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I have seen mention of single span vs. continuous span in various books on building decks, specifically related to the length of the bearer. It says that a continuous span is where the "bearer has intermediate support" - can anybody explain specifically what this means. I am trying to determine what size bearer I need to span 4 metres - I hope to build a deck over a water tank (without touching the tank).
Regards,
knewey,
Single-span means you have a single timber member that is supported at two points only. Double span means it is supported at three and continuous means it is supported at four or more. So if you had a 6 metre bearer with a stump every metre, that would be continuous span.
You can make up longer spans with shorter members but you need to pay attention to the span rating. Three 2 metre members spanning two metres each (and therefore all three are single span) is not the same as a continuously-spanned single 6 metre member with the same stump spacings. Longer continuously-spanned members can generally span wider gaps than shorter single-spanned ones.
Thanks silentC,
I had a feeling it meant supported elsewhere along the beam where and how many times had me stumped.
I have the Decks and Pergolas book by Allan Staines which also says "Do not cripple continuous bearers" - do you know what this means?
Cheers,
I believe that is a cut part way through the bearer above where it sits on an intermediate stump supposed to stop movement. I guess if you did that, you'd be weakening the bearer's spanning ability because it is the tension of the timber on either side of the span that gives it the structural strength. Don't know if this is common practice any more, someone else may know.
BTW, you may need to go to steel to span 4 metres. I'm not sure if a hardwood beam can span any more than 3 metres, although there may be some laminated products that can.
I know the span may be an issue - I would prefer to use timber if possible but 4 metres is a fair distance.
I intend to place the tank and then build around it. The tank is actually 3.66 metres across - so by placing 4 posts around it I may be able to reduce the span to 2.7m - 2.8m. Early days yet - plenty to think about and plan.
Silent C, you're right, crippling is when you cut part way througha a member (and then reinforce it with blocks either side), usually in order to straighten it. I've seen it done with hardwood studs, but never on bearers or joists, although I suppose you could to remove excessive crown. If your timber is bowed enought to require this it's time to pick up the phone and hassle your timberyard (either that or you've left it out in the sun uncovered:mad: ).
Mick
Sounds like you're building a trampoline!
Recalculated:
F8 unseasoned hw 2x 250x40 laminated will give you a max span (single span) of 3400 mm (joist FLW max 2400 mm)
F17 seasoned hw tables you could laminate 2x 290x45 mm in single span and applying maximum joist FLW of 2400mm you could span 3800 mm.
F7 seasoned treated pine 2x 290x45 mm laminated single span max 2400 mm joist FLW could span 4100 mm.
F7 unseasoned oregon = 2x 300x38 mm to achieve 3900 mm span with same FLW as above in single span.
The FLW is taken as half the span between bearers for perimeter bearers. I hope this makes sense.
Also on single/continuous scenarios - another way of saying it:
A bearer supported at both ends and nothing in the middle is calculated as single span. A bearer supported at both ends and with one or more intermediate stumps is calculated as continuous span.