Bhavin Shah conducted very interesting discussion on the concept of stiffness modifiers (SM) as per IS 1893 (Part 1):2016. The participants raised many interesting questions. Few of them are mentioned below: · What is the value of SM for PT beams? · What are the values of SM for composite beams? · What are the impact on analysis due to consideration of SM? · What are the impact of SM on strong column weak beam concept (IS 13920:2016) ? · And many more…
Please refer the recording of video for details. Your further comments/suggestions on the subject are welcome.
The concept of stiffness modifiers is introduced for the first time in IS 1893 (Part 1) : 2016. The clause no. 6.4.3.1 of the code defines requirements for structural analysis. It is mentioned in the clause that for structural analysis, we should consider reduced moment of inertial for RCC structures. For columns, 70 percent of Igross should be considered and for beams, 35 percent of Igross to be considered. This clause has generated many questions among the group of structural engineers. The overall objective of writing this article is to collate views/suggestions from the wider group of engineers. In the following section, I have attempted to give answers to few questions, received from different engineers. The answers are given for buildings having height less than 50 m. The comparison of stiffness modifiers with IS 16700 : 2017 & IS 15988 : 2013 will be done in the subsequent article.
Q-1 Why stiffness modifiers are introduced in the code?
Before this clause, in the structural analysis, we were considering 100% of moment of inertia for RCC beams and columns. In RCC member, the cracks will generate in the tension zone of concrete due to application of different loads. Due to these cracks, the moment of inertia of RCC member will be lesser than the gross moment of inertia. Hence, to account for the reduced moment of inertia of the cracked section, the concept of stiffness modifiers is introduced in the code.
Q-2 Why stiffness modifier value is higher for column than beam?
The pattern and extent of cracks will vary substantially from structure to structures and members to members even in a similar structure. It is very difficult to assign the unique values of the stiffness modifiers for different members. The values which are suggested in the code are based on the numerous experiments and might have been referred from different international standards. The stiffness modifier value for column is higher than the beam since the columns will have higher axial compression than the beam. Hence, the extent of cracks would be lesser in the columns as compared to the beams. Hence, the stiffness modifier value for column is higher than the beams.
Q-3 What impact stiffness modifiers will make on overall analysis?
There will be two major impacts in the structural analysis results as compared to the model using 100% of gross moment of inertia:
1) Since we are considering the reduced moment of inertia, the overall stiffness of the structure will reduce. Due to the reduced stiffness, the structure will be relatively flexible and hence would attract the lower seismic forces.
2) Drift of the structure will increase because it is relatively flexible.
Q-4 Are stiffness modifiers required only for analysis or for design as well?
The stiffness modifiers should be considered only for the structural analysis. The analysis results to be derived considering the stiffness modifiers. The structural design to be done with the conventional procedure considering the analysis results.
Q-5 Should we consider stiffness modifiers for all load cases or only for seismic loads?
The stiffness modifiers mentioned in IS 1893 (Part 1) : 2016 are only for ultimate condition, i.e. for seismic load case. The same modifiers should not be considered for the other load cases.
Q-6 Is it required to consider the value of torsional stiffness modifiers?
As per IS 1893 (Part 1) : 2016, no reduction to be considered for torsion constant, i.e. 100% of torsion constant to be considered for the structural analysis.
Q-7 What will happen if we consider unique stiffness modifiers for both serviceability and ultimate conditions?
Generally, the stiffness modifiers are different for serviceability and the ultimate conditions. As discussed above, the stiffness modifiers defined in IS 1893(Part 1) : 2016 are for the ultimate condition. If we consider the same stiffness modifiers for the serviceability condition as well then the moment at beam column junction will be higher and the span moment will be lesser as compared to the model without stiffness modifiers. In my opinion, the span moment may err on the unconservative side, if we consider the same stiffness modifiers for the serviceability condition.
Q-8 Is it required to consider stiffness modifiers for the shear wall and slab?
At present, I think it is not required for the shear wall & slab as per IS 1893 (Part 1) : 2016. It is only required for RCC beams and columns.
Q-9 Should we consider stiffness modifiers for design of foundations?
As mentioned above, the stiffness modifiers to be considered only for seismic load case. The structural analysis is to be performed with the seismic loads having stiffness modifiers. Thereafter, the analysis results should be used for design of the foundation.
Concluding remarks:
Consideration of the stiffness modifiers will reduce the seismic demand on the structure. But at the same time, it will result in relatively higher drifts due to increased flexibility of the structure. The stiffness modifiers value should be different for the serviceability and the ultimate conditions. The stiffness modifiers are to be used only for structural analysis. The comparison of stiffness modifiers with IS 16700 : 2017 & IS 15988 : 2013 will be done in the subsequent article. The above mentioned answers are based on my understanding. If you have further questions / difference of opinion, then please share details in the below comment box.
As a part of a “weekly live technical discussion” series, Bhavin Shah conducted a live session for design of the gravity columns as per IS 13920:2016. We had a highly interactive session today on design of gravity columns as per IS 13920:2016. The recording of the same is attached below.
Your views/suggestions are welcome for the next topics.
Bhavin Shah – Live Technical Discussion on design of gravity columns
For design of steel structures, IS 800:2007 was published before ~13 years. However, till today there are practical difficulties for performing the design in total compliance of the code may be due to several reasons like unavailability of detailed handbook with practical examples, gap between current software and IS code, doubts related to certain clauses of the code, interface gap areas between different IS codes, etc.
To bridge this gap area, we are glad to announce an online workshop which is specifically designed to address some of the above-mentioned practical difficulties. We intend to share the collated discussion with the concerned BIS technical committee for their perusal. The areas related to the software will be shared with the concerned software developer for further action at their end.
