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Notching Bearers into posts

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  1. #1
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    Question Notching Bearers into posts

    Hi, I'm a fitter by trade, so I'm new to working with timber and I'm building my first deck. It's 3600 x 4800 and only 300 off the ground and it will be under an existing pergola. I'm using 190 x 45 bearers with 190 x 45 joists hanging between the bearers on joist hangers.

    Because all the timber I bought are 3600 long I'll need to join the bearers. I thought that notching the bearers into the posts would give these joins greater strength. My posts are 90 x 90, so I'm wondering how much I can notch into the posts. Is 40mm too much? Also, what type of joint is best, a scarf joint, lap joint or a simple butt joint? I'd really appreciate your help.

  2. #2
    Senior Member brynk's Avatar
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    gday battyman & welcome -

    what type of timber are you using? at what spacings are your joists and bearers? how are you fixing your posts into the ground?

    the timber-framing-code makes no provision for 45mm wide bearers but don't be discouraged. it just means you need to get the engineer's certificate for your development approval

    r's brynk
    "Man got the opposable thumb - woman got four opposable fingers." - Rowdy

  3. #3
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    Quote Originally Posted by brynk View Post
    gday battyman & welcome -

    what type of timber are you using? at what spacings are your joists and bearers? how are you fixing your posts into the ground?

    the timber-framing-code makes no provision for 45mm wide bearers but don't be discouraged. it just means you need to get the engineer's certificate for your development approval

    r's brynk
    Thanks brynk.

    Excuse my ignorance re all things timber , but all I know is it's treated pine CCA-DAR (I bought it at an auction because I'm a fiscally challenged uni student atm). My bearers are 4800 long (as my timber is only 3600 long, I'm looking to join two pieces together housed in a notched post). They are being supported by 90 x 90 posts every 1600. I have 3 bearers, spaced 1800 apart (1 each side and 1 in the midde). My joists will hang between the bearers with joist hangers and are spaced at 450. My decking timber is 85 x 19 karri. I was planning to fix the stumps into a 300x300x500 footing. I've tried to ensure that it's over engineered... Should any of this pose any problems for the engineer?

    Steve

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    Hi battyman

    I too am interested in the answer to this one, particularly how to "join" the bearers.

    Re the notches in the post - the Allan Staines book everyone refers to only shows a minimum of 45mm (from memory as I don't have the book in front of me now) of timber left on the post after notching. However it makes no reference to the size of the post in the first place! I always viewed notching the post as a means of providing support for the bearer and removing load from the bolts. This could be done with a notch as small as 10mm. On my deck recently completed I notched 20 - 25mm on a 90mm post. It would be interesting to hear from someone with a structural background how notching a post affects it's strength. (Can you comment brynk?)

    Re the posts and footings - the favoured method is to use galvanised post stirrups embedded in the concrete footing with the post bolted to the stirrup. (eg http://www.pryda.com.au/catalog.php?...Post%20Anchors)
    This alleviates termite problems, and there has been some discussion about timber swelling in the wet concrete in the footing, then shrinking as the concrete dries leaving the post slightly loose.

    Re joining the bearers - nothing from me aprt from these ramblings!
    Scarf joint would take a bit of work to taper the end of the timber and is probably "finer" in detail than anyone would use on deck construction.
    Butt joint - the easiest but you only end up with half of the 90mm post to support the bearer and trying to fit two 12mm bolts through 45mm would be too hard.
    Lap joint - which type? You could cut the bearer half thickness for the end 90mm and lap the two bearers together. You could then easily fit two bolts through both bearers at the same time. Or instead of half thickness you could cut the end in an "L" shape. ie half width and match the two bearers together that way. You would then need to use a total of 4 bolts however on a bearer total width of 190mm they would fit easily.
    The "L" shape joint would also be easy to cut - you could do it all with the circular saw. The half thickness joint however would require chiselling or routing and more time.

    FWIW I used both lap join methods. I also found the quickest and most accurate way of notching the posts and doing the half thickness lap joint was to run multiple cuts across the timber with the circular saw set to the correct depth. Most people would then use a chisel to cut out the rest, however I found that I could hit the timber with the hammer and break off sections, then quickly run over the top of the remaining timber with the router for a nice smooth finish.

    Or being a fitter by trade you may be able to make your own brackets for joining the bearers and mounting them to the post in the one go!

    Cheers and good luck,
    Loki

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    Hi Loki,

    Thanks for the info. I might have to get a copy of this Alan Staines book because now that I have the building bug I hope to make this deck the first project of many. I'll post photo's as I progress.

    Steve

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    Senior Member brynk's Avatar
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    my apologies - i should clarify before somebody jumps on me! the timber framing code makes no provision for a *single* bearer of 45mm's wide - indeed, bearers of treated structural pine are generally doubled (one on either side of the post).

    anyway, as for joining bearers, here are some options - there is no one best way - it depends on your situtation & what you have available, what materials you have allowed for, etcet - please bear with me as there is a bit of physics in here but i promise to keep it confined to the keyboard

    joining mid-span

    the code allows for joining mid-span (not over a support: stump, post, etcet) as long as the resultant join is as strong as the material being joined. there are nailing (aka gang-nail) plates for this purpose - this nail plate is a gal-steel piece with machine-perforations into the steel that are shaped like nails; you lie it flat over both pieces and hammer these 'nails' into the material - the larger the contact area the stronger the join.

