Civil Engineering Association

I strongly request to be corrected if I am wrong on the observations below:
I realised some issue with ETABS but did not go much into its intricacies. Its mathematical though not that much,the simple type,just units conversion.
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.
Now if you open a new file with KN-m as your working units, and go to define Material properties, you will notice that by default the unit weight is 23.5616KN/cubic metre and unit mass as 2.4007.....(here meaning that unit weight/9.81=unit mass).
Now i would want to assume that 2.4007 is actually ton/cubic metre. But then i go ahead and make a further check. What would this be? I go into the calculator mode of ETABS by shift clicking to access the calculator. Here i change my units temporarily to ton-m. I then enter a value of 2.54 ton/cubic metre, and calculate and then OK the results. Surprisingly, i find that the the unit weight and unit mass are actually the same value. I am still not satisfied cause i feel this should be the correct input but requires more confirmatory testing. I then exit the forms and change my working units for the entire project to ton-m. Now here comes the catch!!!! ETABS defaults the value of unit mass to 0.2448 ton/cubic metre.

Observations
Defaulted inputs are not correct as expected.

Conclusions
ETABS or any other program Users should not take default values for granted. Check them and see if they are correct. If you doubt them, consult. CivilEA is there to help!!

Hi Sir,

I agree with your conclusion.

When using a computer program, it is more safe to keep always the same pack of homogen units. It is not recommanded to change units in the middle of introducing the values. I prefer to transform the units by hand and stay homogen in the program. It is very dangerous to change, especially in dynamic analysis.

Best regards.

Zipatt

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.

Hi,

I'm surprised by the mail of Coolestbliss wich seems complicate the problem. The situation is simple : The mass notion is different than the weight notion. In dynamic analysis, just be care to get the ratio of the gravity between the weight and the mass (9,81 m/s²). And ETABS knows that. If you declare that the masses are given by the weight, it makes the division inside.
But in static analysis, it doesn't matter. Juste be care to get homogen units.
That's all folks.
Best regards
Zipatt

Etabs make it seems complicated by allowing weight and mass values to be defined.
In this way you can design a building also on Mars or the Moon, where the g value is different. :yes:

If you try SAP2000 you will see that the mass input box is inactive but the value inside is changing automatically, function of weight, making all easier.

To be sure that you input the mass correctly:
- define the weight taking into account the units;
- divide the weight value to g (m/s2);
- input the resulted value in the mass input box, ignoring the units.

In the end, mass value must be approx 10 times less than weight.