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Author Topic: King Post Restoration  (Read 395 times)

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Offline EPops

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King Post Restoration
« on: May 22, 2020, 08:13:33 PM »
Hello fellow timber framers.

I would like to ask opinions on restoring the Tie Beam/Rafter connection in the below frame.  I have some good ideas but am looking for other ideas as well.

There are double king post trusses (one king post stacked on another).



 

Top Chord (Rafter) frames into the Lower Chord (Tie beam).  I believe it is a simple birdsmouth.  Where the rafter birdsmouth sits on top of the tie beam.  The connection goes into and bears on a brick wall, so it is hard to see the exact details of the connection.

My question is: depending on the amount of rot at each connection, how would you repair or replace the connections, even going so far as to replace the rafters or beams themselves?  How much rot (in terms of depth, overall section loss, etc.) constitutes repair or replacement?

In my first opinion, I would think to cut out the rotted sections.  How far back would you cut out the rotted portions?

Then, a scarf joint or splice joint can be used.  Would you agree?  And also, which kinds of joints would you use?

There is most likely no damage to the beams at midspan.  Also assume no damage to any other members, including the upper king post.

Lastly, if repair needs to happen, how would you jack or crib the structure to be able to cut out and replace with a new joint?

Thanks!

Offline Don P

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Re: King Post Restoration
« Reply #1 on: May 22, 2020, 09:44:01 PM »
Hi EPops

Interesting frame. It screams "Get an Engineer!" this appears to be a large commercial building, liability carries the day.

I'm assuming the center row of posts under that is load bearing, if so this isn't really a kingpost truss. If what I'm describing is correct this act quite differently than a kingpost truss and that is definitely in your favor if the heeljoints are in trouble. So I guess start there, is the post and beam row under the "trusses" load bearing and is it in line under the kingposts? Can you post a clear photo showing all the joinery possible at the compromised heeljoint? Is the tie beam spliced or continuous? If spliced get a pic of the splice.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline EPops

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Re: King Post Restoration
« Reply #2 on: May 25, 2020, 09:49:55 PM »
Don - thanks for adding in some advice.  Yes, we'll certainly need an engineer for this one.

Yes, those center posts are load bearing.  That center beam is basically in the same plane as the truss tie beams.  Looks like it is there to have some lateral support down the longitudinal axis.  Those trusses are certainly unique.

I'm actually just working off of pictures right now.  Heading to the site tomorrow to check it out and I'll get back to you with more detailed photos on the heeljoints.

I think the tie beams might be spliced, or otherwise not continuous.

Appreciate the guidance and I'll check in again tomorrow.

Offline Don P

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Re: King Post Restoration
« Reply #3 on: May 25, 2020, 10:25:45 PM »
@EPops
Hopefully this will ping you before you head out. I'm guessing you'll be there with your design team. Some things to think about and chew on while you all are looking it over. If it is designed to act as a truss the heeljoints and bottom chords will be designed to resist significant tension. The joinery will reflect that.

What I suspect though is that the post and beam row is supporting the kingpost(s) and then the top chords are designed to hang from that central support. The kings will then be under and propping the top chords in compression rather than hanging from them in tension. The top chords will be resting on the tie beams at the heeljoint rather than being very rigidly connected to them to resist thrust. The tie beam splice will not need to be as robust.

I'm not trying to drive your thoughts just another thing to think about as you look it over.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline EPops

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Re: King Post Restoration
« Reply #4 on: May 27, 2020, 09:51:54 PM »
@Don P : Excellent advice, thank you!  If you do not mind, I would like to share some inspection photos and observations, and ask your opinion on what types of joinery to use for scarfing at the heeljoints.

It's a very odd frame.  There's no mortise and tenon joinery.  In fact, none of the connection points have any joinery at all.  No steel plates, nothing.  It's all mostly timbers butted up against each other.  The only type of joinery is at the heeljoints (simple birdsmouth, with the Rafter sitting on the Tie Beam).  There's also a few places with a simple seat.

First, here is a general view down the longitudinal axis.  The longitudinal beams have a simple, somewhat angled seat into the lower kingpost, and are not continuous.



 

Next: overall view of the "bent" or truss.  No joinery (mostly), members just butted.  There are additional longitudinal beams in the upper "truss".  The upper side posts have a top and bottom longitudinal beam.  There is also a ridge beam above the top kingpost.



 


Here is a view of the Tie Beam connection.  These beams are just sitting on the corbel, and have no joinery to the kingpost or to each other.  So, there is thrust from the Rafters applied at the heeljoints, but the Tie Beams are not even connected to each other at midspan of the truss, so they do not share any tension.  How they are sitting there without being pulled apart or shooting out the side of the building is baffling.  I have a hunch that the sheer weight of the Tie Beams, combined with the bearing pressure at the walls, is keeping the thrust in check.  But it's just really strange.



