This is the question i am asked:
Determine a suitable size for the (gantry girder) beam selected from standard section tables.
Prior to that is some information:
The cross-section of the beam shows that of a Gantry Girder. The maximum bending stress in the girder musn't exceed 32MN/m^2 and the modulus of elasticity can be assumed to be 280GN/m^2
What has been looked at in class before being given the homework follows:
Section Modulus: Sigma max = M / Z
Theory of Simple Bending: E/R = sigma/y = M/I
The Attempt at a Solution
I believe I need to find Z to help locate in the standard section table the chosen girder.
Looking at the information, i have the following values known:
sigma max = 32MN/m^2 = 32 x 10^6 N/m^2
Modulus of E = 280GN/m^2 = 280 x 10^9 N/m^2
Thats all i have, i know it's a gantry girder so in the section tables i look in the table for gantry girders only.
I remember doing something in class to find the Z value, which when known you look into the Zxx column of the table and find the nearest values to yours and then read the row to find the breadth and depth.
What does M represent in this equation: sigma max = M / Z? I know it isn't the modulus of E.
Thats why i'm looking at this other equation: E/R = sigma/y = M/I - because both values known are in this equation, but how to i find M from this if this is how i do it?
I have questions following this one such as:
Determine radius of curvature and Determine the actual maximum stress value in the chosen beam.
I don't want help with these, i just need reminded of how to find the right girder or am i mis-reading the question and i should just chose one instead?
The sooner you reply the sooner i can thank you :)