Let’s work together for improvement in our IS codes.
Bhavin Shah will be conducting the unique workshop named as “Decoding of IS 800:2007 for design of steel structures”on 18-SEP-20 at 3:30pm. The event is organised by Steel Structures & Metal Buildings (SSMB). The three-hour online workshop is specifically designed to address some of the practical difficulties being faced while design of the steel structures.
We are going to initiate “Weekly Live Technical Discussion” series on the topics for which industry is facing difficulties. The topic will be decided in advance and the technical discussion will be conducted through the live stream at youtube. The sessions will be organized on every Sunday at 11:00am.
The topic for upcoming Sunday (13-Sep-20) is “Gravity Columns”. The concept of gravity column is included in the latest IS codes related to earthquake resistant design. I have received many questions from the practicing engineers like how to model the gravity columns, design of gravity columns, which columns can be considered as gravity columns, etc. In the upcoming session on Sunday, the technical discussion will be focused on the gravity columns.
Live Technical Discussion on Gravity Columns
The brief note prepared by Bhavin Shah can be downloaded from the below link. The note was prepared and shared with a purpose of initiating the discussion in the structures group at telegram ( link for joining the group : https://t.me/structuresgroup ). Bhavin Shah will give overview of the note and take up each comment/question/suggestion, etc. sequentially. You may post your comments/details in advance. While, posting the comment, pl mention your name, city & organisation (optional).
Quality assurance is one of the extremely important area for the design engineering consultancy firms. The Quality Assurance covers the broader aspects like compliances to the prevailing regulatory standards, meeting functional & aesthetics requirements, ensuring strength and stability of structures, effective interface to avoid rework at site, generating error free design documents, inclusion of sustainability, safety, constructability aspects, etc. In my opinion to achieve the quality objectives, it is required to define them clearly at initial stage of the project so that the required aspects can be covered during appropriate stages of the projects such as conceptualization, basic engineering, detail engineering, execution, operation, maintenance, etc. In this blog, we are going to talk specifically about the structural engineering firms from the perspective of quality assurance. In upcoming blogs, we will talk more about the multi-disciplinary engineering firms.
Why Quality Assurance is so much important for the Structural Engineering firms or structural department in the multi disciplinary firm?
The prime role of a structural engineer in any project is to ensure the strength & stability of the structures, effective interface with the other disciplines to understand the functional requirements, meeting functional requirements along with the structural requirements, ensuring error free design and documents, compliance to the latest IS codes, generating drawings by taking care of the constructability, sustainability & safety aspects, etc. As structural engineers are taking the prime responsibility for strength and stability of the structure, the quality assurance is extremely important for structures function in the company. In the current scenario, Work From Home is becoming now new normal which again require increased attention on the quality assurance.
What are the common challenges which are being faced by the structures group for quality assurance?
Some of the areas where structures group are facing the challenges are mentioned below:
The projects are becoming increasingly fast-track, so the availability of time for the detailed design process is one of the major concern.
Apart from any definite statistics, I am of the opinion that the geotechnical failure is one of the major reason for the failure/collapse of the structures. Hence, interface between geo-technical engineer and structural engineer is very crucial for entire project cycle as well as during operation & maintenance.
IS codes which are generally used in the design are getting revised gradually to meet the global design practices. The typical revision cycle for IS code is in range of 10~15 years. So, whenever the standards are revised, the consultants have to upgrade the knowledge and the design practices to meet the requirements of the latest codes.
With the revisions of IS codes and due to internal bugs or improvements, the software are now revised more frequently than before. With every new version of the software, the design engineer has to become acquaint with the same and at the same time, it has to be ensured that the software is free of major bugs. Authentication / validation of the software is extremely important and the engineer should be aware about limitations of the same while performing the analysis & design.
Generally, the excel sheets are used in the design offices for manual design for the items which are not designed in the software. If the control design sheets are not maintained then after few years, there will be multiple versions available with the design engineers may be with nominal changes. Validation and control of these design means is also important for the quality assurance.
Frequency of revision of documents is increasing due to internal and external reasons. The chances of errors tend to increase while revising the documents.
For keeping pace with the global design practices, the engineers should be trained in the areas like performance based design, wind engineering, concepts of structural dynamics for earthquake resistant design, design of steel structures using Limit State Design method, etc.
Due to higher attrition rate, mid size company might stay in the continuous recruiting mode. Handover of documents and training of newly recruited engineers require substantial investment of time and efforts.
How SQVe can help the company for the above mentioned challenging areas?
Our unique mixture of technical depth and proficiency of organization development brings on board the effective and practical solutions for the above mentioned problems. The solutions will be highly customized to meet the requirements of the company. We strive towards making the companies strong internally.
We will associate with the company on need basis and the exact scope of work will be detailed out mutually. The entire work will be divided in three phases, firstly, identifying gap areas, secondly, working out the action plan for improvement and finally providing the hand holding support for the implementation.
Please get in touch with us to know more about thisunique service : contact@sqveconsultants.com . We will schedule online meeting to discuss it further.
ICE (Delhi) has invited Bhavin Shah to deliver expert talk on the above mentioned subject.
About the Technical Talk:
India has already witnessed devastating earthquakes in past and may experience even higher intensity earthquakes in the future. Also, Indian Standard codes related to earthquake resistant design are getting revised to align with the global design practices gradually. With these, there is a huge requirement for retrofitting of the existing structures related to earthquake resistant design. In the session, we will discuss briefly overall methodology for seismic assessment of the existing structures as per IS 15988:2013 and will explore usage of the Buckling Restrained Braces (BRB) for the same. The session will cover brief history of BRB, key advantages, limitations, design methodology, etc.
Organised by : Institution of Civil Engineers (ICE) Local Association Delhi.
