RESPONSE SPECTRUM DOUBTS

HI. I'M DAVID LINARES FROM VENEZUELA. I'M A CIVIL ENGINEER. I'VE BEEN USING STAAD PRO FOR ALMOST 1 YEAR. I HAVE DOUBTS WHEN IT COMES ON SEISMICS LOADS.

I'M GOING TO ADD A SIMPLE EXERCISE HERE OF WHAT I DO WHEN I DESIGN ANY STRUCTURE. I WILL ADD SOME PICS SO YOU CAN SEE HOW I WORK AND SEE WHERE MY DOUBTS ARE. HOPE YOU CAN ANSWER MY QUESTIONS.

AS YOU CAN SEE AT THE EXAMPLE PIC 1, I HAVE A STRUCTURE THAT HAVE APPLIED SELF WEIGHT AND DISTRIBUTED FORCES ON 2 BEANS FOR DEAD LOAD.

EXAMPLE PIC 1

NOW AS YOU CAN SEE AT EXAMPLE PIC 2 I ADDED MY SEISMIC LOAD CASES AT X, Y AND Z DIRECTIONS

EXAMPLE PIC 2

THEN I STARTED TO ADD THE SEISMIC LOADS AT EVERY LOAD CASES AS YOU CAN SEE AT EXAMPLE PIC 3

EXAMPLE PIC 3

SO IF YOU NOTICED, AT EXAMPLE PIC 3, YOU CAN SEE THAT ON MY SEISM (X DIRECTION) LOAD CASE I ADDED SELF WEIGHT POSITIVE AT X, Y AND Z DIRECTION. I ADDED TOO MY DISTRIBUTED FORCE AT EVERY DIRECTION TOO (X, Y, Z) AND POSITIVE. AND FOR LAST I ADDED MY RESPONSE SPECTRUM.

THEN AT SEISM (Y DIRECTION) LOAD CASE AND SEISM (Z DIRECTION) LOAD CASE I ONLY ADDED THE RESPONSE SPECTRUM.

I DO THIS WAY BECAUSE AT THE PLACES I'VE WORKED PEOPLE DO THIS WAY. ALSO I'VE SEEN VIDEOS ON YOUTUBE AND PEOPLE DOES THE SAME. BUT I REALLY DON'T KNOW WHY. I HAVEN'T BEEN EXPLAINED WHY PEOPLE DO IT THIS WAY.

SO MY QUESTIONS ARE:

1.- WHY DO I HAVE TO ADD POSITIVE SELF WEIGHT AT X, Y AND Z DIRECTION?

2.- WHY DO I HAVE TO ADD ANY LOAD (IN THIS CASE MI DISTRIBUTED LOAD ON BEANS) AT EVERY DIRECTION AND POSITIVE?

3.- WHY DO I HAVE TO ADD ALL THE SELF WEIGHTS AT EVERY DIRECTION AND LOADS AT EVERY DIRECTION FIRST AND FOR LAST MY RESPONSE SPECTRUM?

4.- WHY DO I HAVE TO DO ALL OF THIS ONLY AT X (DIRECTION) AND AT Y AND Z DIRECTIONS ONLY ADD MY RESPONSE SPECTRUM?

PLEASE I WOULD REALLY LIKE TO KNOW WHY IS DONE THIS WAY.

I REALLY APRECIATE YOUR ANSWER AND EXPLANATION ABOUT THIS TOPIC, BECAUSE MEANWHILE I'M DOING THIS I FEEL LIKE A ROBOT DIS KNOWING WHAT I'M A DOING.

DAVID LINARES.

ligonza.david@gmail.com

  • David : Here are the responses to your queries

    1. The selfweight has to be applied in ALL directions because from these weights the software will calculate the masses which can vibrate in all 3 directions when subjected to the response spectrum loading. Only positive value is considered as masses can only be positive.

    2. Any load, that needs to be accounted for in the calculation of seismic masses, has to be applied in all 3 directions ( and with positive sign ) for the same reason mentioned in point 1.

    3. From the loads applied in all directions, the software will calculate the seismic masses for the dynamic analysis whereas the spectrum data represents the seismic load input which tells the software how the acceleration is varying with the time period.

    4. The seismic mass is only calculated based on the load provided as part of the first response spectrum load case. Any load defined in subsequent response spectrum cases are not used for the mass calculation and so adding only the spectrum data for those cases is good enough.



  • Thanks for your answer sye. I have another question for you about masses.

    As you now, on staad pro software if you go to Loads & Definitions you will find Reference load definitions.

    I've seen it's possible to add here a load case and select mass as a loading type.

    If i do this, and add here all self weights at any direction and distributed force at any direction, am i doing the same thing?

    I mean is this enough to make the mass definition by this way and don't do this at my seims (x direction) load case?

    will the software do the same?
  • Correct. If you define the Reference Load type mass and apply in all directions, you do NOT need to model the mass as part of the first spectrum case anymore.



    Answer Verified By: David Linares 

  • i understand that in staad ,if you want to do response spectrum,you can use referenced loads,and.all loads should define in x,y,z three directions ,but want can I do with floor loads(not floor weight,it is only the weight of steel plate,and i use floor load to only to consider the weight),and can I define the floor load in three directions? somebody help me,please?