obvious.)

I'm not defending one system vs. another. Just wondering what directions were given.

Regarding the original question (about how to read the garbled page) - Just looks like a PDF reader issue has caused two pages to print as one. Not ideal but not insurmountable.

I generally like to see the CSI numbers and the deflection check/statement is extremely generalized (seems likely to be way better than the L/240 or L/180 limits)

 

[RontheRedneck]

I'm a bit late jumping in here, but I just stumbled onto the thread by accident.

I've been a truss designer since the mid 1980s, almost continuously. I started working a plant building trusses, and worked my way up from there. I have also built houses and been hired by contractors to help set trusses. So I have well rounded experience.

Lots of issues were brought up, so let me hit a few.

The truss drawing -

The software produces the drawings, and we have no control over it. It's not like they're produced in AutoCAD and we can edit them.

To the OP, I would suggest contracting the truss designer and asking them about the reactions. They can look in the software and see what load case produced the maximum reactions.


The rather large reactions -

Assuming a value of 565 PSI for compression perpendicular to the grain and a bearing area 3.5" by 7.5", the maximum reaction would be around 15,000#. The design must be assuming the addition of some bearing blocks.

Where there's a "U" next to reactions, that's the max uplift based on unbalanced loads.

BTW - The max downward reactions are also based on unbalanced loads.


Someone questioned building and shipping trusses this large. For most fabricators I've worked with, a 60' truss is common. Not a big deal. The place I work at now has a trailer that will stretch to well over 70'.


Someone mentioned a "Manufactured Truss Engineer". There is no such thing.

Trusses are designed by truss designers like me. I don't have a degree, but have a lot of training and experience. And enough sense to know when I'm in over my head.

Truss manufacturers as a general rule do not employ engineers.


The company that makes the truss plates (Alpine in this case) supplies the software. They also have engineers on staff. If we need seals we send them down electronically for them to put seals on.


Back to this specific design - As the OP said, this was a preliminary design. Isn't one of the reasons for doing preliminary work to discover potential problems and solve them?

Seems to me that's what happened here. A significant issue has been brought to light so it can be dealt with.


I'm happy to discuss things further if I can be of assistance.

 

[RontheRedneck]

Been thinking about this overnight. Something struck me that I missed at first.

The bearing area didn't make sense. In the calcs I did in the previous post there wasn't enough bearing area for the huge reaction.

If you zoom in on the bottom center of the truss you'll see there's an H1315 plate on each joint. (H is heavy gauge. The plate is 13" tall and 15" wide) That's a really large plate.

I tried attaching a screen cap from the PDF in the OP to show that area.

The large plates are there because they ran the 2X6 vertical through the BC to the bearing. The reason for that is to make an over-stressed bearing work. Or at least appearto work.

Running the numbers again - The value for 2X6 SYP 2400 in compression parallel to the grain is 1,975. So take 1,975 X 3.5" X 7.5 inches, and you have 51,844# capacity.


This stunt is commonly used, and I think it's totally wrong. It pisses me off to no end.

If a contractor or architect or engineer without a lot of experience in truss design sees this, the drawing shows that 3.5" of bearing is adequate. I'm guessing in this case the wall below has SPF top plates. No way can it handle that kind of PSI. So technically the truss works. This just pushes this off as "It's not my problem".

I would have no problem calling this truss designer and chewing his ass out. This is amateurish, half-assed, lazy, stupid, and unhelpful truss design.

Did I mention that it pisses me off ???