critique my truss replacement plan

[Orionrising]

Note I said replace.

Built by a company that went out of business 7 years ago, so no staff engineers to hire to tell me the original design specs on spreading load held back by the floor portion. (if there is any, I am not sure there is as the floor truss meets standard floor truss sizing for the span)

I have a gambrel open truss roofed garage that they did not leave room for stairs inside. The trusses are 16" oc so to move a stair and material hoist inside I need to modify the floor truss part of two of them. My plan to replace them with conventional framing is as follows.


truss 2x4 members represented by lines


replacement plan ( one side shown)

comments?

couple hundred bucks on lumber for this is cheaper than having an engineer model the whole truss methinks. Putting in stairs etc already so the posts dont matter to space use really.

 

[theoldwizard1]

The whole point of ceiling/floor joists is not allow the outward forces from the roof load to displace the top of the wall.

I'll never be able to find it, but one GJ member was able to remove most (all?) of the joist by attaching a heavy steel C-channel (2x3?, 3x4?) point up along the inside edge of the top plate Then he sistered 2x6 on to the existing rafter. The bottom of these new rafters were notched to land on the on the inside edge of the top plate.

Each of these C-channels was weld to a header piece at the front and back.

 

[Orionrising]

yes, and by this plan I would be changing those forces to vertical loads on the post, from the main top part of the roof where the load is via conventional framing. I could effectively remove all the truss members at that point (well other then the outer members sistered to the new conventional framing, as that is what the roof is attached to.

Metal would certainly likely substitute for reinforcing the truss without all this... but again... paid engineering math vs tables and 200 bucks in lumber.

It would likely also be effective to forgo the rafters and just leave the top triangle and ceiling truss as its own remaining truss.. but yet again expensive math

 

[Orionrising]

hmm actually I do not need to buy floor joists... I will have two perfectly good floor trussed to move over there...

 

[lakeroadster]

I'll never be able to find it, but one GJ member was able to remove most (all?) of the joist by attaching a heavy steel C-channel (2x3?, 3x4?) point up along the inside edge of the top plate Then he sistered 2x6 on to the existing rafter. The bottom of these new rafters were notched to land on the on the inside edge of the top plate.

Each of these C-channels was weld to a header piece at the front and back.

Post #72 by LX-Markham on this thread: Truss Modification for Car Lift

 

[Orionrising]

hmm interesting method. Would work here definately... much harder to calculate though due to the multipitch roof I have...

 

[lakeroadster]

Questions;

1. How big is your building?
2. Snow loads and high wind loading?
3. How big a stairway are you planning?
4. Replacing trusses as in remove and replace? Won't that involve re-sheathing the roof?
5. Wondering.. Could simply make a rectangular structure, possibly a steel structure, for the stairway opening and then attach it to the existing trusses, having to only modify the lower chord of one truss?

 

[Orionrising]

26 wx 36 garage

Minimal wind loads- high snow loads * with a caviot that its got a metal roof and doesnt accumalte much ( which may change with insulating)

need to remove two spans of the floor truss.

Replacing is easy, have room to slip header right in and rafter just sister the rafter to the truss out members which will hold the roof on still.

litterally 200 bucks in materials for my plan, just mostly wondering if there is a force I am not considering.

 

[lakeroadster]

So if I understand correctly, you aren't replacing the existing rafter, you're sistering the new to it?

Some photo's of what is there now would help us immensely.

 

[PugetDude]

need to remove two spans of the floor truss.

Can't you just move each of the floor trusses over and sister it to the existing ones, then frame the opening between them with dimensional lumber as required for the floor load? Each of the doubled trusses will be capable of supporting twice the current load. (Similar to what you would do with conventional framing for stair openings, dormers, etc.)

 

[Orionrising]

What I have currently is the truss in the first post 16" oc. I can use the floor trusses part of it as the joist on the side but it is part of the roof truss so I cannot just remove that part.

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[PugetDude]

What I have currently is the truss in the first post 16" oc. I can use the floor trusses part of it as the joist on the side but it is part of the roof truss so I cannot just remove that part.

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Are you sure you can't cut the entire truss loose one at a time and slide them over? (With temporary bracing in place, of course...) Pry the sheathing plywood up a bit from underneath and cut the sheathing nails with a sawzall or multitool. When you frame in between the displaced trusses with 2x's, use construction adhesive and framing clips (or full-length garage door angle) and 5/8" long screws to tie the sheathing to the displaced trusses and new framing.

