Areeb

Dear Engr. Salman, As you have said yours is a SMRF system, it would be better if you try to avoid reducing the torsion modifier to 0.1, however in some cases even if you increase the beam depths to 1000 mm it still fails or increase the width to say 400 mm or 500 mm. So try to increase your beam cross section but if it is still failing then reduce the modifiers. The practice of reducing the modifiers is not wrong as you are in a way transferring the torsion from beam to other structural members. Thanks and Regards, Areeb Ahmed.

Well as its sudden reduction in width of the column my advice is to consider this solution. Don't consider the reduction of column and instead if possible provide a beam and floating column thereby the floating column will act as point load on beam and you could design your beam for that load. Also stop that 5.4 m width column at that level and don't consider it for upper levels.

Hi, The attached pdf shows the column reducing from 5.4 m to 1.8 m. Please correct me if I'm wrong, but what I understood from the pdf is there is reduction in column width by 66% not 33%. Well that is huge and sudden reduction in width. Please Confirm.

Hello Israr, As madam Ayesha said there are areas of high stress concentration in raft. Could you specify the location where you are getting 10 Kip/ft2. As specified if this pressure is at corners in small concentrations then you can ignore, however if the area is large then need to apply changes to raft design. Thanks.

Hello Hafsa, Instead of adding negative value in mass source, why don't you change the value of load. I mean enter the load as negative and don't change the sign in mass source. Thank you, Regards, Areeb Ahmed.

Hello, Well as you said as per dimensions of the plot that X direction is smaller, it becomes necessary to add shear walls in X direction as it is critical in that direction during seismic analysis. Regarding the system you have asked you can provide core wall around the lift and staircase and columns, however you have to see which zone the plot is located. if it is till zone 2B then i think this system is acceptable as you can have high response reduction factor value. Also even if you are using walls you can reduce the size of walls in terms of thickness as well as length as you go to higher stories. This reduction also should be done in a reasonable manner. There is also another system of haunch girders which could be economical and can be used in zone 3 or 4. There are many systems also like frame tube system or shear wall frame system etc. which could be economical. Thankyou, Regards, Areeb Ahmed.

Hello Fatima, Well the procedure you have explained is correct for finding out the shear strength of columns, thereby ultimately finding out the story strength. Still I find it cumbersome and we have direct formulas for knowing the shear strength capacity of wall or column thus rather than going by the method of flexural strength we can directly find out shear capacity of wall/column and find out story strength. Thanks for the update regarding the method used by you. Although cumbersome something new to learn today.😃 Regards, Areeb Ahmed

Hello Zain, The method mentioned by Fathima is new to me. Flexural strength is nowhere related in finding out weak story. As it is clearly defined in ASCE about weak story i.e, the shear strength of story A is less than 80% of shear strength of story B. Shear strength or story strength its same. Then the system is said to contain weak story irregularity. Thanks. Regards, Areeb Ahmed.

Hello Zain The method which you have mentioned above is for finding out whether the system has soft storey or not. You can use the method mentioned above or ETABS latest version directly gives the stiffness for each storey thereby its easy to find out whether the system has soft storey irregularity.

Hello Fatima, Well that's a first for me. I think rather than elaborating by calculating flexural capacity and so on you can directly calculate the shear strength capacity of all walls/vertical components in a particular story in particular direction and compare it with the story above thereby finding out whether the system has Vertical Irregularity 5a. or 5b . Thanks

Umar, Yeah I have checked it and by discussing it with other engineers came to find out that ETABS 16 explicitly doesn't provide data/excel format for identifying the weak story. There is a table for finding out stiffness of each story thereby helping us identify Soft Storey, but Weak Story needs to be done manually. Software MIDAS provides a table in results clearly displaying Shear Strength of particular storey, but in Etabs it is not present. Yeah by using displacements of adjacent floors it is possible to identify Soft Storey, but that method is not needed now as ETABS clearly gives us Stifnesses of each storey thereby easily helping us to find out Soft Storey in a system. Sorry, I have quoted ACI for irregularities its ASCE Regards, Areeb Ahmed

Hello I would like to check my structure for Vertical irregularity 5a or 5b (according to ACI) i.e check for weak story. It is mentioned that structure has weak story if shear strength of story is less than 80%of shear strength of story above. Also extreme weak story if shear strength of story is less than 65% of shear strength of story above. How to view story strength in ETABS?

Thanks

Hello Question regarding choosing between shear walls and moment frame system. Both the systems are provided to resist the lateral forces. Shear wall acts as lateral force resisting system same as Moment frame like IMRF, or SMRF. The value for Response reduction factor is different for different systems according to UBC. If i have a choice to select shear wall or moment frame system which system is better to choose. I know SMRF or is used in areas of high seismic zone thereby less R value and a ductile system. But in case of moderate seismic zone like zone 2A or zone 2B if I have option of providing only moment frames or only shear walls then what is the criteria for selecting such system?

Thanks for the reply. Umar Makhzumi, HafsaAzmat

Thanks

Hello, One question regarding Collector elements. The code specifies that we need collector elements to transmit the lateral loads from the diaphragm to shear walls or vertical elements.True, but there are cases where we have a flat slab or due to architectural limitations cannot provide beams or collectors in that region. How are lateral forces transmitted in such cases. Where there are no collector elements the lateral forces are not transmitted to vertical elements effectively. Or we have another solution apart from Collectors to transmit the lateral forces from diaphragm to vertical elements. Thanks.

Hello I have a question regarding stiffness modifiers in ETABS. The code(ACI318) specifies us to use stiffness modifiers for walls column etc. What if I use 0.35 as stiffness modifier and not 0.7. What is the effect.I know that we have these stiffness modifiers based upon cracking of section and using these modifiers results in lesser stiffness thereby reductions in attraction of lateral forces.But suppose I want to reduce the lateral forces on walls therby using stiffness modifiers 0.35 for walls and not following the ACI. Thanks