Thursday, April 12, 2012

L / 16 ... continued ...

 .. .your word `constant' is too strong.

it is more of a `starting place'.

in all cases we are looking at `Live' load and a corresponding Live load deflection of L/360 ...

all loads are uniform, and the beam is simply supported.

so, for steel, 50 ksi ... 50/800 x Span = Span / 16

for concrete... check out ACI Table 9.5(a) simply supported beams (lower left hand corner of table) ... L / 16

for wood, take yourself, for the moment, out of the glulam world in general, to wood with a bit less stiffness ...

e.g., go to the AWC max span calculator, put in DFL No. 2, Live load, say, 50 psf, dead 10, Limit L/360, ... not wet, not hot, etc., etc.

try 2 x 12 (1 ft deep)

press the max span button ...

and, whoa! max span is ... drum roll SIXTEEN feet ...

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Douglas+Fir-Larch&size=2x12&grade=No.+2&member=Floor+Joists&deflectionlimit=L%2F360&spacing=16&wet=No&incised=No&liveload=50&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span

play around with it a bit!

...

and then embrace the idea of ...

depth for deflection

more width for wood for more strength (or greater spacing)

more steel for steel (beam or flange steel for W shapes)

more reinforcing steel for reinforced concrete

check shear ...

...

these spawn from the more common properties of our more common materials ...

wood ... E = 1.5-ish million

steel ... E = 29 million and Fy = 50 ksi

concrete = E = 3 million ish

...

See what we miss when we use computers!!!

At 07:46 AM 4/12/2012, you wrote:
What assumptions are you using for your wood and concrete beams? Unless
I am missing something, the depth is a function of the load, E, and
width in addition to the deflection limit. I don't see how you are
reaching a constant depth of L/16.

Linville

1 comment:

Pancho said...

John says: yeah, I admit. I would have never seen this by using my computer software. but I will still use my computer :)

Jeff says: that's why I am the `Doc' ... and you are the `P.E.'