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.

vest4R Mar 5, 2020

#2

#3**+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

edited by ElectricPavlov Mar 5, 2020

edited by ElectricPavlov Mar 5, 2020

#4**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**+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

#8**+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