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# Round error? units not right?

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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
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How do you go from 6.85 dn   to   6852300 dn

shouldn't it be    6852.3 dn ?

Mar 5, 2020
#2
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sorry, I wrote in the *10^-3 at the last second.

I've used 6852300 as Newtons

Mar 5, 2020
#3
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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
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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
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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
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unfortunately dn cannot exceed 460mm in this problem

vest4R  Mar 5, 2020
#8
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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
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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.