In the next session of 27-SEP-20, we are going to discuss the stiffness modifiers as per IS 16700, IS 15988 and IS 1893. Earlier, I received interesting questions at LinkedIn regarding the stiffness modifiers. I have given the answers based on my understanding. Few of them are reproduced below.
Link for attending live session of 27-SEP-20 : https://youtu.be/wEom_O1i7LY
Your views / suggestions / comments are welcome in the comment box below.
Why no stiffness modifiers for wind and other load cases, cracks can occur in other loads cases also which in turn increase the deflections ?
his is a very good question. Let me try to explain this with the simple language based on my understanding. For wind resistant design, the structure is designed to resist wind load under elastic limit. However, for earthquake resistant design, it is not economically feasible to design the structure to resist the earthquake loads under elastic limit. Hence, during extreme seismic event, the structure is expected to experience the inelastic displacements. In view of the above, the structure will be under elastic stress for the wind load case and will be designed to respond inelastically under the effect of earthquake loads. Due to the same, stiffness modifier concept might not have been considered for the wind resistant design for buildings less than 50m height (IS 1893 (Part 1) : 2016). IS 16700 : 2017 defines stiffness modifiers for wind load case as well (table no. 6).
stiffness modifiers are independent to the nature of the force but they are dependent upon the extent of cracks in the member. The structures are designed to resist the wind force within elastic limit whereas for earthquake resistant design, the structure has to undergo inelastic displacement. Due to this the extent of cracks will be lesser in the structural members for wind resistant design as compared to the earthquake resistant design. Hence, the stiffness modifiers values are different for wind load and earthquake load.
Clause no 1.4 says, IS16700 can also be used to design building under 50 m but in no case, it should be used to design building having a height greater than 250m. Please do comment on this.
For buildings having height up to 50m, IS 1893 (Part 1) : 2016 to be referred. IS 16700 : 2017 to be referred for buildings of height greater than 50m but less than or equal to 250m. It is mentioned in Cl. no. 1.4 of IS 16700 : 2017 that the standard may be used for buildings of height equal to or less than 50m. It is recommended that IS 16700 : 2017 may be used for such buildings as it may add values to the design. However, it is not mandatory to use IS 16700 : 2017 for design of buildings up to height of 50m. It is mentioned in Cl. no. 1.4 of IS 16700 : 2017 that the standard is not applicable for buildings of height more than 250m. Buildings of height greater than 250m are considered as Super Tall Building (Cl. no. 3.13). I believe that special issues associated with the Super Tall buildings like safety of structure, serviceability criteria, different structural systems, wind effects, etc. might not have been covered in the code. Hence, it might have been mentioned that the code is not applicable for Super Tall Buildings.
Ultimately as per my understanding from the discussion, we have to prepare 2 nos frame analysis model from which 1st model is used for satisfying serviceability criteria in which we have to assign stiffness modifiers as per IS 1893 :2016 for building having height less than 45m.and as per IS 16700 if building height having height more than 45mt. and check serviceability criteria. In 2nd model for design in which don’t assign stiffness modifiers if building height less than 45m and assign Ultimate stiffness modifiers if building height greater than 45mt. Make me correct if, am I wrong?
Let me correct your understanding based on my experience. As per IS 1893 (Part 1) : 2016, only one model is required and the stiffness modifiers to be considered only for seismic load. As per IS 16700 : 2017, two separate models will be required as there are different stiffness modifiers for wind load (serviceability state) and seismic load (ultimate state). I hope it is clear. May share further doubts, if any.
In IS1893 we have different stiffness modifiers for slabs, beams, columns but in IS16700 table 7 that shows stiffness modifiers to be used during Sensitivity Analysis mentions same modifiers for slabs, beams and columns. Why so?
In IS 1893 (Part 1) : 2016, there are separate stiffness modifiers for beams and columns. IS 16700 : 2017 defines separate stiffness modifiers for slab, beam, column and wall in table no. 6 (Cl. no. 7.2). As per IS 16700 : 2017, the sensitivity analysis is to be performed for multiple tall buildings connected with a common podium (Cl. no. 8.1.3). Table 7 indicates stiffness modifiers to be used for upper and lower bound solution while performing the sensitivity analysis. The sensitivity analysis is to be performed in addition to the analysis based on the cracked RC section properties, as defined in Table no. 6 (Cl. no. 7.2). I hope it answers your query.
Though with respect to IS 16700: 2017 table 7 modifiers are in addition to table 6 modifiers but why same modifiers are considered for slabs, beams, columns, walls? While table 6 mentions different modifiers for slabs, beams, columns, walls. Table 6 has different modifiers for different structural elements because we consider slabs will have more cracks than beams and beams more than columns. But in table 7 the modifiers are same for all elements. Why so?
