Timber size to avoid sag

[havabeer69]

Hi,

Not sure if this should be in the home reno section below but...

Is there a way to calculate timber size to avoid sag, waiting to put some hard wood (blackbutt) arbor/pergold beams up that will go over a span of 3.5m

These will not be structural at all purely decorative but i don't want to come out in a few years and see a 4 bowed timbers. Engineering tables will obviously give me what I want to degree but are also over kill as there will be no load on them so trying to work out the sweet spot between the two. Ideally I'd like to just put up a 115x115 post of timber but again have no idea how to know if it will sag over time.

[r3nov8or]

AS1684 is effectively the minimum safe standard for timber framed structures, so I would suggest just going with that. Haven't looked for a long time, so can't comment on whether your 115x115 is suitable, and perhaps use the tablets related to verandahs

[mic-d]

My gut feeling is that 115x115 will sag due to the weight. If you were to treat your members functionally as similar to rafters then 150x38 f11 or 125x50 f14 should be ok.

edit. I pulled these dimensions from an old pergola span table by Allan Staines. Also you should know that seasoned timber is rarely straight, if you can pick 4 with a small spring and mount with the spring up that will go a long way to help

[Bushmiller]

HAB

A little difficult to assess as 115mm is not a standard size so the span tables tend not to give exact information, plus, as you say, there is no load. really you are looking at whether the beam will sag under it's own weight. I'm sure you realise that timber gains strength from it's depth rather than the width. Other factors are whether the timber is green or seasoned.

Blcakbutt is in the second highest strength category and would range from F11 to F22 if green and F17 to F34 if seasoned. However, I would work with the lowest strength grouping. That still doesn't answer your question. Somebody may have some information on this, but if not i would suggest you place the timber across that span sat up a little and place some weight in the middle. See if there is any noticeable deflection. I would start with about 50Kgs, which will be about the weight of the beams and then load it up from there. Perhaps use the other beams to do this.

Not very scientific.

Regards
Paul

[Avondale]

115² sounds a very strong post.
When I added to my previous house in 1987 I used 100² clear Oregon for 2 corner posts.
One supports the corner of a 3.5 wide carport, the other the verandah corner at front door.
They're still standing
The old Blackwood verandah posts along back were 100²
I replaced the corner one with Oregon.

As to your beams, can you get them and air dry them then when up prop them for a couple of months in middle?
I was advised to do that with a massive centre beam (truss) Imade from green hardwood supporting the ridge of my workshop
It was 900 deep, 7 metres long and made like a truss
Once the beams are exposed all round they will dry quickly but can't sag under their own weight if unsupported.

Only issue anyone would have to face is if any of this becomes structural. Putting on laserlight for egs.

You'd want mature, knot free timber whatever you use to avoid too much movement

[Lappa]

Have you tried this.

The Sagulator – WoodBin (https://woodbin.com/calcs/sagulator/)

I know it’s for shelves but I fed your dimensions into it and it came up with a sag figure.

[havabeer69]

yeah it is a hard one, especially being the non standard sizes.

the "bridge" style expiriment suggested by bush miller sounds ok but its hard to order the timber and then try it and if it doesn't work work I'm sure old mate isn't going to be happy with just taking the timber back. the other hard part is the ends will be supported/locked in place which also helps to avoid the flex.


good suggestion by all to get some with a small amount of "hog" in them, or at least point the sag upwards.

[Bushmiller]

yeah it is a hard one, especially being the non standard sizes.

the "bridge" style expiriment suggested by bush miller sounds ok but its hard to order the timber and then try it and if it doesn't work work I'm sure old mate isn't going to be happy with just taking the timber back. the other hard part is the ends will be supported/locked in place which also helps to avoid the flex.


good suggestion by all to get some with a small amount of "hog" in them, or at least point the sag upwards.

I didn't realise you don't have the timber already. A way around the sag would be to rout a trench the length of the beam on the underside and insert a steel strap. Say 25mm x 5mm. Screw it at regular intervals and that will resist sag as the strap will be under tension. Paint it to disguise it or use as a feature.

Alternatively, choose a section that is deeper such as 150mm x 75mm for example.

Regards
Paul

[havabeer69]

I didn't realise you don't have the timber already. A way around the sag would be to rout a trench the length of the beam on the underside and insert a steel strap. Say 25mm x 5mm. Screw it at regular intervals and that will resist sag as the strap will be under tension. Paint it to disguise it or use as a feature.

Alternatively, choose a section that is deeper such as 150mm x 75mm for example.

Regards
Paul

Sounds like the strap could make a nice feature actually.

[ian]

Given that the "beam" will only be loaded with its own weight, an option would be to plane the potential sag out of the beam by inducing a reverse spring -- is that really a term?

a straight beam will look to have a slight sag -- the potential solution is to plane a slight spring (2 to 3mm) into the beam's underside so that it looks straight. Unless someone measures the beam's dimensions, no one will notice that a 115 deep beam has a small amount of material is missing from its underside.

[Mountain Ash]

Hi HAB69. I am pretty sure it will sag. I would expect a 90 x 45 piece of graded pine to sag over 3.5M, which is much lighter and dried out. Something approaching 240mm in height would be fine but that is double your dimension.

[DTH1]

Hi, this is the amount of sag for Kiln Dried Hardwood

Timber Deflection Self Weight – 3.500m Span.
F17KDHW 115x115 =2.11mm
F27KDHW 115x115 =1.88mm

If you intend to use green timber - fresh from the sawmill, design your fixings so in 6 months you can rotate the timber 180° so the sag is now a bow.
Then 12 months later, check to see if the bow has leveled out, or sagged down. If so, rotate the timber again to place the bow up. Once dried, it should be OK.

[Mountain Ash]

Hi DTH1. Where did you get your info from? That could come in handy. And a 2mm bow over 3.5M would be hardly noticeable.

[DTH1]

Hi DTH1. Where did you get your info from? That could come in handy. And a 2mm bow over 3.5M would be hardly noticeable.

Hi Mountain Ash,
It's a proprietary programme we use in the Truss and Frame Industry. It also gives the option for I-Joists, Steel PFCs, Universal Beams. The only thing it doesn't cover is Green Timber.

[havabeer69]

Hi, this is the amount of sag for Kiln Dried Hardwood

Timber Deflection Self Weight – 3.500m Span.
F17KDHW 115x115 =2.11mm
F27KDHW 115x115 =1.88mm

If you intend to use green timber - fresh from the sawmill, design your fixings so in 6 months you can rotate the timber 180° so the sag is now a bow.
Then 12 months later, check to see if the bow has leveled out, or sagged down. If so, rotate the timber again to place the bow up. Once dried, it should be OK.

Thanks for that... now to find straight lengths

[Mountain Ash]

Hi Mountain Ash,
It's a proprietary programme we use in the Truss and Frame Industry. It also gives the option for I-Joists, Steel PFCs, Universal Beams. The only thing it doesn't cover is Green Timber.

Aah! I use something similar from a local LVL manufacturer but I can't customise dimensions. It sticks with standard framing sizes.