Trying To Understand Structure/Loads Of Existing Building

[Pepe_Silvia]

Hey all.  I was wondering if anyone has seen this type of construction, if there's a name for it, and if it's integral to the roof structure.

I've drawn up the building as it stands in Sketchup to hopefully better provide a picture of what I'm working with.


Here it is.  I've left out a few details because I wasn't sure if they were pertinent.  For example, there are additional 2x4 vertically between the beam and the rafter on the right hand side of the picture.  The left side is connected to another structure.

 

 

My questions mostly revolve around the pieces highlighted in red.  The post is cemented into the slab floor, but does not attach to the rafters.  I've shown the detail of how the beams attach to the post in the middle.  They are then nailed to the perimeter posts.






Presently there is a loft built on top of half of the red beams.  I did not show all the framing for that in the drawing, but I can add them if it is pertinent.

Is this post and beams adding structure, keeping the trusses from 'flattening' under load?  Or was it likely added only to support the loft?

I'm happy to provide any additional details or pictures that help.

[mike_belben]

I think its a "man with hammer" job.  


The spreader from plate to plate helps keep the snowload from flattening the rafters and ballooning the frame.  They missed the chance to extend the post up to the ridge and make it a sort of load bearing ridge beam.  It would be a lot stronger if the ridge was sitting on that post.    I dont think the other horizontal member isnt doing a whole lot for snow load but maybe some wind bracing.   It is lacking in crossties.


Whats with the misalignment between the upper and lower posts on the gable end?

[Pepe_Silvia]

I think its a "man with hammer" job.  

That's a great way to describe it.

The spreader from plate to plate helps keep the snowload from flattening the rafters and ballooning the frame.  They missed the chance to extend the post up to the ridge and make it a sort of load bearing ridge beam.  It would be a lot stronger if the ridge was sitting on that post.    I dont think the other horizontal member isnt doing a whole lot for snow load but maybe some wind bracing.   It is lacking in crossties.

My theory is that it was built where an existing structure already stood, and the post in the middle was there already.   If I were to run some 2x6 rafters on the trusses would I be able to eliminated the center post?  For what it's worth, we basically never get snow and then it's not enough to create a significant load.

Whats with the misalignment between the upper and lower posts on the gable end?

The pitch is asymmetric. The upper post hits the peak and the lower is centered.  Here's a better look.


 

[mike_belben]

Man with hammer is DonPs line.  And its his opinion you really need here.



  If you take out that red cross tie you only have the outside ends keeping those walls from balooning.   I cant tell you if thatll live or not.  But the post isnt preventing it at present.  The crosstie is.  I see no need for the post if youre willing to add crossties at the top plates. Just my man with hammer opinion.

[Don P]

You got it Mike. Current building codes say we have to put a tie across from rafter to rafter, in the lower third of roof height, on every rafter pair, or size and support the ridge as a beam to hang the rafters from to avoid that spreading. At the end of the span tables in the codebook is a table that tells how many nails to put in that heel joint connection based on roof pitch, building width, and roof load. 

[Pepe_Silvia]

Current building codes say we have to put a tie across from rafter to rafter, in the lower third of roof height, on every rafter pair...

So if one were to do that to this drawing, it would look something like this?  Or do I have "rafter pair" wrong? 

 

[Don P]

That's it. "Rafter couple" is another term you'll see used for that. Technically there should be a tie on every pair.

Down into the weeds; As the tie is raised above the plate it puts a higher bending load on the rafter and increases the connection stress at the heeljoint. The rafter begins to get deeper and more nails are required. In the span tables and heeljoint table there are correction factors as the tie is raised. You're in chapter 8 of the IRC around R802.5, use the shallower pitch for design, the stresses are higher doing that so a bit more conservative than absolutely needed but that isn't a bad thing.

Space a string along the beam on each side and confirm that you don't need to pull them back together, I wouldn't be surprised if it is bowed out some, this is an opportunity to bring it back in before locking it up. 

[mike_belben]

An exaggeration but this is the idea with cross tie height.  The snow load of a full triangle will push straight down on posts or plates.  A raised crosstie leaves legs like a picnic table and theyll want to eiffel tower.