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Seismic Load definition

Dear respected members,

The staad example shows in the definition of seismic load as "joint weight", can any one show me how to calculate the joint weight of the structure.

I have calculated the seismic design load manually and then applied on nodes at

different level and by staad seismic load generation. I found vast difference in value of loading as can be seen in the two different uploaded files.

My procedure for seismic load generation is, 1. self weight of structure 2. member weight, which includes weight of brick/ block walling 3. floor load which includes 50% live load.. Is this correct way of load generation. If yes, then wat will happen to dead (beam, slab, shear wall,column) load. How will we define this dead load in staad.

So, the summary of my questions is;

  1. How can we calculate joint weight of structure (for seismic load definition).

After going through my attached files;

  1. Why is this big difference of seismic design loading in my attached models.
  2. what is the correct way of defining seismic loading.
  3. is the process of my seismic load generation correct.

Waiting for early response.

Regards,

4 Replies (Most Recent Reply)

  • Dear Sir,

        I believe that the seismic weight calculation for each code will be different. But I think methodology will be same. For example, I am using IS 1893. In IS code, specification for seismic weight is Dead Load + Factored Live Load where factor for live load is 0.25 for LL<=3, 0.5 for LL>3 and 0 for roof LL...

       The methodology I adopt is, I believe, the easiest to use for STAAD.

    Step 1. Create reference loads, definining (a) dead loads (some may prefer to create separate reference loads for self weight, wall loads and floor dead loads),  (b) live loads except on roof (may need to split into two if some floors have LL<=3 and some have LL>3) and (c) live load on roof. Additional loads may be required in case of cantilevers projecting from floors, water tanks, etc where higher contribution is specified in the code.

    Step 2. In creating the load definition for eq, call the reference loads with corresponding factors

    Step 3. Create load cases for DL, LL, LL-Roof using the reference loads factored by 1.

    Step 4. Create load combinations as required.

      By the above method, loads are defined only once (instead of once in load case and once in seismic weight definition) which reduces chances of errors.

    Arun

  • In reply to Arunkumar Srinivasan:

    Dear Mr. Arun,

    Thank you very for your valued information which made my calculation easy.

    I would also like to know that how can we generate the L/C (0.9dead load - 1.5earthquake load) in Staad Pro.

    Regards,

  • In reply to Arunkumar Srinivasan:

    Dear Arun,

    I have the understanding that Reference Load needed for defining IS 1893  should be assigned Mass. Ref load has the provision for assigning mass. While loading for analysing the structure need not be assigned mass. Will this not make difference if the same Ref load( having mass  unassigned) is called upon both for analysing the structure as also for defining IS 1893?

  • In reply to sureshprsharma:

    Dear Er. Suresh Sharma,

       As I understand, the loads assigned in reference loads are automatically converted into joint and member weights by STAAD. Even floor loads are automatically converted. There is no need to assign them as Mass loads. You can also refer to an Intro. on Reference load at communities.bentley.com/.../reference-loads-an-introduction-cs.aspx where, near the end, it is noted that the reference load for primary load can be used in defining seismic weight.

      I have used this method in one project already and compared with earlier results (when I defined seismic weight by entering the loads again). There was not much variation. Hence, to my knowledge, there is no issue in the above methodology.

      If anybody has other experience, or perhaps if the members of Bentley Techincal support who frequent the forum know otherwise, they may please post their comments on the same...

    Arun

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