As per clause no. 8.1.3.2.1 of IS 16700 : 2017, the stiffness modifiers are defined for upper-bound and lower-bound cracked section properties of floor diaphragms and diaphragms & perimeter wall of podium. Hence, there are unique values of the stiffness modifier. For more details, pl refer to the clause.
In case if shear walls are only the lateral force resisting element then shall we not be considering reduced stiffness? ACI also suggests 70% stiffness for the walls. Hence stiffness modifier for the walls shall not be ignored.
I agree with your views that stiffness modifiers may be included for shear wall as well. I believe it may be included in the next revision of the code.
Please correct me if I’m wrong, “stiffness modifier is a bi-product of stiffness reduction due to crack formation in concrete structures”. If we’re well positioned in terms of assuring conformance with crack-width criteria, in a sense resulting crack-width is well with in the permissible limit, under such scenario would you be still advocating such a tremendous reduction of portal stiffness?
As per my understanding the values might have been derived based on testing of the structures which are designed using conventional limit state design method principles. I think if we are limiting the crack widths as per requirements of un-cracked concrete then the values of stiffness modifiers may be increased. However, at present in the code, there are no different values of the modifiers defined for the structures which are designed considering the principles of un-cracked concrete.
Is this stiffness reduction “part-and-parcel” of serviceability checks since crack formation by-and-large activates such reductions in stiffness? Looks like, strength checks [Flexure/Shear etc] would remain unaffected by this substantial stiffness reduction.
As I understand that the stiffness modifiers are to be used only for structural analysis. After deriving the analysis results, the flexural / shear design to be performed using the conventional procedure of limit state design method.
From Analysis Point Of View Can We Take I as 1 and Run The Model And Take Out The Base Reactions Using That Model?? From Design Point Of View We Can Save As Model And Apply Stiffness MODIFIERS As Mentioned In The Code And Carry Out Design Procedure??
In IS 1893 (Part 1) : 2016, stiffness modifiers are defined for structural analysis for seismic load case. The analysis results to be derived considering the stiffness modifiers. Thereafter, the design to be performed as per the conventional procedure.
I would like to ask that if there are cracks on the structure in service life (I am talking about old structure), then how to calculate its effect on the strength reduction of that member or in building analysis. Can we use stiffness modifier in that case by considering it for serviceability? Presume that I have analysis and design model of the original structure. Or any other Idea that how to analyse the old damaged structure (after obtaining required NDT results) and redesign it for Jacketing or wrapping?
Stiffness modifiers are defined with a fact that the RC structural elements would have cracks during their service life. For seismic evaluation and retrofitting of the existing RCC structures, you can refer IS 15988:2013.
By providing smf we can distribute the loads also. Is it true?
The intention for introducing the stiffness modifier is not for load distribution, instead it is to account for reduced moment of inertia of different members due to cracking.
If overall stiffness of building reduces and mass remains same then how overall seismic force on building going to reduce. Please clarify
The seismic force values are inversely proportional to time period of the building, i.e. higher the time period lesser is the seismic force. The time period is again inversely proportional to the stiffness of the structure. Hence, even if for the constant mass values, relatively lower stiffness will result in the higher time period and the higher time period will reduce the seismic force on the structure. I hope it will clarify your doubts.
Bhavin Shah
Founder & CEO, SQVe Consultants
__________________________________________________________
Part values of modifiers from different IS codes are given below. For more details, please refer relevant clauses and table of the above mentioned codes.
IS 15988:2013 Table 2 Some Effective Stiffness Values
(Clause 7.2.3)
Flexural Rigidity
- Beam, non pre-stressed……………………….. 0.5
- Beam, pre-stressed ……………………………… 1.0
- Column in compression(P > 0.5fc’Ag)…….0.7
- Column in compression (P > 0.5fc’Ag )…..0.5
- Walls — Uncracked ……………………………. 0.8
- Walls — Cracked ……………………………….. 0.5
- Flat slab To be determined based on rational procedure
IS 16700:2017 – Table 6 -Cracked RC section properties
(clause no. 7.2)
Element Unfactored loads Factored loads
- Slab 0.35 0.25
- Beams 0.7 0.35
- Columns 0.9 0.70
- Walls 0.9 0.70
IS 16700 : 2017 – Table 7 Stiffness Parameters
Clause no. 8.1.3.2.1 (a)
Diaphragms & Perimeter walls of podium and below the level
Of backstay – Ieff / Ig – 0.5 (upper bound) , 0.15 (lower bound)
____________________________________________________
An online course related to stiffness modifiers will start from 22-JUN-22.
Click below for more details and registration for EQ-STR-003
Feedback of participants from the previous online courses:
Dr. Raksha Parolkar
Topics covered in WIND-STR-002 were new for me to understand, as it has never been covered in syllabus. But the way Dr. Abhay Gupta sir taught, explained the problems and shared his experiences, the subject got easy to understand. I feel proud of being his student.