 


Next, details of the Rafter connections.  The lower braces are simply sitting on the Tie Beams, and have a simple birdsmouth into the Rafters. The Rafters are just butting up against each other, sitting on top of the angled corbel (notice the gap between the Rafters).  And the upper kingpost and braces are also just sitting on this corbel.  I noticed a few toenails into the braces!  That's it!



 



 


Lastly, here are pics of the heeljoints.  They are simple birdsmouth with the Rafter sitting on top of the Tie Beam.  And the joints just bears on the double layer brick wall.  Thus the Rafters are thrusting the Tie Beams.  Also notice the picture from the outside of the wall.  Several of these joints are quite deteriorated.



 


 


 


This brings me to asking: what kinds of ideas does anyone have on replacing or repairing these connections?  I know these are only opinions and I won't be taking any advice as an official plan.  We have an engineer and anything we feed to him will be verified and stamped by him.  But I value any ideas you may have.

I would think to brace (both longitudinally and laterally), then peel off the entire roof.  It seems that the only thing holding this building together is the roof planks and the gravity loads.  Once those are removed, it could easily just fall apart.  Since there's no joinery!  It almost reminds me of the asian timber framed bridges, where gravity locks things in place.

Then, I would peel off the upper truss, with only the lower truss remaining.  Then, one can repair with scarfs at the heeljoints.  The parties involved want to keep as much as possible for historic preservation purposes.  If it would be better to replace whole members, that would also be considered.

But lastly, my main question:  which kinds of scarfs or splices would be most appropriate for this scenario?

Thank you again for any advice or ideas you have.  Hoping I can return the favor to this community in the future.

Offline Nebraska

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Re: King Post Restoration
« Reply #5 on: May 27, 2020, 10:29:47 PM »
 popcorn_smiley That building is cool! I hope we get to watch this project unfold, better than most of modern television.

Offline Don P

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Re: King Post Restoration
« Reply #6 on: May 27, 2020, 11:18:41 PM »
Pretty cool old building.
It appears to be what I suspected. Everything is hanging from the posts. There is no thrust. Stand up a greeting card tent fashion on a slick desk top, push down on the "ridge". The feet slide out, there is thrust. Put a vertical prop stick under the ridge, push down, nothing, everything is hanging from the prop. That greatly simplifies things. As load is applied the posts want to buckle out of plane, your longitudinal beam is bracing that buckling tendency, doesn't take much.

This appears to be type IVb Heavy Timber construction, check this out for reference;
https://awc.org/pdf/codes-standards/publications/wcd/AWC-WCD5-HeavyTimber-ViewOnly-0402.pdf
Hit pg9 fig 6, notice the internal dowels and large staples hidden in some of those bolsters and ties. I suspect they are in there in yours. Take a handheld metal detector next trip and start sweeping the joinery areas. I'll betcha they are in there in every joint. The webs are directing the load from the monitor's beams back to that central post. Look at the upper end of the webs into the top chord, there is a seat cut locking them in, the bottom end is trapped, compressing into the tie and post so the nails just hold it in place.

The ties really only need to support their weight, I would do a vertical bladed scarf, a long vertical tongue and groove joint out at the decayed heel, the tongue is on your outboard end repair, the groove is in the existing. The top chords need to be capable of resisting the vertical dead and live loads in bending, you are out at a point of low moment. The same repair will work to a point but be prepared to replace those, engineer land there. When it is all open and if you expose raw wood soak it all heavily with BoraCare to halt bugs and decay. It will not penetrate finish but if you expose wood you can get it in. Abatron makes restoration epoxies for wood and masonry, might want to check them out.

If I may, you just took me along on a virtual consult on a commercial job, and I'd be happy to keep this conversation going. I'd love to see what you do with this building. I think I've given you some solid advice. If you agree, bill me through as a consultant, scroll down the left column to the donate button or contact Jeff and consider sending this fine website that allowed us to do this a donation equal to what you would have paid me to come along today :).

More thoughts. Fire, I imagine the building dept will require turning this into a IVa, sprinklered, due to area. Either way it would be a good idea and would probably pay back in reduced insurance premiums. I did see rust on a box in one pic and it has been leaking. At that age the insulation in that armored cable is probably breaking down and possibly damp. If you haven't done so I'd decommission the old panel and work off a new temporary.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline EPops

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Re: King Post Restoration
« Reply #7 on: June 14, 2020, 06:35:13 PM »
Hello all, nothing new so far in the last couple weeks - still waiting on others to decide the next steps.


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