Knee Wall..."Hinge Joint", site built truss ???

Started by reverendxlt, March 22, 2007, 08:25:14 PM

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reverendxlt

Hi John and others...

I am getting ready to start the roof framing, and am having some level of disagreement with my knowledgeable assistant (he's argumentative, but free  ;), my dad)...

Have a 3 foot knee wall on the 1.5 20x30 cottage. He says that less outward pressure will be exerted on the vertical walls using a modified kingpost/ridge beam combination.

Two problems with this are...1) I don't think that the load transfer is adequate to support a beam that size 2) that beam is going to be extremely cumbersome to mess with, and pretty dangerous, not to mention that every ridge beam I have seen spanning anywhere close to that has sagged pretty noticeably.

I say, that we need to lower the cross member on the site built truss (roughly proportionate to the amount we have added with the knee wall to keep the center ceiling height about the same) and that the truss will be better at holding the outward stress in check. Lowering the cross member seems to provide better support/leverage to hold things together in my mind.

So, at risk of sounding like a Dear Abby letter...what should we do? Am I thinking about this at all correctly? Thanks,

Reverendxlt

reverendxlt



PEG688

#2
I'll agree with Dad , with a post centered under the rigde beam, allowing that said post provides for a path downward to direct bearing , would lessen any outward thrust put on those knee walls in turn lessening the hinge point effect.



 If the ridge can't move downward there is no outward push/ thrust .

 This ridge beam won't sag , 5 1/2" x 14" IIRC.  W/ 6x6 fir post under both ends , down to the window and door headers , to the trimmers ,  box sill and foundation.  

Be sure if you go that route that the headers , if any , are large enought to carry that load.  



Done in this case purely for the visual effect of a large beam , the ceiling will be 1x6 T&G Cedar, smooth side showing , with a "V" match.

 
When in doubt , build it stout with something you know about .

reverendxlt

#3
DOH!!!! Any other takers....!!!  :-/

Thanks, PEG, I appreciate it. I would also appreciate any dissenting opinions however...  ;)

PEG688

#4
So your not looking for the truth eh ::) Just ammo to beat up dear ole Dad :'(

 ;) ;D
When in doubt , build it stout with something you know about .


reverendxlt

#5
Not at all, always seeking truth...

I guess I just visualized whirled peas...oh wait...wrong forum... I visualized the truss as actually self contained/self supporting and the larger triangle made by lowering the cross member as actually adding positive resistance to the outward pressure...

Whereas I guess I saw the the kingpost/ridge beam as just passively not adding to the outward pressures...

the beam that you mentioned won't sag, even across an almost 30' span? I know it doesn't have the same load as the beams for the foundation, but it is basically the same dimensions and they are only 7.5 feet between supports... 30' seems like pretty far.

And it seems like sag would = extra outward pressure

PEG688

You'll Have to free span 30' ??  No interior partions ?? Just a large open room?

Site built truss's are hard to judge , grade of wood , quaility of gusset materials  and fasteners , quaility of jointery etc are all subject to , well you the builder.

The bean I show is only spanning 12 ' , if you have to span 30' you'd have to consult with some one to size it for your span and load / conditions specific to your area.  

A truss,  even a manf. one used it a 1 1/2 storey building would have to have a lot of lower anti spread strenght built into it. As your ceiling joist  / collar tie/ triangler area /  cross piece,  would be high up / a long way for the bearing point / heal area.  How would you build in / design that anti spread part of said truss??    
When in doubt , build it stout with something you know about .

reverendxlt

well, yeah, for the most part, one wall about 10 feet fromthe end. But that still spans 20 feet... We have four boys will be using the upstairs as sleeping quarters/play area, so it will be mostly open.

If we need to add so intermittent posts that would be ok fo the layout of the room, but the supports couldn't just sit there, they would have to have posts under them on the 1st floor which would have to have extra piers underneath to transfer the load. Right? Or is load light enough to just rest on the floor joists?

John's plan for this shows a site built truss that I presume is sufficient, without the knee wall present. I was going to drop the cross member down by the height of the knee wall (3') to compensate, keeping the ceiling height at the center consistent with the plans. Haven't done any math on the thing, but trying to get input into whether this is worth pursuing.

I f I added a wall right down the middle, would that be enough support for the beam, without adding posts and piers on the lower levels?

The beam I calculated needed for 30' clear span would weigh in the neighborhood of 12-1400 lbs...not something I want to really do.

PEG688

Quote
#1: If we need to add so intermittent posts that would be ok fo the layout of the room, but the supports couldn't just sit there, they would have to have posts under them on the 1st floor which would have to have extra piers underneath to transfer the load. Right?

#2:   Or is load light enough to just rest on the floor joists?

#3:     John's plan for this shows a site built truss that I presume is sufficient, without the knee wall present. I was going to drop the cross member down by the height of the knee wall (3') to compensate, keeping the ceiling height at the center consistent with the plans. Haven't done any math on the thing, but trying to get input into whether this is worth pursuing.

#4:   I f I added a wall right down the middle, would that be enough support for the beam, without adding posts and piers on the lower levels?

