Best joint to choose

[RogerS]

I've replaced an 8 x8 inch oak beam with a new one 8 x 10 inches (10" width). The original beam had joints for the joists made like (A) - clearly easier to make. Another old beam has joints made like (B) and I can see that it has advantages as the integrity of the beam seems to be better as the top surface of the beam is continuous compared to (A). Trouble is, to produce (B) will take much more work than (A).

Can I get away with (A)? Or is (C) a compromise? Any other joints to consider?

The oak beam span is 4m. Joists are approx 4 x 3 inches.

 

[jasonB]

As you have got a bigger beam there will be little loss in strength if you go for option A. I would also not have a notch out of the bottom of the joist as this reduces the joist strength.

Option C will only give you about half the joist strength but the beam will be stronger.

Check what your engineer or BCO wants before cutting wood.

Jason

 

[Lukey]

Jason you have slightly confused me.

Are you saying that A is stronger or C is stronger?

 

[OLD]

I have always understood that the stress is in the bottom part of the joist so thats where the support is needed, think joist hanger and were the rebar is in a lintel. so B would be the strongest .

 

[Chris Knight]

B will be the strongest by far. However, since A has presumably survived quite a few years previously, I'd probably go for that. It will make fitting the joists easier too.

 

[jasonB]

I said that as the main beam has been increased in size by 25% this will offset the weakening of the beam by using method A over method B but B is the strongest method.

As far as joists go, B is again the strongest, A will reduce the effective cross section of the joist by a third as drawn and the inside of the notch is a stress raiser from which a split can start, As drawn C only has 1/3 the cross section of the joist but as the bottom is solid will probably give a load bearing capacity of about 50% of what an uncut joist could take.

Jason

 

[RogerS]

I considered cutting out a complete housing for the joist and leaving the joist untouched but felt that that would take too much out of the main beam. other suggestions have been to stagger the joists but cosmetically I don't like the look of that.

 

[jasonB]

This is probably the best compromise

The least amount of material removed from the top of the beam, Joist will not be weakened and a minimum of work cutting the joint. give the joist 50mm bearing on the beam.

Jason

 

[dirtydeeds]

the sketches and dimensions look like medieval carpentry (or at least very old)

i wish i could do simple sketches like youve done to show you

both joints are valid and neither takes any strength out of the beam or joist because of the outsize dimensions

if its replacement or repair the first method is easier if the floorboards are already taken up

the second joint type is a medieval "prefab" joint

offset and irregular spacing of joists "look" wrong to our eyes but it was perfectly natural to medieval carpenters.

they saw EXACTLY what we see ( 2x8 joists timber on edge)

they just didnt remove the excess timber becaue it cost too much to do it,

it needed costly tools and labour (a two man saw and a pit) to square timber up

 

[RogerS]

if its replacement or repair the first method is easier if the floorboards are already taken up

I think it is!

If you're curious, you can see more if you click on my weblink

 

[andy king]

Hi Roger,

I saw this posted on the other forum where you show a picture of the beam after you've notched it.
I would maybe consider one of these options to gain some strength back. The first one with the bevel will maybe be more tricky to control as it will dop slightly if the joints aren't spot on, so the tops of the joists may not align. They could also sperad the beam, deflecting it in the centre of the span, but as you've checked both sides on your picture, one should keep the other n place.
The second option allows you to shorten the housing, and in doing so, regain some strength, and also align the joist tops easily as the shoulder carries.
With the bevel as well, the joist can't drop, and with the beam checked both sides, you introduce a dovetailing effect, so they can only get tighter.
Hopefully there is enough meat in the beam to allow this type of joint?


Hope this helps!

Andy

 

[dirtydeeds]

yes, agreed

thats repair and repalcement

 

[andy king]

Oops! Just noticed you've posted the same pic here now.
Anyway, the image I posted shows the end of the joist, so you would have to run a bevel through the bottoms of your housings on the beam to allow these options to slip in and do the job.
Does this make sense?

