Hi Timbeal,

Ooops! - I hadn't spotted that this frame has a single 25' clear span bay between the bents (cross frames). That's really a tad too much and it might have been better to split this span into 2 bays of 12.5ft. For future reference 16ft is approaching the maximum length per bay and more usually bay lengths are in the range 8 - 12 ft. This frame is very "empty" and as such it has very little in the way of second lines of defence for load shedding should there ever be a major component failure.

Given that the frame has a fairly low pitch roof and a fairly light roof covering and that the gable ends are to remain open (the sides as well ?) then I would anticipate that this frame will most likely be vulnerable to wind damage. Frame design is not all about beam sizing to satisfy dead loads.

Regards

Ken Hume

Actually it is only 22 ft between posts. The end to the left (north) will be filled in as well as half of one side. In the end however the client did want a clear span of 24ft. I tried to persuade them to add a post to no avail so I made it 22 ft in order put full size tennons on the post tops that could have two pegs. The client also wanted the end beams to be the same hieght as the side beams. That would have been a whole different ball game and I didn't have the confidence to do that properly.
My original design for this frame had the posts four feet in from the ends reducing the span to 16ft. It also had braces on both sides of the posts and it looked better. That design was rejected by the client. I did what I could to make it as strong as possible.
Thane

Hi Thane,

Who is liable if this frame fails and kills someone i.e. who is responsible for the design ?

At what point do you walk away from a prospective job when the client is not listening ?

You will have to learn to pick your jobs and your customers.

Regards

Ken Hume

There is a lot of value in what your saying Ken and in the future if I don't feel right about something a client wants I may say no. In the mean time I really do hope no one gets hurt as a result of my apparently inadequate frame design.
Thane

Again, fine looking frame Thane.

As I mentioned in another thread, before we jump to conclusions and get all didactic on Thane, maybe we should have some information pertaining to the structure?

What species did you end up with? I know you used EWP#2 for the calcs.

What will be sheathing the roof?

What is the roof material (metal, shingle, slate?)

Are there concerns for absurd wind loads? (on the coast? hurricane zone? tornado alley?)

What did you do for a hold down to the piers or slab? Are there footers under the slab?

What size are the posts?

I see through tenons at the bents - how thick? how long? wedged?

How long are the tenons into the plates and ridge? How long?

Pegs and species?

What will the materials / build up be for infill sections - will they be sheathed in plywood and sided? stud infilled?

Will there be anything else in the structure that will act as bracing - a bar, storage, seating, etc. etc?

Just some questions that would really paint a clearer picture before we should be talking about it falling over and someone starting a law suit.

Ken, I disagree. The rule of thumb should be tied exclusively to function, desired form, and how the two inter-relate about the engineering to make it work. A blanket thought on 'emptiness' is quite subjective, and it would be a sad thing to see most things designed with 8-12' bays with little to no reference to function.



As I understand it this wants to be an open air pavilion. The plates are probably OK, depending on snow load and roofing material. My guess is that there is more a worry of uplift here that failure from roof load - so critical information will be how the rafters are held down, and if the bracing along the ridge and plates will be enough to keep things rigid (this also is highly dependent on roof sheathing choice).

Hold downs to the foundation are another potential area that play into this, if the 'kite' effect with the partially closed structure is strong enough to tip it over.

I think to put design on autopilot and drop posts every 8-12 feet and give someone a rule such that (and I quote) "For future reference 16ft is approaching the maximum length per bay and more usually bay lengths are in the range 8 - 12 ft." is doing a disservice to the trade and to what wood can do... especially in a design that wants to be an open roof to enjoy drinks lake or poolside.

Hi Mike,

You and others are perfectly at liberty to disagree with my thoughts and opinions but that does not necesarily make you right and me wrong (in my opinion). History and traditional craft practice is on my side in respect to what I have preached whereas you seem to be promoting the idea to a new entrant that he should take a walk on the wild side. That's a fine thing to do as long as you are equipped, aware, educated, experienced and have very deep pockets but I very much doubt this to be the case in question.

Thane, by his own confession, has already agreed with me that a centre bent was considered and rejected by the owner and it was this that resulted in the adoption of the extreme scantlings seen at the ridge and wall plates plus also the lack of long wall and cross frame infill timbers.

I note that you have actually started to follow the same analytical path that I trod but have not chosen to be so economical with your words. You have now asked so many questions of Thane (as yet unanswered) that this might well now be a cause of concern to him. I tried to resist that temptation in order to let him discover and hopefully gain confidence in how to contribute towards the decision making process hopefully helping him to achieve better (future) outcomes.

We need to keep in mind that most subscribers to the TFG Forums are in discovery mode and therefore its not entirely unreasonable to provide them with a few rules of thumb that will help guide them until they gain that all important experience.

My current project involves the use of 68 foot long (that's the width of Westminster Hall hammer beam roof) single piece beams, simply supported at the centre only and subject to both bending in two planes and torsion. This would not be a good timber frame introductory project nor one where rules of thumb would be appropriate but it is one where previous craft based design solutions have been verified through a process of trial and error. This is a perfectly valid approach providing people don't get hurt whilst the experiment proceeds. We are today more aware of our responsibilities towards others to say nothing of today's culture of litigation so its not entirely unreasonable that the general craft should evolve to become risk averse and it seems to me just plain common sense to adopt well tried and proven solutions and practices.

Timber Framing is not a trade it is a craft and your remarks about me doing a disservice to same are frankly insulting.

Regards

Ken Hume P.Eng.

Thanks Ray, I agree

Wood species is pine with cherry braces and pegs.
Roof sheathing is 2x6 T&G pine plus 1/2 O.S.B. Plus ashphalt.
Rafters (6x6 with bird's mouth) screwed with 1/2" x 6" screws.
Posts are 8x8 with 2x7.5x6" tennons.(2 pegs)
Tie beams are 8x10 with 2x9.5x10" tennons. (3 pegs)
Post in steel saddles with 2 1/2"x 4" screws each.
Saddles anchored to 16 "x 4' piers with 4 1/2x3 lags.
Snow load here is 30 psf.
I allowed 5000 LB/beam total load in my calculations.
North end filled. Board & batton
Partial fill on east side. Same
Just a note. This thing is open for the most part. If there is a wind strong enough to blow this thing over, there will not likely be anyone inside.
It's true. I'm fairly new to this. I raised my first frame in '03. (An 8x20 wood shed with 4"x4" timbers) It stands open to north west winds. Loads were not calculated. Looking back now, the 1x3.5 tennons were too short. It probably breaks all the rules in the book. It's still there.
Thane