10-26-2010, 12:47 PM
THIS IS NOT A BUG
First some rules for ETABS
1-weight is used for self weight calculations.
2-Mass is used for MASS SOURCE only if mass option is selected.
So in general mass is only used when you want to include advance analysis. Generally even in sesimic calculation we do not include this option.
"Lets say our working units are KN-m, this means that approximately i may use 25KN/cubic metre and 2.54 ton/cubic metre as the unit weight and unit mass respectively for reinforced concrete."
For weight units are okay (KN/m³) because you have selected these units but for mass, your specified units are wrong (ton/m³). because you have already selected KN/m³ as your current units. Now everyhing will be in these units. ETABS does not have seperate units for separate fields within the same form. except in case of ct value in earthquake which is always in feet. Now you may be wonder how you can have same units for mass and weight? How you will convert ton/m³ to KN//m³ which will become weight? see below!
if your weight is 25 KN/m³ then your mass will be 25/9.81 = 2.55 Ton/m³ but you have to put this value in KN/m³. This 2.55 Ton/m³ is mass value scalar to make it Ton-f or KN you need to multiply and divide by 9.81 getting the final value 2.55 KN/m³.
just imagine ft-lb system in which concrete density is 150 lb/ft³ and g = 32.17 ft/s² so mass is 150/32.17 = 4.66 lb/ft³. So you see lb is used for both mass and weight.
I know this is confusing but try to understand.
First some rules for ETABS
1-weight is used for self weight calculations.
2-Mass is used for MASS SOURCE only if mass option is selected.
So in general mass is only used when you want to include advance analysis. Generally even in sesimic calculation we do not include this option.
"Lets say our working units are KN-m, this means that approximately i may use 25KN/cubic metre and 2.54 ton/cubic metre as the unit weight and unit mass respectively for reinforced concrete."
For weight units are okay (KN/m³) because you have selected these units but for mass, your specified units are wrong (ton/m³). because you have already selected KN/m³ as your current units. Now everyhing will be in these units. ETABS does not have seperate units for separate fields within the same form. except in case of ct value in earthquake which is always in feet. Now you may be wonder how you can have same units for mass and weight? How you will convert ton/m³ to KN//m³ which will become weight? see below!
if your weight is 25 KN/m³ then your mass will be 25/9.81 = 2.55 Ton/m³ but you have to put this value in KN/m³. This 2.55 Ton/m³ is mass value scalar to make it Ton-f or KN you need to multiply and divide by 9.81 getting the final value 2.55 KN/m³.
just imagine ft-lb system in which concrete density is 150 lb/ft³ and g = 32.17 ft/s² so mass is 150/32.17 = 4.66 lb/ft³. So you see lb is used for both mass and weight.
I know this is confusing but try to understand.
WS