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How to calculate max load for a VLV beam? 
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#1
Jul809, 09:03 PM

P: 21

Hello,
I'm looking for a formula to determine the max load on a piece of Laminated Veneer Lumber (LVL) supported at each end. Although I've calculated the load on each component of bridges a thousand times way back in my Statics class, nobody ever showed us how to find the max allowable load given a specific material. I have the MOE and MOR values. Does anybody know any useful formulas? Thanks. 


#2
Jul909, 07:38 AM

P: 175

If you're looking for max allowable loads, you're probably looking for some kind of a published table from the manufacturer. Otherwise the best you can do is guess... laminated beams are composite and their strength depends not only on the strength of the individual materials but also on the interface between the materials (between each layer and the glue used to stick them together).
Kerry 


#3
Jul909, 08:31 AM

P: 63

This question is hard to understand. Can you post some picture with value you have no idea how to calculate? In example, easy formula like R>N/A, you can calculate N<R/A [Nmax load in kN, Asection area m^2, R max elements strenght in compresion in kN/m^2]. If you know R and A get the value neededN.



#4
Jul1909, 10:31 AM

P: 21

How to calculate max load for a VLV beam?
Sorry it took so long to get back...been kinda hectic here.
Anyways, I'm simply looking for the maximum load a beam can take before failure. The beam is horizontal and supported on each end, like a bridge. Let's call it a bridge and say I'm trying to figure out the heaviest truck that could go over this beam. Here is a PDF with values for the material I'm looking at: http://www.mccauseylumber.com/masterplankbroc050106.pdf Can I just convert the compression values to metric and solve for N or am I looking for the Max Moment, or even something else? Thanks again. 


#5
Jul1909, 12:14 PM

P: 63

I assume that the beam is loaded with point load wich could be placed anywhere on the beam (truck).
1) Max design moment M will be when the truck will be in the midle of the beam. M=qL/4 M<R*W  > Your max load for moment is q<(R_{b}*W)/(4/L) q design load kN W  design section modulus [w=bh^2)/6] hsection hight, bwidth R_{b}  design beam resistance in bending. (2700PSI from link) 2) Max shear force Q will be when the tuck will be near the support. So Q=q q*S/(I*b)<R_{v} Your max load for the shear force is q< R_{v}*(I*b)/S R_{v}  design beam resistance in ''chopping''. (320PSI from link) I design moment of inertia [I=bh^3/12] S staticmoment [(b*h/2)*h/4 if i remeber right] 3) Deformations... do not remember the formul for point load correct me if i am somewhere wrong 


#6
Jul2009, 12:41 AM

Sci Advisor
HW Helper
P: 2,124

(1) The equation in item 1 of post 5 is incorrect, and should instead be q < 0.6667*R_{b}*b*(h^2)/L, where q = applied midspan point load (N), h = crosssectional height (mm), b = crosssectional width (mm), L = beam length (mm), and R_{b} = allowable bending stress (MPa). From the link, R_{b} = 20.00 MPa.
(2) Item 2 of post 5 is correct, and simplifies to q < 0.6667*R_{v}*b*h, where q = applied point load (N), h = crosssectional height (mm), b = crosssectional width (mm), and R_{v} = allowable shear stress (MPa). From the link, R_{v} = 2.206 MPa. 


#7
Jul2009, 03:12 AM

P: 63

''The equation in item 1 of post 5 is incorrect, and should instead be q < 0.6667*Rb*b*(h^2)/L''
Agree. Should be q<Rb*W*4/L 


#8
Jul2009, 08:56 PM

P: 21

Hmmm....with equation 1 I get that a 30 ft beam (8" X 12'') can hold about 6200 lbs. I know LVL is strong, but that seems impossible with a span of 30 feet.



#9
Jul2109, 11:22 AM

P: 63




#10
Jul2109, 12:10 PM

P: 21




#11
Jul2109, 04:36 PM

Sci Advisor
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P: 2,124

giant016: No, the safety factor is already included in that allowable bending stress value, R_{b}. Your math in post 8 is correct. If the crosssectional dimensions you listed are accurate, the beam would hold the value you computed, if you provide one lateral brace at midspan. (Let us know if you are not familiar with what lateral support to the compression flange is.) If you do not want to provide the midspan lateral support, and you still want to use a span length of 9144 mm, then you must use R_{b} = 18.70 MPa, instead of 20.00. (Note that this reduced R_{b} value is not a general rule and only applies to your given crosssectional dimensions and span length.)
The rounding errors in numbers posted by archis in post 9 happen to be all additive, giving a 6.8 % cumulative error in his answer. 


#12
Jul2309, 03:31 PM

P: 21

The main beam is split into three sections with a total of 320 inches. The middle is 120 and the end sections are 100'' each. The support beams make a right angle. The design needs to be updated since I will be using 3 4X12 beams for the main support and also the right angle support beams will be shorter. Thanks for all the help so far. 


#13
Jul2309, 08:40 PM

Sci Advisor
HW Helper
P: 2,124

Just to clarify, in post 11, I was referring to the hypothetical, 9144 mm beam you commented on in posts 8 and 10.



#14
Jul2409, 05:05 AM

P: 63

Nice bridge design.



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