By Ray Delaforce

PV Elite Software Engineer, Intergraph

In last month’s newsletter, we covered the definition of load cases. Read the article here.

In this installment, we will discuss how load cases are handled by PV Elite. But first, to unravel the mystery of load cases:

1. Weight:

a. OW – Operating weight – in the corroded condition

b. EW – Empty weight

c. HW – Hydro test weight

d. CW – Un-corroded weight

2. Pressure:

a. NP – No pressure

b. IP – Internal (design) pressure

c. EP – External pressure

d. HP – Hydro test pressure

3. Moment:

a. WI – Moment from the operating wind load

b. WE – Moment in the un-corroded condition empty

c. WF – Moment in filled with the operating liquid

d. EQ – Moment from earthquake loading at operating condition

We can ignore the other loads, which are forces or moments to which the user can subject the tower. Note that we are still only considering three loads: Weight, Moment, and Pressure.

What equation can we use to compute the stress from these three loads applied to the tower? Here is a simplified equation, that is very easy to apply, but remember, we are considering axial, or longitudinal stresses, not hoop stresses:

Where:

W = Weight

P = Pressure

M = Moment

The plus sign means tensile stresses, and the minus sign means compressive stresses. In the case of the moment, there is a ± sign. This is because the right side of section X-X could be tensile, and the opposite side could be compressive. In the case of pressure, the ± sign means that the pressure could be internal pressure, or partial vacuum.

Now, let consider just the first load case as displayed by PV Elite:

Pressure:

0.9IP is 0.9 x the internal or design pressure

Weight:

CW is the operating weight

Moment:

1.1WI is 1.1 x the wind load at the operating condition

All you have to do is to go down the list determining which is the pressure, which is the weight, and which is the moment. Then apply the simple equation shown above.

PV Elite does the calculation for each load case (combination of pressure, weight, and moment) and displays the result in a table like this, as shown for the first load case:

f1 – f2

This means the hoop stress minus the axial stress

f2+0.5p

This means the axial stress plus 0.5 x (0.9 x design pressure)

Hoop+0.5p

This means 0.9 x Hoop stress + 0.5 x (0.9 x design pressure).

You can work your way down the list looking at the various load cases to see how the stresses are derived.

Looking at the table below, we can see, for example, that we have a problem with node 30. It is in red font with an asterisk at the end of the line.

This is an indication that the item is over-stressed. Toward the right-hand side of the table, you see these headings:

The word “Rat” stands for “Ratio.” Toward the left-hand side of the table, we have the actual computed stresses. However, those stresses are to be within the allowable stresses, imposed by the code.

Ratio simply means:

If the actual stress is greater than the allowable stress, the ratio is bigger than 1.0. That is an indication that overstress occurs. For node 3, the ratio is 1.017, indicating we have a condition of overstress.

The article has attempted to simplify the understanding of load cases. The concept is simple, and I hope that the explanation is understandable.

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