+0  
 
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avatar+251 

Hi there,

 

Having a bit of a problem with this one; hope you can help.

 

My lecturer did this problem in kN/mm while I like to use base units: N/m

 

We need to find dn.

 

Here is the lectuers final equation to solve for dn:

dn = 61.2mm

 

Note; the units for the values are as follows:

6.85 = kN

132 = unitless

40 = mm

465 = kN

dn = mm

 

Here's my final equation:

dn=0.06785m

 

 

I think the problem is lies with finding the term 6.85dn or 6852300dn

 

Here is the lecturers equation to find 6.85dn

Note; the units for the values are as follows:

0.802 = unitless

32 = MPa

300 = mm

0.89 = unitless

1000 = unitless

 

Where as my Cc is as follows:

 

 

 

Any ideas?

I've run it by the lecturer but they didn't know why either. They put it down to a rounding error.

 Mar 5, 2020
 #1
avatar+22043 
+2

How do you go from 6.85 dn   to   6852300 dn

    shouldn't it be    6852.3 dn ?

 Mar 5, 2020
 #2
avatar+251 
0

sorry, I wrote in the *10^-3 at the last second.

 

I've used 6852300 as Newtons

 Mar 5, 2020
 #3
avatar+22043 
+1

I am referring to your first equation.....it starts as 6.85 kn   which is   6852.3 N   not    6852300 N

 

Wait....now I see you converted dn to m   from mm which would make it   6852300.....

    looking some more....

 

Wait...again.....when you converted dn to m  you MULTIPLIED by 1000.....you should DIVIDE by 1000

 

Then I get   - 6.8523 d^2 + 464868 d + 5.28  = 0     resulting in dn = 67841.2

ElectricPavlov  Mar 5, 2020
edited by ElectricPavlov  Mar 5, 2020
edited by ElectricPavlov  Mar 5, 2020
edited by ElectricPavlov  Mar 5, 2020
 #4
avatar+251 
0

yeah I don't understand how it's 6.85 either. I was hoping someone could shed some light.

The third image shows how the 6.85dn was calculated

vest4R  Mar 5, 2020
 #5
avatar+22043 
+1

I think the problem is lies with finding the term 6.85dn or 6852300dn

 

6.8523  kn   dn      when dn is in mm

    when you convert to m   and N

       this just equals    6.8523 N  dn  

ElectricPavlov  Mar 5, 2020
 #6
avatar+251 
0

unfortunately dn cannot exceed 460mm in this problem

vest4R  Mar 5, 2020
 #8
avatar+22043 
+1

I don't think we are talking about the same things:

you have:

Here's my final equation:

465000 = 132/d (d- .04) + 6852300

 

This should be

465000 = 132 (d-.04) + 6.8523

 

solving for d n     = 67841.2    N   =   67.841 kn

 

then cc = .802 x 32 x 300 x .89 dn  / 1000  = 464.86 mm

ElectricPavlov  Mar 5, 2020
 #7
avatar+251 
+1

Thanks for your help pavlov. I've just found my error.

Upon calculating the term 132/dn * (dn-40). I used E as 200MPa for steel instead of 200GPa.

 

I appreiate your time.

 Mar 5, 2020

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