Expert: Babak Esmailzadeh Hakimi - 1/19/2012

Question
QUESTION: Dear sir

1) A medium rise building having adequate shear walls in X & Y
direction to be modelled using ETABS. What is the correct method to be adopted relevant to lateral loading out of the following and pl give the reasons

a)Model all the lateral load resisting elements(columns and shear wall)then apply the lateral load in each direction to the
center of diaphragm

b)model the structure without shear walls and analyse only for vertical(gravity) loads.then shear walls are separately analysed for lateral loads

2) When analyzing shear wall what assumption can  we made to bottom of foundation as pin or fixed in above case a&b pl give the reason

ANSWER: Dear Priyantha

This is what I prefer to follow:

1st step : To model the structure completely and to load it totally according to codes of practice including lateral loads.

2nd step: To remove the shear walls and load the frames laterally for 20 to 25 % of standard seismic loading.

This procedure ensure the structure to get equipped with dual structural system which is suitable to confront earthquakes.


No reason to assume hinge support for your shear wall basement. If your shear wall is reinforced in more than or equal to 2 layers in  X and Y direction, then you will have to assume it being able to transfer bending moment to the foundation. Hinge support will need suitable structural details.

Please feel free to ask me further if necessary.

Regards
Babak E Hakimi



---------- FOLLOW-UP ----------

QUESTION: Dear Sir

1)Normally I assume column bases as pin. is it correct to assume column bases as pin and shear wall bases as fized.

2)how to decide the no of fixed joints required for the basement of shear wall(is it enough one  at each end)

3)What is the criteria of incorporating a shear wall to a building

4)I am in Srilanka and doing designs according to BS8110. can you explain why this code does not addressed to dynamic loads and earthquakes

ANSWER: Dear Priyantha

1- While you have at least 2 layers of reinforcement entering inside the foundation, you have a fix end column. May be this will be influenced in ultimate loading , and deformation of the support will make a plastic hinge , but in normal loading this will not be the case. A hinge need a hinge detail.

In steel structures you can assume hinge support easily but in reinforced concrete not.

2- This is up to you and the accuracy of your calculations, I suggest a maximum  of t ( thickness of shear wall ) for the dimension of finite elements. As a generel rule it is recommended to try t and t/2. If no significant difference you exploited between the results, using t is accurate enough.

3- Shear walls are better to follow in the situation you have great concern about your columns size. In a situation of no quality control present in construction procedure it is better to use a shear wall than rely on frames and their connections. As a general view it is better to have a dual system ( shear wall + frame)

4- BS codes are specific of UK . There is no earthquake hazard in UK territory, then BS codes are free of seismic related regulations.

Feel free to ask me further if necessary.

Best Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir

1) When the column is assumed as fixed , moment is transferred to foundation.then is it required to design foundations(including pile foundations) for both axial load and moment.
         is it not conservative to assume column bases as pinned and foundation design only for axial loads

2)Can you explain difference between "in-plane membrane stiffness and out-of plane plate bending stiffness"

3) Slab elements are assumed as shell(out of shell ,membrane and plate)
element is it correct and why.In case of shear wall what assumption to be made of and why

4)When modelling a shear wall of 5000mm width and 225 thick shear wall no of fixed joints to be something around (5000/225),23 and 46 is this correct(to clarify previous  answer -2)

5)When modelling a RC shear wall structure, first model the frame and then place shear walls as infill walls in between columns and beams, is it correct or shear wall to be modelled continuously throughout the structure

Answer
Dear  Priyantha

1- For foundation design , considering moment and axial load is more conservative than axial load alone.

2- you can have 2 types of surface elements : A- one with only in plane stiffness a bit like a balloon, but with compression and tension stiffness (shells)  B- one with additional shear stiffness like slabs (plates).
In diaphragm identification and design you will need  in plane stiffness and strength then a shell system ( membrane having axial stiffness and not shear stiffness).

3- The main assumption in shear wall is to have an in plane stiffness and neglect the shear stiffness ( out of plane behavior) , the it is more than  enough  to choose shell elements in your structural model , while you can have plate too.

4-yes it is correct to have 23 nodes fixed. You can try 46 to check the appropriateness of your calculations.

5- Shear wall can be modelled in any instance of your modelling process.


Please feel free to ask me further.

Best Regards
Babak E Hakimi