REVA VENU V M
I am a budding geotechnical engineering professional. For three years I have worked as faculty in NIT and recently joined as project fellow- geotechnical engineering in a research Institute. Now I am in the transition phase of my career ie, shifting from academics to industry. My desire to become a geotechnical engineering consultant inspired me to join this course. As I don’t have much industrial exposure, attending the course GEO-STR-001 helped me to gain a lot of practical knowledge related to geotechnical engineering. Especially the discussions related to soil investigation, interpretation of soil report and interactive sessions were really amazing. Queries of other participants and the discussions associated with it,were really informative, as it mainly focussed on current industrial practices. Thank you Vivek sir and Bhavin sir for giving this wonderful opportunity. Hoping for a better support in my future endeavours too. Also expecting further courses related to this, in the coming future.
Alok Rathore
After attending the course WIND-STR-001, I change my prospective towards structure and learn many concept.
Dipal Trivedi
The course GEO-STR-001 effectively addressed the gap between geotechnical and structural engineer. It not only covers important interface areas but also discusses consequences of poor interface. Subject experts have in depth knowledge and experience to address expectations of various participants ranging from fresh engineering graduate to highly experienced professionals. The course is interactive and experts are more than willing to address any queries of the participant. The course content is appropriately divided into multiple sessions over the weekend to allow flexibility to participants to participate and reflect during the week at their convenience. Overall, this course is highly recommended for structural and geotechnical engineers for gaining and enhancing the understanding of the very important interface between geotechnical and structural engineering work.
Hiren Desai
The course WIND-STR-002 is very much informative as well as useful for practicing engineers considering the non availability of easy to detailed literatures and practical examples.
Sujata Gupta
It gives a good awareness about software as well as corresponding codal provisions. So one can get technical knowledge as well as how software using clauses giving in codes. Teacher is very patient, soft spoken and knowledgeable person. Topics selected in the course STEEL-STR-001 are very relevant, useful and to the point for design and analysis of steel structure.
Tanuja Singh
STEEL-STR-001 : Excellent initiative to bring clarity of concepts to professionals.
Parth Gandha
The course STAAD-STR-001 was really amazing an knowledgeable. As I fresher I gained so much knowledge about all the little things I didn’t know that were so much important. Troubleshooting of the software was the most important topic as a single wrong interpretation of the warning can totally change the outcome. I heartily thanks Shri Bhavin bhai and SQVE consultants for introducing such wonderful courses so that we can perform good in practice.
Praveen Vasishth
STEEL-STR-001 is very useful course for updating to new code for steel design of buildings.
RATHISH V R
I really enjoyed throughout the journey of the course STEEL-STR-001 wherein I improved by Fundamental Knowledge in the Design of Steel Structures. Bhavin Sir’s explanation on Lateral Torsional Buckling and other critical areas, really helped me to become sound in manual calculations. Keep continuing the good work!
Akash Solanki
STEEL-STR-001 was the best package for understanding the limit state design of steel structures using IS800:2007 along with its application in STAAD.Pro. Tremendous wealth of knowledge from Bhavin sir combined with queries by other participants really facilitated a great learning experience. Thank you for such an amazing course. I hope more such courses are on the way.
Mainak Ghosal
The Course WIND-STR-002 was very effective. It changed my perspective on not only wind engineering but also seismic loading analysis. Dr.Gupta also referred some excellent books which would not have been possible otherwise. Last but not the least, Dr. Gupta was kind enough to give me the charges of highlighting the errata for the Explanatory Handbook on Wind Engineering.
Ashish Sawant
The course WIND-STR-001 is very informative. It clears basics of overall concept of wind flow. Thanks to Bhavin sir and Gupta sir for your guidance on wind loading.
Kunal Patil
At first I never expected the sessions which are going to be conducted would be so knowledgeable. Day by day listening to Abhay sir touching every aspect from the codal point of view, few from his own experience. The mixture of everything made this course WIND-STR-002 very valuable and useful. Before this course I was trying to learn from everywhere how does this wind dynamic forces are to be calculated but through SQVE consultants arranging this webinar and learning under experience fraternity was worthwhile.
Chokshi Hem
I heard somewhere that we shall never use STAAD for dynamic analysis. But that wrong interpretation of the software was cleared through the course : STAAD-STR-001.
Rajen Dharia
On attending to STEEL-STR-01 course with the multi discipline participates from across India is the best part of this course. As the questions raised and its solutions, are many a times very new for each other during the sessions which enhance our knowledge in different practical applications and approach towards it’s solution. I Recommend to join STEEL-STR-01 to all upcoming Structural Engineers.
Praveen Upadhyay
Bhavin Shah is a fantastic trainer, very friendly and encouraging, it was a well-delivered and presented course (STAAD-STR-001) that was well received by the group. There was a great balance between theory and practice and the concepts were clearly and professionally explained.
