Hardwood equivalents for HYspan beams

[blak]

Guys

I have taken out a small load bearing wall and am putting in a beam which will span 2750mm.

The HYspan span tables for my application indicate I should be using either a 170x36 or 150x45 beam.

Is there a way of figuring out a hard wood equivalent beam size? I have a good piece of hardwood which is 95x75 and will use this, rather then buy a HYspan beam, if it is up to the task.

Thanks in advance.

[journeyman Mick]

If you need a 170x36 or 150x45 in a hyspan beam I'm fairly certain that a 95 x 75 isn't going to suffice. What is the beam going to support? I could look it up in some span tables but it will depend on what sort of load is on it: roof, tiled or sheet, half span thereof, upper storey floors?

Mick

[blak]

Thanks for the reply.
The house is single storey.

The 2750mm beam goes across a widened walk way between the lounge room and the sun room.
The beam will support a ceiling and schilion gal iron roof on one side (sun room)with rafters 1700mm long and support a ceiling and schilion gal iron roof (lounge room) on the other side with rafters 3500mm long.

I have used the HYspan span table document, under roof beams on page 26 to come up my the spec of the beam I think I require.

[ian]

blak

a 150x45 hyspan is advertised as being equivalent to 200x50 F7 Oregon

unless your hardwood is strength graded I wouldn't use it.


ian

[journeyman Mick]

The tables don't really cover a situation where a beam is carrying two loads, but just as an illustration if the beam spanned only 2700 and carried a sheet roof with a 3500 span it would require a 250 x 100 @ F7, or 225 x 100/250 x 75 @ F8, or 200 x 75/250 x 50 @ F11, or 175 x 75/225 x 50 @ F14, or 175 x 75/200 x 50 @ F17. Even if your lump of timber went F27 it would need to be 140 x 70 or 190 x 35 and that's before you take into account the fact that it's 50mm over the span and should really jump up to the next table and that it's carrying a second skillion roof load.

Mick

[journeyman Mick]

blak

a 150x45 hyspan is advertised as being equivalent to 200x50 F7 Oregon

unless your hardwood is strength graded I wouldn't use it.


ian

If that's the case I don't think you're reading the Hyspan tables correctly Blak. Mind you the tables I'm reading are for cyclonic areas and account for more uplift than you would require, but then there's the small matter of the second roof load.

Mick

[ian]

Mick

I was reading Hyspan's 4 page "why we're wonderful" blurb were they provide that example.
I must admit that my "gut feel" was that a 4 x 3, even if hardwood, was nowhere near strong enough.
Your span tables just confim it.
from memory, all other factors being equal, beam strength increases at the (aprox) third power of the beam depth


ian

[journeyman Mick]

..............from memory, all other factors being equal, beam strength increases at the (aprox) third power of the beam depth.............

Ian,
sounds like the sort of thing I should file away mentally for future reference if I knew what "third power of the beam depth" meant. Could you please explain for someone who struggled with maths in high school. Thanks,

Mick

[ian]

Mick
at the risk of getting us both into deep trouble, the strength of a beam is related to a property called the Second Moment of Area (often refered to as the Moment of Inertia)

For a rectangular beam the second moment of area is calculated as
<DL><DD> </DD></DL>

• b = width (x-dimension),
• h = height (y-dimension) (thanks to Wikipedia for the formula)

this value along with the strength of the material used for the beam is used to work out how much deflection (bending) a beam will experience when loaded.
I'd have to go back to the books to find the exact formulas used but a bit of number crunching with "standard" sections and assumed "normal" loads then gives you the span tables we all look up.


but back to your question the I(x) value for a
225 x 100 is (100 x 225 x 225 x 225 / 12 =) 94,921,875 which would usually be rounded to 94.9 million
250 x 75 is (75 x 250 x 250 x 250 /12 =) 97,656,250 which would be rounded to 97.6 million

94.9 and 97.6 are near enough the same, so the span tables consider them to be equivalent sections


ian

[journeyman Mick]

Umm, I sorta understand but I might come back to it when I'm a bit more awake. Thanks,

Mick

[blak]

The tables don't really cover a situation where a beam is carrying two loads

Correct me if I am wrong, but dont the Bressumer beams in the HYspan software relate to my exact application ?

[glock40sw]

G'day.
Go to www.timber.net.au and download the span table software and find out for yourself.

[journeyman Mick]

Correct me if I am wrong, but dont the Bressumer beams in the HYspan software relate to my exact application ?

I don't have the Hyspan software so can't tell you one way or the other, but it seems pretty unlikely that a table will have allownace for two sets of loads on the one beam. If it's actual software where you enter all the variables it might do. However given that my tables show a 250 x 100 beam at F7 for a 2700 span it wouldn't suprise me if you've perhaps read it wrong.

Mick

[journeyman Mick]

G'day.
Go to www.timber.net.au and download the span table software and find out for yourself.

Whereabouts? I had a look but couldn't find them.

Mick

[journeyman Mick]

Okay, I found the HYspan tables and it appears you are reading them correctly. If you go to 41Ms wind speed though the beam size jumps up to 200 x 63 which puts it more in line with the tables I was using. But anyway, to cut a long story short, there's no way that your 100 x 75 is going to be adequate, especially as you have no way of knowing its F rating.

Mick