dear honorable engineers,
I am learning staad.pro on my self.please someone write to me how can I assign seismic load(both static and dynamic) in staad.pro especially through is1893.Please tell me about seismic defination. When we assign self weight, joint weight,member wt,floor wt in seismic defination, shall we again assign them in Load Case Details?or What is actual meaning?
thanks in advance
To assign a seismic load in a structure there are two steps. First you have to define the seismic load and then you have to assign the load to the structure.
1> Seismic definition:
To calculate the seismic load acting on the structure, there are different parameters which should be defined at first. On basis of those factors using the standard formula, defined in any standard code, the seismic load is calculated. In this particular part, those parameters like zone factor, performance factor or the soil type have to be defined to calculate the “Response acceleration coefficient”. In IS-1893 the corresponding values are given. Also in the technical reference manual in section 220.127.116.11, you will find some information regarding these.
Next you have to define the seismic weights. It is needed to calculate the total weight of the structure to calculate the “base shear”. There are different types of loading. The self weight option defines the self weight of different members. If there is any load at the nodes you can define it by joint load option. The load coming from walls can be defined under member load option. The weight of the floor can be defined using the floor weight option.
You can select the code you want to follow. The "Include accidental load" option helps to determine the effects of accidental torsion to account for the uncertainties in terms of centre of mass or centre of rigidity as there position can not be properly located.
2> Load assignment:
Next the seismic load has to be assigned to the structure. For this a new primary load case has to be created in which the seismic load has to be added. The seismic load may be applied in X direction or may be in Z direction. There must be two separate load cases to define seismic load in different directions. The load factor must be assigned as 1 to define that the load is acting in the positive direction. You also have to add all other loads like dead load, live load or wind load. You can do it either by adding those loads separately in the two different seismic load cases. Or you can create a different primary load case where you add these loads and then create another primary load case where you have to use the “repeat load case” option to combine the loads. Load combination option, in this case, should not be used as this is a non-linear case.
The loads due to self weight or member weight should be assigned in load case details also. Else the structure will be analyzed only for seismic load cases.
One most important thing is that you have to assign the seismic load cases as the first load case before defining any other load case. Else during analysis it will give some error message.
You can perform “P-Delta analyze” as this is a non-linear case.
You can also go through Example 14. The problem will help you to understand.
dear Surojit sir,
thanks a lot for your information.It helped me a lot.Also could you please tell me when we offset the members those does it changes the cross section of member that we have previously defined.Is it necessary to offset the members.What is the effect of offset in analysis?
Member offsets do not change the cross section of members. An offset is a way of saying that the point where a beam starts or ends is at some distance away from the node associated with that location. For example, if you consider a portal frame drawn using 3 lines like in a football goal post, the node at the start and end of the beam is usually specified at the intersection of the centerlines of the beam and columns. However, in reality, the connection between the beam and columns may be located at the two inner faces of the columns. So, the distance from the intersection point to the connection point is not as flexible in bending as the rest of the beam, but is instead considered rigid (high stiffness). To convey this information to the program, you could specify an offset at the start face of the beam (as well as at the end face) with a value equal to half the depth of the steel section used for the column at that location.
Please go to Help-Contents-Application Examples-British Examples-Example 7 for additional information.
It is not absolutely necessary to specify offsets. The feature is there just in case you want to take advantage of the rigid portion and reduce the bending moments on the beam.
"here must be two separate load cases to define seismic load in different directions. The load factor must be assigned as 1 to define that the load is acting in the positive direction." Does this mean you need a combination with those given a negative value for seismic to be applied in the other direction?