iDRY Vacuum Kilns

Sponsors:

Round log truss design

Started by RMS, July 03, 2007, 11:18:58 AM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

RMS

I'm in the process of building a log home and I'm looking for a little direction in designing two king post trusses for my roof system. Here are the design parameters:

32 x 22 ft great room/kitchen.
Roof pitch is 7/12, span is 22 feet.
Tie beam is 14in average diam.
Principal rafters and king post are 12in average diam.
Struts (webs) are 12in at the tie beam, 11 in at the principals.
Material is western lodgepole pine.

I'm assuming 20psf dead load and 50psf live (I need to confirm these numbers).

There will be two trusses, the first located 12 ft. in from one gable and the second located 10 ft. in from the other gable. The trusses will support a 15in ave. diam. ridge log and two purlins (all full length and supported by framing on the gable walls). The purlins will support 2x12 common rafters spaced at 24in O.C.

My gut feeling is that this will be more than sufficient as a strong roof system, but I'm hoping that some of the experts on this board can let me know if I'm going in the right direction here. Some of the designs I've seen call for through bolts at the rafter foots and where the rafters meet the king post and also some other type of connection (knife plate, perhaps) at the king post/tie beam connection. Is there a rule of thumb for sizing these connectors?

Eventually I'll have to have an engineer sign off on the design, but for now I just want to know if I'm on the right track.

Thanks for any advice.

RMS

sawdust


Hey RMS, I think there are some tools and things in the www.logassociation.org site.
I will be in the park in a couple weeks for a 40 anniversary.

sawdust
comforting the afflicted and afflicting the comfortable.

Don P

How bad do you want to be accurate?

The book says to take the area of the tapered column's ends and use a square of the same cross sectional area. There is a formula that takes the taper into account and gives a value to use for "d" or depth in calculating.

I would take the small end's cross section area and use that to make a square with the same area, use that first try.

You can draw the truss and get a quick check of the forces with this calc
http://www.jhu.edu/~virtlab/bridge/truss.htm

Then the forces shown for each member in the truss can be checked against size and species to see if you have enough wood;
https://forestryforum.com/members/donp/columncalc.htm


Chapter 6 Round Timber Poles and Piles of the NDS lists base design values for lodgepole pine as
Fb 1350 psi
Fv 85 psi
Fc perp 240 psi
Fc 700 psi
E 1.08
Emin .570

That takes care of the members. The connections would be checked one by one next. The rafter foot is probably the best place to start. You either need to have enough wood to comfortably resist the crushing (Fc perp) and shearing forces or need to add steel. When a joint comes together at something other than paralell or perpendicular you can interpolate the 2 Fc numbers (Allowable fiberstress in compression). Design values for bolts and plates are in the NDS.

RMS

Thanks for the info, Don. Sorry for the late reply but I had to take some time to mull all this over - a lot of info to digest, especially for a greenhorn like me.

I think I need to step back here and start by determining the distribution of loads on this roof system. Here's where I get confused: do I need to consider the purlins and the trusses as two separate components? That is, should I calculate loading on the purlins first to determine proper sizing and truss spacing, and then calculate the loading on the trusses (including the load of the purlins) as a second step?

As for accuracy, I just want to make sure my roof doesn't fall in on me  ;). Seriously though, most examples I've seen for log design simply use the midspan diameter, but to be safe I think you're right in using the small end as the starting point.

I tried the bridge truss calculator -- really cool -- but I need to figure out what the numbers mean. I'll try it again once I get a little more educated. I'm off to do some more reading ;D.

Thanks,

RMS



PS to sawdust: I bought the "Log Span Tables for Floor Joists, Beams and Roof Support Systems" by A. Mackie, N. Read, T. Hahney from the ILHBA some time ago and have been using this as a reference as well.

Don P

I'm taking your description to mean loading the nodes of the truss, that is the ideal way. There is a ridge and one purlin on each side, over the strut.

I would want each purlin to be able to support the section of roof over it. That would be an area halfway to the next purlin (the ridge) and halfway to the wall, or a tributary width of 5.5 feet. The center purlin will have the same trib width. The worst span is 12'. So 5.5 X 12 X 70 psf = 4620 lbs on each purlin.
I checked it with this calc (Formulas from the back of the log span tables  ;))
https://forestryforum.com/members/donp/logbeamcalc.htm
That was one of my first calcs, it could use an overhaul.
........................................................

To get the truss load;
I took half the 12' to the front gable, (6'), and half the 10' distance to the truss behind it,(5'), and assumed a tributary width of 11'.