Andy

 

[RogerS]

Andy...don't really follow you..it looks to me as if they'd slip out?

Roger

 

[jasonB]

Andy on your sketch to the left the small flat area on the joist will be carrying all the load so the joist will only have an effective CSA of 1/3 and as I said above the internal abngel is a likely source of a split. Take a look at the pic showing the floor joist going in on this timber frame and note how the bottom of the joist gently curves down to full depth to avoid the stress raiser.

If the two sketches were the othe way up they would give full capacity for the joists and remove timber from nearer the center of the beam where it has the least effect.

Jason

 

[andy king]

Ah, it might be because I'm assuming the joists will be checked into a beam in a similar way at the other end? The skethces show the ends of the joists to go into the beam you show, so the housing needs to be altered to accomodate it. If the beams aren't checked into a beam at both ends, (built into a wall maybe) then they can drop and slip, but trapped between two beams they should continually tighten as weight goes onto them.
I wouldn't use the first option to be honest, the second one, or a similar version of it with a dovetaled housing rather than a simple square one, I used in a boat project for the deck beams and hatch combing openings.
Does this sketch explain it better?

Off out with the wife and kids now, so if not, i'll try and explain tomorrow!

cheers,
Andy

 

[OLD]

You seem to have settled on method A as its easy to do, remember that they did not understand lines of stress years ago and the floor timbers were thick .You could always fit a timber as a ledge under the joists fixed to the beam this could be done later and only if required.There are lots of black and white houses in Nantwich and some have this arrangement .

 

[dirtydeeds]

old

i dont agree with your assessment of understanding lines of stress by earlier generations (or by implication accuracy of work)

they were just as able to set out square buildings and square joints as we can, there is NOTHING in my tool belt (apart from my mobile) that our fore fathers in medieval times didnt have, mine are just modern copies

yes somethings might have got better. our tape measures might be set out in millimeters, or 16ths of an inch but regularly spaced knots in a piece of string are just as accurate if you can read them.

you can halve the distance between each knot by (just like imperial units)

at first sight (to our modern eyes) medieval joists in vernacular buildings are set out chaoticly at totally irregular centres

this is completly wrong MODERN thinking (modern as in the last 200 years or so)

look again. the centre lines of medieval joists are as accurate as you and i can set them out

what most people these days cant past is that the joists are (aparantly) laid flat and they arnt straight (because they were just cut from the tree) and they only bothered to make only two edges paralell (top and bottom)

this hides the truth to modern eyes

the joists are wide and wavy BUT the centres and shapes of the BEAM shape are exactly the same as ours

deep webs and narrow widths

what they didnt worry about was spending extra time and skilled labour to produce nice square 2x8 joists. why bother

 

[jasonB]

DD I don't think Old is saying that they couldn't do accurate work.

What he means is that they could not calculate the bending moment and plot the line of stress through the bearing part of the joist. BUT they knew from years of experience what type and size of joint worked and what failed and what size of timber to use. BTW most timbers in a framed building of this type are sized to accomodate the amount of joints and are far larger than they need to be to carry the loads.

Its 25yrs since I last did calculations for steel,concrete and to a lesser extent timber structural members but I know that some of the joints being suggested put cuts right accross the line where the shear loads are at their highest on the joists.

The talk of dovetailed housings is getting a bit far from the original question of a quick to cut yet strong joint.

JAson

 

[RogerS]

Thanks guys for all the advice and suggestions.

As Old says, we opted for A for the central beam...as that was what was there originally. Hindsight is wonderful as it's only when I started looking at other beams in the property that I saw B. B also has the advantage that as the oak shrinks, you won't see any difference from below. You will see a gap around the shoulders in A (that's assuming that they were cut tight to begin with :wink: ).

There are still the joints left to make in the other new beam and B is a becoming more attractive.