Sharad Gupta
I am very grateful to Er. Dr. Abhay Gupta and Mr. Bhavin Shah for sharing your knowledge with us even though we have the same profession. Your course WIND-STR-002 helped me to clear many of the doubts I had sine the past 20 years.
Ravichandra
STEEL-STR-001 is very wonderful course. I have learned many things through this course. During session Bhavin sir interaction with the participants is very great with clear explanations. We are expecting these type of courses in future also.
Anvitha Yadama
It has been an excellent course and given me an in-depth understanding on ETABS and STAAD.Pro software . I feel more confident in my understanding and it helped me complete my major project in my B. Tech course .Thank you for all the support and excellent teaching. During the training program all my queries were answered quickly.
Akshay Thakur
REC-STR-001: The recorded sessions are more than anything for tackling practical difficulties…. and you bring some other professionals like dr. Jain and dr. Gupta is just like awesome experience for the student like me who are from tier-3 college who won’t get such a precious information… thanks to you also for the knowledge you impart… best luck to you and your team for this your initiative to resolve practical difficulties among aspiring structural engineers… 🙏🏻 once again thank you so much… because nobody in today’s era from structural engineering domain is wants to share the information, but you do… kudos!
Srinivasa Reddy
Dear Sir thank you so much for your Course ETABS-STR-001. We learn Lot from here. Happy to Join this Course we are waiting for next Upcoming Courses.
Dhirender Singh Bisht
I am pursuing masters in structural engineering and steel was the topic that I found always challenging since my B.Tech times. Moreover this is also a missing topic for the syllabus of many universities in masters program. To understand its theory ,and execute it practically in software is really challenging and need a mentor for that. I’ve been following Bhavin sir from last few months and was it was my good decision to choose him as a guide. By the course STEEL-STR-001, now I can feel that I am in a position to explore boundaries of IS code, and also I’m enthusiastic to see how Euro code, AISC code and other countries code tackles the problem which sounds to be complicated in IS code.
Shreyas Barad
STEEL-STR-001 is much awaited course. Finally the industry expert teaching the right thing with right methodology. I had never seen such an excellent online course like this. The amount of knowledge and engagement was extremely good. Thank you so much for the wonderful course.
Tarnpreet Singh Gill
The course content for STEEL-STR-001 has been thoroughly devised to be effective in best manner. Sharing of Recorded Sessions was the best part as I myself missed few end sessions due to office pressure but I could complete these later on. Making a permanent whatsapp group for discussion was also a good idea.
D. Nivetha
The Staad Pro Training sessions were extremely fruitful, informative and it gave deeper insights into the field of structural engineering. Bhavin sir had not only taught us the software of Staad Pro, but he had explained us several technical concepts based on his field experiences. It was an interactive session where sir had asked us questions and it helped us think a lot and come up with solutions. Overall, it’s been a great experience and this 25 day of training has taught me a lot and has inspired me to develop greater passion towards civil engineering.
Er. Sachi Parekh
I appreciate the course structure designed by Er. Bhavin Shah , which includes the theory and implementation of coddle provisions to software and interpretation of results. By attending it, I am able to learn and explore the analogy of a software – Staad Pro. Thank you very much for sharing the knowledge and make my doubts crystal clear…
I must say that he is a true professional academician…
Like a bridge between Institute and industry…
Shefali Thakkar
STEEL-STR-001 was the best interactive course ,wherein all practical facts and limitations those are related with prevailing standards, software and practices were discussed. Vision was provided to participate towards solving such prevailing limitations rather following the same collaboratively. Course content was described very effectively and constant encouragement on experiments of the course deliverables and raising doubts and solving the same. Effective time management by faculty to satisfy all participants’ queries along with effective delivery of course content. 24 hour interaction facility was made available for solving course related queries.
Anal Shah
The course WIND-STR-002 was so interactive and Dr. Abhay Gupta was so kind enough to listen to all queries and answer them in very great detail. Also time spent by Dr. Gupta is highly appreciated. Great job Dr. Gupta in spreading the knowledge.
Nitin Sharma
I would like to thanks and appreciate for this helpful course comprising of STAAD PRO Training and Design concepts for RCC and Steel. In this module I have learned in detail about essential concepts for designing industrial buildings and gained an understanding of different types of Analysis. I also acquired the knowledge of different types of Codes. Moreover, l was introduced with FEM, design of foundation, anchor bolts and connections. All the sessions were very interactive and engaging. The valuable knowledge that i have gained in these two months will certainly be propitious to start my career as a structural engineer.
BHARATENDU PATEL
This course related to STAAD.Pro and case studies was beyond the conventional software training. It helped me conceptualise the idea behind the behaviour of any structure, understand the special requirements between steel and concrete structures. Real life case study and interactive sessions made the learning more impactful.