The beam I calculated needed for 30' clear span would weigh in the neighborhood of 12-1400 lbs...not something I want to really do.

#1: That would MTL be best.

#2:  Maybe if that wall where sheeted on one side , to make it into a sort of box beam , a ridged wall that couldn't sag. That placed on the floor joist in theroy could transfer that load over the full width .

Maybe  the beam could break over that wall so it would be a two piece ridge beam ??

#3: The 3' knew wall and hinge point would seem to be the issue. That hinge point.

So that cross member would be a collar tie in essence??

 #4: Length wise ???  Or cross ways ?? When you say down the middle .  :-?

The cross pieces are collar ties in the photo , at the dormer the ceiling joist are also collar ties . This place has a let in ledger so no hinge point at that short pony wall.  


 
 
When in doubt , build it stout with something you know about .


glenn kangiser

Are you semi-balloon framing the pony wall?  That could cut down on the hinge effect.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

PEG688

QuoteAre you semi-balloon framing the pony wall?  That could cut down on the hinge effect.


I think line 3 of the Rev's first post sez he has a 3' high knee wall as she sits Glenn , I could be wrong  :-[ But that would be a first eh :o ;D
When in doubt , build it stout with something you know about .

glenn kangiser

Yeah - you aren't wrong often, PEG.  I was just thinking that maybe it was speculation and hadn't been done yet so could be built stronger that way but I am pretty sure you are probably right.  I'd have to get up pretty early in the morning to beat you.  Everyone knows its the early bird that gets worms. :-/
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

reverendxlt

technically, the knee wall is not in place, no, it's not ballon framed. i would like the extras usable space in the loft, but I could eliminate the hinge joint at this point by just setting the truss on the existing walls.

yes, the collar ties are what i was referring to as dropping down three feet. This would appear to add more resistance to the outward tendency of th walls, as the site built truss.

"Right down the middle" I meant that lengthwise, right down the 30' span, or at least enough to break up the span into sections supported at least at 12' intervals, based on the beam dimensions you gave me earlier, PEG.

glenn kangiser

Moving the collar ties down would resist the outward force on the walls but add a bowing point in the rafters at the end of the collar ties.  Loads down and out would still be there.  Posts and beam would stop that.

I see it commonly done but am not an engineer so don't know the result of how the loading would all interact with each other.  I can picture the way the forces go though.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.


reverendxlt

I am not an engineer either (could you guess...  :D) but I guess I don't see how the truss isn't a self supporting unit with the plywood gussets and lower collar tie...

I like the theory of the post/beam, but I don't know if it is manageable...

John Raabe

#15
OK - Are you doing the full height stud wall shown on the Cross Section sht. 4?. That has a short extension up to form a kind of pony wall, but much stronger. You can extend the 10' studs I show into 12' footers if you want a higher "pony wall". I do not suggest platform framing the loft floor as then you are building a short unsupported wall that becomes a hinge joint.

Then, if you do the balloon framed walls, you can use the site built truss shown on sht. 6 of the 20' wide 1- 1/2 story plans. This is somewhat similar to the photo PEG posted above (but with plywood gussets)

That diagram builds a very sturdy truss and the position of the cross-member (collar tie) can be adjusted to match your needed ceiling height (7'-6" min) and the sidewall height you have settled on. If the inspector is not happy with that sturdy truss you may need to have them built by a local truss company who will do the needed engineering to take the the load of the tail of the inspector (and the outward thrust at the tail of the truss).  ;)

If you do this, I am willing to bet you will think the manufactured trusses you will get are pretty wimpy compared to the one I've designed.

If you have platform framed and decked the loft floor already, then you're best option might be to visit the local truss company as they will likely be able to save your bacon. They may also be able to build the pony wall stud into the truss and make the whole thing stronger as well.

If you are doing a dormer you will normally double the trusses under the dormer sidewall. Ask the truss company.

All this can be replaced by a beam at the peak if you want full cathedral interiors, but that is expensive and you will need to have solid bearing all the way down to the foundation which may involve modifications to the work you have already done. That option will need to be worked up locally by a designer or engineer.
None of us are as smart as all of us.

reverendxlt

#16
I would rather do almost anything to using the manufactured trusses I have seen. I have no inspector, we are not in any zoned area, only septic is regulated in the county, so no worries there. I just want it to be sturdy and stand up straight...  :)

Thanks for the input, John, I appreciate it.

MJ

Interesting thread (I just discovered this forum).  Can someone help me understand why platform framing a short second story wall creates a hinge joint?  How is this different from a full height second story wall (as far as the outward force acting on the top of the wall, short or tall)?

MountainDon

Perhaps either of the topics in these links will explain the problem,,,

http://countryplans.com/smf/index.php?topic=11261.msg164940#msg164940

http://countryplans.com/smf/index.php?topic=12073.msg155420#msg155420

It comes down to the absence of a rafter tie when the short knee walls are used.
Just because something has been done and has not failed, doesn't mean it is good design.

MJ

OK.  I get it.  You can't use rafter ties with a one and a half story, because that would eliminate necessary head room.  Thanks for the quick reply!