iDRY Vacuum Kilns

Sponsors:

Kingpost load bearing calculation

Started by trhill, March 20, 2014, 03:05:01 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

trhill

How would you guys figure the load bearing ability for a kingpost loft area?  My design is currently a 20x20 or 20x30 kingpost, 10ft bays, 9 or 10 ft sidewalls, framed mostly in ponderosa pine 8x8s. The kingpost would be a through wedge, 12/12 pitch. The purlins may be peeled logs with 4-6 inch tops. Foundation is rubble trench.
I am in South Dakota, so about 40 psf snow load.  Should I use struts in the truss for sure? I want to use the loft for work and not just light storage.  Thanks for the help.
-TR

Thehardway

TR,

We need more information.  What you provided is a bit cryptic.  Is the proposed span of your king post truss 20'?  What would the 30' dimension be?  An additional truss/bent?   Where are you proposing to put the "through wedge"?  Is this at the rafter/tie beam connection or the kingpost/tie beam connection?  Is this a timberframed roof over conventional 8X8 log walls or all timberframe?


A traditional kingpost would use a wedged through tenon at the kingpost/tiebeam joint and wedged half-dovetail at the rafter/tiebeam connection.  There are several styles of joint that can be used at the top of the kingpost depending on your taste and skill level. 

The load bearing ability will depend upon a lot of factors like timber sizes, and species, joint selection, thrust control measures at the wall as well as local design requirements such as snow and WIND load which can be significant in a 12/12 roof.  Designing without struts will increase useable room in the loft as well as simplify design, joinery and assembly but will require larger members in the top chords. Kingposts have been used for railway bridges so it can be very strong it just depends on what parameters you design it to.
A primitive sketch will help.  What kind of "work" do you plan to do in the loft and what kind of loads do want to design for.  20' is not an unrealistic span but I would recommend an engineer review whatever you come up with.

What kind of wood and and what lengths/dimensions will you have access to?  8X8 pond pine will not be sufficient to build the kingpost for the span you propose IMHO although it would be sufficient for stacked log walls.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

trhill

Thanks for the reply.  To clarify, it will be a 20' span, the length of the shop will be either 24 or 30'.
The kingpost to tie connection will be the wedged through tenon like you mentioned. It will be entirely timber frame, with 10 ft bays. I have only ponderosa pine; I planned on 8x8s for most of the framing members but maybe I'll try to get 8x10s for the tie beam.
   Thanks for the help on struts, I didn't fully realize their purpose. I thought they were mainly for rigidity but it sounds like they are only to prevent the rafters sagging.
  I want that loft area for storage but also to setup my setter and sharpener. Just not sure how much storage or how much of a workspace I could have up there. Thanks again
-TR
   

Thehardway

TR,

You'll need to have the load calcs done but I'm betting the 8X8 is going to be a little small for your rafters (top chord of kingpost truss) as well, especially with 10' spacing.  In a kingpost truss design, all members are in tension except the rafters which are in compression and bear the entire weight of the assembly down to the walls.  The struts work to prevent sagging of the rafters or in extreme cases buckling of the rafters.  In many cases they are decorative.

If it is an option, you would be better off to cut 7X10 or 6X12. either would give more strength than the 8X8 and possibly utilize the same log.  You could also use scribe rule and leave the bottom of the logs natural round, squaring up just the tops and the sides for additional depth and strength in the rafters.  I have seen this done with very nice effect.  I left the bottom of my main floor bearing beam natural peeled finish while milling top and sides. same goes for kingpost leave sides natural round and mill just the front and back.

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

trhill

Thanks, that's great help. I will increase the timber sizes. My shop will go through more revisions in the next couple months so the experience of the guys on here will be invaluable.

Thank You Sponsors!