attn: question for maggot builders

[train07]

Huh? Do you even know what the difference between bending stress and shear stress is?

yeah. and I know gimpy is stressing out needlessly.

[Below Zero]

yeah. and I know gimpy is stressing out needlessly.

train07, why are you so curt with me? Everytime I try to have a conversation with you, you are a dick to me. In this thread too, when I was just trying to ask questions: http://tetongravity.com/forums/showthread.php?t=83668.

[Atlas Q. Powmonger]

tech talk!

[train07]

Ok you've made me laugh. I'm sorry. It won't happen again.

[Chemical Ali' The Wax Master]

The manufacturer is usually pretty cool with questions and specs on their products. if you are happy, they are happy

[Below Zero]

Ok you've made me laugh. I'm sorry. It won't happen again.

Cool! Maybe you just hate Architects!

Can't we all just get along!

[Chemical Ali' The Wax Master]

I'm never happy

[DougW]

Nerd Homework Review

Given:

-A wood beam spans 20'-0" with a 400#/ft uniformly distributed load for 10'-0" on the left side of the beam, and a 600#/ft uniformaly distributed load for 10'-0" on the right side of the beam.
-Allowable deflections: Total - L/240, Live Load - L/360

Find:

-Draw Shear and Moment Diagram
-Find Point of Inflection (point of maximum moment)
-Select most economical floor beam of douglas fir-larch No. 2

Formulas:

Fv = 3/2 x Vmax/A
Deflection = 5WL^4/384EI
S = M/Fb

OK........go!

me me .. . well my program couldn't find it for 600lb per sq foot so I think you put in a typo and meant 60 psf and 40 psf over 10 foot widths.

Got a 16 x 16 D Fir - Larch SS, assumed all dead load. I think steel would make more sense.

[runethechamp]

me me .. . well my program couldn't find it for 600lb per sq foot so I think you put in a typo and meant 60 psf and 40 psf over 10 foot widths.

Got a 16 x 16 D Fir - Larch SS, assumed all dead load. I think steel would make more sense.

Nono, not psf, pounds per foot distributed load.

[RootSkier]

Got a 16 x 16 D Fir - Larch SS, assumed all dead load. I think steel would make more sense.

A 16x16 just looks so badass, though.

[runethechamp]

Since I don't have the UBC here, max moment is about 29,792 ft-lbs, at 10'-10" from the left end (assuming the beam is suimply supported, which BZ's deflection formula says it is).

[Below Zero]

me me .. . well my program couldn't find it for 600lb per sq foot so I think you put in a typo and meant 60 psf and 40 psf over 10 foot widths.

Got a 16 x 16 D Fir - Larch SS, assumed all dead load. I think steel would make more sense.

-10 for not providing the shear and moment diagrams.
-10 for using a computer
-5 for noting a Select Structurual Beam when the homework called for "the most economical Douglas Fir Larch No. 2".
-5 for incorrect load values.
-5 for not showing your calcs for Section Modulus, Horizontal Shear, and Deflection.
+10 for effort!

Sorry, DougW but I'm going to have to give you an D+ on this homework assignment.

Since I don't have the UBC here, max moment is about 29,792 ft-lbs, at 10'-10" from the left end (assuming the beam is suimply supported, which BZ's deflection formula says it is).

Yes, sorry, the beam is simply supported. Also I made a mistake when I said find the point of inflection. The point of inflection is the point of zero moment, not point of maximum moment. I meant to say find the point of zero shear.

The right reaction is 4500#. The left reaction is 5500#. This puts the point of zero shear at 9.16' from the right reaction (or 10.84' for the left reaction).

So rune's 10'-10" is correct, however, I calcuate the maximum moment as 25,217 foot-pounds. This puts the section modulus at 345.8 inches to the third about the X axis. Lek's try a 6 x 20 beam douglas fir larch No. 2. The for a 6 x 20 is 70.89 which is less that 85 allowable, so that is OK.

So now let's check deflection. Our allowable deflection for total load is 1" and our allowable deflection for live load is 0.67". Using our formula, we compute that the deflection for a 6 x 20 with the given loads at 20'-0" will be approx. 0.33" 0.33" is less than 1" and 0.67", so that is OK.

(Note: Cf was not used, although it should be. The Cf factor (size factor) would put the section modulus over the 6 x 20 allowed 348.6. Allowabe shear stress and deflection still works out. Let's go with a 6 x 22)

6 x 22 Douglas Fir Larch No. 2

[RootSkier]

6 x 22 Douglas Fir Larch No. 2

Finding a decent quality 6x22 will be tough, and you will pay a lot for it. More troubling is how much that thing will shrink and move across the 22". Is 3/4 inch of settling OK with you, or are you going to spec a reclaimed timber?

Even a microwave kiln dried 6x22 is going to move a fair bit.

This is all from a timberframer's perspective, I am definitely not an enginerd.

[runethechamp]

Yes, sorry, the beam is simply supported. Also I made a mistake when I said find the point of inflection. The point of inflection is the point of zero moment, not point of maximum moment. I meant to say find the point of zero shear.

The right reaction is 4500#. The left reaction is 5500#. This puts the point of zero shear at 9.16' from the right reaction (or 10.84' for the left reaction).

So rune's 10'-10" is correct, however, I calcuate the maximum moment as 25,217 foot-pounds. This puts the section modulus at 345.8 inches to the third about the X axis. Lek's try a 6 x 20 beam douglas fir larch No. 2. The for a 6 x 20 is 70.89 which is less that 85 allowable, so that is OK.

, missed on the formula for the second part of the shear diagram (that was integrated to get the moment). I do however get 25208 ft-lbs; not that it matters anyway

[Below Zero]

Finding a decent quality 6x22 will be tough, and you will pay a lot for it.

Yes, I see your point. Of course this little "homework assignment" was strictly theoretical. If it was the real world, you would of course know where the beam was being installed and what the availabity and cost of timber is for your area. In this case you might go with a glu-lam beam if you are looking for dimensional stability. Maybe you go with a built up beam or a flitch beam. Heck, steel might even be a better choice, depending on it's location and the project. Also, you might go with a different commercial grade, like a douglas fir select structural, where you might be able to go with a 6 x 14 or an 8 x 12 assuming these work for deflection.

[Below Zero]

, missed on the formula for the second part of the shear diagram (that was integrated to get the moment). I do however get 25208 ft-lbs; not that it matters anyway

Yeah, close enough!

OK, now let's find out what the most economical steel wide flange beam is for the same problem/ Everyone pull out their AISC books!

[boarddad]

6x22 DFL??

Ever hear of engineered lumber? I'd go with a glu-lam only because a beam like this is usually exposed (as well it should be). 2000 psi F'b is pretty common these days but I've spec'd them @ 3000 psi F'b on some projects (Superbeams out of Canada).

[Below Zero]

6x22 DFL?? Ever hear of engineered lumber?

Dude, it's theoretical. See my above post in response to RootSkier.

[boarddad]

Theoretical or not, don't forget your size factor (in this case it knocks your allowable fb down about 7&#37.

Edit:
Damn! Just noticed your mentioned that.

Boy, my pocket protector is really getting a workout today!