I assume there will be 2' overhangs loading the 22' truss for a 26' span X 11' trib width X 70 PSF = 20,020 lbs on the truss.

When you load the truss nodes, I distributed 5000 lbs on each node and 2500 lbs on each heeljoint. (In the model I used 25 and 50 and remembered to add 2 zeros after).

That's just how I see it from here, make sure you're satisfied  :)


RMS

Thanks, Don. There is now a glimmer of understanding here. I had a little difficulty figuring out how to distribute the loads in the roof system.

I just rediscovered an article I had on designing log trusses. It's a chapter in Robert Chamber's "Log Construction Manual" (an excellent resource, I might add). Although the analysis in the article isn't complete it does offer a lot of insight into what should be considered when designing these trusses.
This, along with your comments, has started me down the right path here.

Once again, thanks for your guidance Don.

RMS.

Edit: Just had to add that this is definitely one of the most informative sites I've stumbled upon.  :)

Don P

For those who have it, Parker/Ambrose"Simplified Eng'r" sections 6.4 and 6.5.
I came across the section on poles while in the reading room.

Poles are round timbers consisting of the peeled logs of coniferous trees. In short lengths they may be of relatively constant diameter, but have a typically tapered profile when long-the natural form of the tree trunk. As columns, poles are designed with the same basic criteria used for rectangular sawn sections. For slenderness considerations the d used for L/d computations is taken as that of a square section's side dimension. the square having an area equal to that of the round section. Thus, calling the pole diameter D,

  d2 =(pi*D2)/4 ...or, d=0.886D

For the tapered column, a conservative assumption for design is that the critical column diameter is the least diameter at the narrow end. If the column is very short, this is reasonable. However for a slender column, with buckling occuring in the midlength of the column, this is very conservative and the code provides for some adjustment. Nevertheless, because of a typical lack of initial straightness and the presence of numerous flaws, many designers prefer to use the unadjusted small end diameter for design computaions.

Poles are used as timber piles and are also used as buried end posts for fences, signs, and utility transmission lines. Buildings of pole construction typically use posts as building columns. For lateral forces, buried-end posts must be designed for bending, and if sustaining significant vertical loads, as combined action members with bending plus axial compression. For the latter situations it is common to consider the round pole to be equivalent to a square sawn section with the same cross sectional area and to use the code criteria for solid sawn sections.

The wood framed structure utilizing poles has a long history of use and continues to be a practical solution for utilitarian buildings where good poles are readily available. Accepted practices of construction for these buildings are based mostly on experience and do not always yield to highly rational analysis. If it works and many long standing examples have endured the ravages of time and climate, it is hard to make a case on theory alone.  :)

I need to reread the back of the "Log Span Tables" there is a section on truss design there also. Looks like you can convert our loads to pounds per lineal foot on the truss and check it there also.

WyomingCabin Builder

 

  

  I have a 60 year old log cabin in Wyoming. It has three log beams spanning 18'6". The width of the cabin is 13' 8". I want to remove the support column. Can I? What truss would anyone recommend if needed?

beenthere

Welcome to the Forestry Forum.
Nice project you have in mind.
I take it you want to build the truss out of logs to fit the cabin. 

What species of wood do you have available? Probably Lodgepole pine?
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

WyomingCabin Builder

Yes, lodgepole pine. The same wood that you see in the picture.

beenthere

How are the walls where the truss will sit? Or will you need to put in posts at the truss ends that go to a foundation at ground level?

Are you going to try to do this within the present wall/ceiling structure?
or willing to cut into the ceiling and above the walls out to the eaves?

Give us some parameters to work with. ;)

From the pic, the center post doesn't seem to align between the top and the bottom..  or is it just the way the pic is taken?

Where do you want the support, possibly to the side of the picture window at about where the wheelbarrow sits outside?
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Thehardway

I would certainly want to investigate further and see more pictures and know what the roof stucture atop those beams consists of and where there are splices in the log walls if any as well as all window placement in side walls before moving forward.  That disclaimer aside, it looks to me like what you need is 2 "false kingposts".  By false Kingposts, I mean they look like kingpost trusses but function the opposite way.  They would consist of a large diameter tie beams between the walls which rest on the second log course and tie the two walls together.  A ridge post would then rise from the middle of this beam to support the ridge and two struts would tie into it to support the mid span purlins.

I would place these spaced equally on either side of the window and the current post and once they are in and everything is secured, I would then remove the central support post.  It would look something like this:

 
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Thank You Sponsors!