 

[Orionrising]

I probably could just move a truss to each side, but without a truss engineer from the company around to say thats ok.....

and its not actually sheathed- metal roofing and purlins.

I have soem photos I will attach, but with the spacing and complexity it basically looks like a mess of 2x4s

 

[Orionrising]

attic floor trusses from below

sidewall of gambrel

 

[Orionrising]

my paint skills are not up to par with my sketchup

orange would be post/ wall reinforcement, red header and yellow rafters.
The rafters may be pointless, the top part of this is effectively already a truss on its own, I could possibly just put the header where the strap is on the bottom.

from my understanding, with this structure if I added ceinlin ties to replace the top ceiling truss, I could remove the whole truss as I would have a conventional framed roof at that point. Dont see a need to though.

 

[Strouty]

So are you only making room for stairs or are you taking full trusses out, I am a bit confused.

 

[Orionrising]

I need to remove the portion circled in black on two trusses. I want to do that in a manner that uses conventional prescriptive framing as the engineering would be really expensive compared to living with a post in the corner and $200 bucks in lumber.

I suspect the portion in blue could effectively be left alone without sistering rafters as it is really a self contained truss of its own already, but for $60 in lumber I will sister the rafters.

red for rafter sistering yellow for added posts (two stories) one to attic floor then one below it to garage slab.

 

[Orionrising]

to remove that black circle I need to remove the whole span basically ( or as I plan to move it over and sister it so I can scab members back perpendicular to support the right side of the floor.

 

[theoldwizard1]

Post #72 by LX-Markham on this thread: Truss Modification for Car Lift

Also post #76, same thread.

 

[theoldwizard1]

The whole point of ceiling/floor joists is not allow the outward forces from the roof load to displace the top of the wall.

yes, and by this plan I would be changing those forces to vertical loads on the post, ...

It is not possible to "redirect" a horizontal force to a vertical force. Those post you want to add will bow out once you remove the center of the joist, unless the post are 4x6 1/8" wall steel tubing filled with rebar and concrete buried into new deep footings. I guess you could use steel I-beams also. Those will still bow out, just not very much.

 

[Orionrising]

I will not be redirecting the force, the force will not be created. The spreading force on the floor of the truss is created by the top portion of the truss pushing outward on the steep side members. If the top portion is vertically supported that component of the force will not be created. There will be minimal vertical and horizontal forces on that portion of the roof as it spans a small amount and is incapable of accumulating snow.






Right?

 

[theoldwizard1]

I will not be redirecting the force, the force will not be created.

I guess you never had a class in Statics as part of you Engineering curriculum ! And I am no architect !!

In your first image, you have your "force vector" drawn in the wrong place. It should be associated with the upper part of that roof.

If those ceiling truss joists are "properly" attached to the top of your new posts (NOT joist hangers) it might work. Assuming the truss does not exploded because it is not designed to take that kind of horizontal tensile load. (The top chord of a flat truss is designed to be in compression while the bottom chord is designed to be in tension.)


Steel posts. And they are still going to bow out some.

 

[PugetDude]

Trusses are designed to take the design load when installed at a fixed dimension on center. If you don't alter them, just change the center to center dimension, all you have to do is provide adequate perpendicular framing in between to re-direct the roof load to the doubled trusses. This is done all the time with stairway openings, dormers, etc. Simple, easy solution, doesn't change the design characteristics of the existing engineered trusses.

You're trying to create a new load path with engineered truss members that are not designed to accommodate them, not to mention adding posts that you're going to hate....

 

[Orionrising]

is there a prescriptive method for the perpendicular framing that would no require further engineering?

 

[Orionrising]

If I add table specd ceiling joist green, and sister rafters blue ( probably really unnecessary the existing out member meets the table), at this point I have an entirely conventionally framed roof and the truss is irrelevant. and could entirely be removed ( well other then the outer members holding the roof to the new conventional framing)

 

[lakeroadster]

is there a prescriptive method for the perpendicular framing that would no require further engineering?

For framing I'd follow this guys lead. Carpentry Pro framer

The trusses are 16" oc so to move a stair and material hoist inside

Material Hoist.... tread carefully here. Do you know the 2nd floor load rating?

 

[Orionrising]

http://www.socomp.com/wp-content/uploads/2014/02/FloorSpan.pdf
the floor section of the truss is at least 40 psf live load per this standard table ( and its a modern truss and the industry standard minimum is 40lb.)

Not doing heavy storage expanding my woodworking shop so need to occasional bring up lighter tools, and be able to move sheets of plywood etc. lift will be a hoist on its own vertical posts braced by the end wall, independent of the roof.

