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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.
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
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.
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
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
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.:rolleyes:Quote:
Originally Posted by ian
Mick
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
Quote:
Originally Posted by ian
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. :doh: Could you please explain for someone who struggled with maths in high school. Thanks,
Mick
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>http://upload.wikimedia.org/math/8/c...b3e3297434.png </DD></DL>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.
- b = width (x-dimension),
- h = height (y-dimension) (thanks to Wikipedia for the formula)
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
Umm, I sorta understand but I might come back to it when I'm a bit more awake.:doh: Thanks,
Mick
Correct me if I am wrong, but dont the Bressumer beams in the HYspan software relate to my exact application ?Quote:
Originally Posted by journeyman Mick
G'day.
Go to www.timber.net.au and download the span table software and find out for yourself.
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.Quote:
Originally Posted by blak
Mick
Whereabouts? I had a look but couldn't find them.:?Quote:
Originally Posted by glock40sw
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
Sorry.
Wrong timber site :doh::doh::doh:.
try this
http://www.timber.org.au/menu.asp?se....02%20Software
Thanks glock,
That'll come in handy.
Cheers,
Dr - 307.
That is all well and good but the first line in your post is incorrect. Beam Stiffness is what you are referring to and it is a function of the I and the E (youngs modulus) which is a measure of the material. For steel E is 200000MPa; Hardwoods between 18500 and 14000 and Softwoods less.Quote:
Originally Posted by ian
EI is the stiffness and it determines deflection resistance.
Strength of beams is actually related to the second power of depth but also to the restaints and the grade.
Someone!!! Quick!!! Give me Alan Staines book!!! :D
Now I know what just flew over my head. :D
Doc.
Please do not assume that beam stiffness will always govern design.
DvdHntr
I think we should be careful not to turn this into a too technical discussion.
These forums generally talk about residential construction where beams typically span windows and door openings, or substitute for the removal of a load bearing wall.
in these cases the "visual" aspects of the design usually govern — get the deflection "wrong" and the window / door wont close properly, or the ceiling line will have a noticeable "woof" in it.
Rarely do questions relate to the load bearing capacity of a timber beam
ian