    or if you have some angle lying around (say, 75x50x6 unequal angle) you can do a butt-joint with your bearers that doesn't look too shabby & will provide sufficient strength. put the short edge of the angle on the underside of the bearers and the long edge up the tall axis. stagger the bolts by at least a bolt-hole's diameter so as not to propogate a split out to the end of the bearers. make it long enough so that the first bolt comes is greater than 50mm from the end of the bearer. (see picture 5).

    you can take advantage of the fact that your bearers can cantilever beyond the posts themselves towards the outside edges (see picture 1) - this will allow you to close your post-spacing up slightly and cut the span down on the bearer. a canti can generally be no more than 30% of the overall back-span. the strongest spot to join simple beams is around 10-15% of the overall span inwards from the support. this is roughly the point of maximum shear and minimum bending. joining mid-span is the least-economical way in a simple beam (ie, one without any point-loads).

    having said this, in your instance the cantilever of say 300 will put the join between the 3600 and 1200 lengths at around mid-span between the posts. you will need to make your join stronger than it otherwise would've needed to be. to do this you need to increase the contact area of the join by another 100% to accomodate the bending forces as well as the shear forces. to envisage this, try snapping a pencil by holding it in your two hands with thumbs & forefingers of each hand touching - now push one hand away and pull the other hand towards 'tearing' the pencil in half - this is shear. now snap the pencil by pulling back with your wrists and pushing outward with your thumbs... which is the easier way to break the pencil? if you had to reinforce or repair the pencil so it could resist these forces, which would require less material?


    joining over a support

    if you can increase your number of posts by 1 on each bearer so they are spaced 1200 this will put a post right under your join on the bearers. this will be more structurally sufficient & may leave you with 4 additional lengths that you otherwise would've had to cut (if you aren't doing a join mid-span). the extra cost will be in 3 more holes & posts. see picture 2 & 3 for some joining options here where you can join over a stump or post.

    your best option would notching the post as loki describes so the ends of the bearer rest on the post, and nailing or bolting through the remainder of the post to ensure they don't fall off. remember that once your joists are attached to the bearers then the bearers will no longer move in that axis, and once your decking is attached to your joists then the bearers will be restrained in that axis also.

    number 3 looks like a doozy. in areas of high-uplift you would need to look at that one but because your deck is only 500 off the ground you dont need to worry about that. i just chucked it in to give you a heart murmur ...

    seriously though, the only other consideration is that you should not cut your joists if you can avoid it. make the middle bearer lower than the outer 2. this will make your joists stronger overall.

    regards
    brynk
    Attached Thumbnails Attached Thumbnails bearer-canti.jpg   bearer-join-over-post-1.jpg   bearer-timber-post.jpg   bearer-join-over-post-2.jpg   bearer-bearer-angle.jpg  

    "Man got the opposable thumb - woman got four opposable fingers." - Rowdy

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    hi brynk,

    Wow, thanks. That's some pretty good advice. I'd like to be able to avoid cutting my joists but my deck is only 300 of the ground, so I only have 100 clearance... is there any way I can avoid cutting my joists with such a low clearance?

    When I saw pic 3, I almost went back to metalwork (oops sorry for swearing on this forum ).

    Steve

  8. #8
    Senior Member brynk's Avatar
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    gday again

    is there a way to avoid cutting the joists with such a low clearance?
    in a word - no; but as usual there is more than one way to deprive our feline friends of their outer covering...

    is the deck in a position where you can excavate an area where the centre bearer is, say to a metre either side, to allow for the depth of the bearer to sit under the joist? if you can take advantage of your joist spanning the full width of the deck then you will cut down on costs of using & fixing joist hangers X 2.

    some considerations to do this - is the deck in a position where it receives run-off from the yard? if yes, you must divert this runoff around the deck. if you were to dig this trench, you would need to actually extend it beyond the deck a little and put a bit of a slope on it so that it drains properly and doesnt become a sump-pit for the under-deck area. you might also have to line it with a bed of gravel say 50mm deep to keep the soil where you put it.

    what are you studying at uni?
    "Man got the opposable thumb - woman got four opposable fingers." - Rowdy

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    Quote Originally Posted by brynk View Post
    gday again

    is there a way to avoid cutting the joists with such a low clearance?
    in a word - no; but as usual there is more than one way to deprive our feline friends of their outer covering...

    is the deck in a position where you can excavate an area where the centre bearer is, say to a metre either side, to allow for the depth of the bearer to sit under the joist? if you can take advantage of your joist spanning the full width of the deck then you will cut down on costs of using & fixing joist hangers X 2.

    some considerations to do this - is the deck in a position where it receives run-off from the yard? if yes, you must divert this runoff around the deck. if you were to dig this trench, you would need to actually extend it beyond the deck a little and put a bit of a slope on it so that it drains properly and doesnt become a sump-pit for the under-deck area. you might also have to line it with a bed of gravel say 50mm deep to keep the soil where you put it.

    what are you studying at uni?
    hi brynk,

    I don't have a problem with water run-off. Because I am ripping up existing pavers to lay the deck I have a ready made bed of gravel and sand, nicely compacted and with a slight slope to allow for runoff.

    I have already had to excavate the area when I laid the pavers 3 years ago, so I have a small retaining wall at the end of the paved area, so I'm worried that if I excavated around the centre bearer I run the risk of creating a sump-pit.

    Also the ground is REALLY hard (I excavated the entire area by hand and shudder at the thought of more digging ).

    I'm about 2/3 the way through a theology degree, and became an ordained minister this year. Big career change. I used to work at a steelworks for BHP Steel, and then I worked at Holdens as a fitter in the Stamping Plant and now I'm a minister... I didn't really see that one coming. But I love what I'm doing and it leaves me more time for projects like this (after all, ministers only work on Sundays right? ).

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