 

[theoldwizard1]

If I add table specd ceiling joist green, and sister rafters blue ( probably really unnecessary the existing out member meets the table), at this point I have an entirely conventionally framed roof and the truss is irrelevant. and could entirely be removed.

Now you are headed in the correct direction !

The real issue is the attachment point of the new, green joist to the post. This needs to be very secure. I would use a 3'x3' 1/2" plywood triangle gusset at each connection. One on each side of your new joist. Glued (construction adhesive) and screwed.

Those dark blue rafter will help a lot, but the connection points, top and bottom, have to be very secure. Use gussets.

 

[Orionrising]

Can't you just move each of the floor trusses over and sister it to the existing ones, then frame the opening between them with dimensional lumber as required for the floor load? Each of the doubled trusses will be capable of supporting twice the current load. (Similar to what you would do with conventional framing for stair openings, dormers, etc.)

I have been pondering this more. Theoretically it should be possible. Temp brace the floor between trusses, cut everything free slide to both sides. Will have a look at the roofing fasteners tonight to see how much they would get in the way. As long as I wait till past chance of snow there is effectively no load on the trusses right now. Should be able to pull them togethor- with straps etc. Does doubling trusses generally come with a nailing schedule?


Then frame up ladder sections and nail to the truss pairs in the middle.

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[Orionrising]

I have been exploring the move the trusses to each side and ladder frame concept. It appears feasible, I have tested cutting free a purlin (wack with sledge till minor gap, sawzall nails, it looks like the fascia are should be easy to remove. Theoretically I should be able to wack the decking from below up enough to cut those nails too. (easier then cutting the whole floor across the span out, but that works if i have to)

Will take me a few days of grinding nail head off etc, but should be doable.

I assume its a a hundred pound or so truss. all 2x4s so call it 20 2x4s weight pluss plates, and once the roof pulins and decking and blocking are free I should be able to hammer them to each side, or pull with straps fairly easily. End truss should be remain well supported by the purlins and deck if i can avoid removing it, until bracing is replaced...

I now rate this plan as feasible.

 

[Orionrising]

and i need to go buy a bunch of long thin metal cutting sawzall blades...

 

[lakeroadster]

I would add components... (Carpentry Pro framer) and not remove anything existing until the new components are in place.

It would be a wise move to run your plan by a structural engineer before changing anything.

 

[Orionrising]

yeah that why I am leaning towards the move the suggested "move trusses to the side like a traditional dormer" Will end up with double truss on each side and a ladder framed center.

Will certainly run temporary joists.

I have run briefly both options by a CE friend in concept and they thought they both seemed to be feasible.

Effectively no load on the structure right now as long as its not a windy day. ( roof is 1x8 purlins with metal roof, with is self supporting for the 40" or so open span I would have during modification until I added the ladder frame cross members.

Prescriptive truss bracing pattern would be maintained and exceeded.

 

[johnnyradiant]

So if Orionrising went with the post method would the post be allowed to sit on the slab? Would you not need to cut the slab and create a footing for each post?

 

[lakeroadster]

yeah that why I am leaning towards the move the suggested "move trusses to the side like a traditional dormer" Will end up with double truss on each side and a ladder framed center.

Will certainly run temporary joists.

You keep using the word "move"? Are you really going to "move" trusses, instead of leaving them as is and adding structure? That's opening a can of worms and sounds easier than what it will ultimately be.

So if Orionrising went with the post method would the post be allowed to sit on the slab? Would you not need to cut the slab and create a footing for each post?

Ideally yes, that would be the correct way.

If he knew what the sub-base and compaction was, the strength of the concrete and it's thickness, and if reinforcing steel was used, he might be able to simply use an adjustable Tiger Brand Jack Post

 

[Orionrising]

there would be minimal actual load on the post. We are only talking about supporting a 4 x 15 foot span for the supported section, go high and call it 100 lbs live and dead load PSF, is a total load of 6000 lbs, each corner should have 1/4 that.... 1500 lbs a post, if it was just a 4x4 ( which it wont be for stability)... under 100 PSI on the concrete, far less then say a jackstand,


hmm simple option I had not though of.... get two 4 ft sections of something like 4x4 steel tube, mount across back of two trusses at bottom... connect with something like 1/2 cable on each end. Quick deflection calculator/table says would have a breaking limit of like 25,000 lbs... on the cable and .25 inch deflection on the tube at that load....

that outta overkill it by a bit...