Civil Engineers are supposed to abide by the code of practice. All design and construction are supposed to be done as per the code of practice in each country. In India, civil and structural engineers have to follow IS 456 code clauses for design and construction of RCC structures. Though many engineers due to pressure, ignores certain stringent provisions, there are some clauses that are difficult to interpret. Code clause interpretation is sometimes challenging and sometimes interesting. In this blog, I will introduce you to 5 such interesting or challenging IS code clauses.
Before getting in to those IS code provisions, I will mention some important points that you should consider when interpreting a clause from a code.
Always read notes below the clauses: In many cases, the code clause will have notes and sub notes under the clauses. Never miss to read them. I will be showing examples in the sections below. Please read on the blog below
If you are referring tables, the table will be coming from a clause. Ensure you read the clause in addition to the table. The clause will have some thing more to tell you about something important.
Always remember that reading some IS code provision needs a skill. Don’t just assume you are right if you are even slightly confused. Seek help and discuss to clarify.
DISPROPORTIONATE COLLAPSE CONTROL IS 875 PART 2 Cl. 0.3.3
This code and clause may be surprising to even some of the experienced civil engineers. Most structural engineers refer this code IS 875 Part 2 only for referring to Live load values for different building types. Most miss to appreciate that there are many important clauses too in this code of practice.
Another reason that this clause remains hidden is that, this clause is given before even the tables for load reference is given. Most engineers and engineering students start treading from the page where the load and building classification is provided.
This clause in the live load code is in fact talking about the need of a robust scheme in a rather confusing way. Only a careful interpretation will convey its meaning. For clarity, I will first state the clause and then provide its explanation. The clause states that ‘’The buildings and structural systems shall provide such structural. integrity that the hazards associated with progressive collapse such as that due to local fail caused severe overloads or abnormal loads not specifically covered therein are reduced to a level consistent with good engineering practice.’’
The meaning of this can be really confusing. I have seen even senior structural engineers saying that this clause just means that detailing will take care of collapse control. That’s not the meaning of this IS code provision. This specifically makes a structural design engineer responsible for disproportionate or progressive collapse control. The IS code clause says that the scheme should be in a way that a good detailing automatically takes care of any accidental collapse. Now note that a detailing can take care of this if and only if there is a lot of bridging ability or alternate load paths in the building. What I mean is, if one member fails then a careful detailing should provide an alternate load path so that a continuous or a progressive collapse do not occur. Please see the video for more clarity.
EFFECTIVE LENGTH: ANNEX E - IS 456
This is an IS code provision which is mis interpreted and less understood. Many consultants who understand this too usually compromise for easiness of design and economy of design. The fact is that if structural consultants understand this clause in the right way, it is easy to capture this clause by using software tools like ETABS and still achieve economy.
In design of columns, effective length plays an important role in arriving at slenderness and slenderness moments. You will have to see the clauses Most engineers arrive at the effective length considering the values shown in Table 28. However, the column ends are neither fully fixed or pinned. We have not pinned or fixed all the column beam junctions during analysis. We have allowed the stiffness to play and decide the fixity. In fact, it is not fully fixed. Then why use this table 28 in IS 456. If you read section E.3 under Annex E, it says ‘’ For normal usage assuming idealized conditions, the effective length may be assessed on the basis of Table28.’’ Code is not explaining what is idealised conditions and normal usage. In my opinion this table can be used only if you are designing less significant building for gravity alone. However, this was applicable when a time IS 1893 was not introduced. Now all buildings small and big is required to be designed for seismic forces and hence this table or E.3 don’t come in to picture.
Even if you look at clause 25.2, it asks to use Annex E for effective length calculation in the absence of more exact analysis and not table 26. Here more exact analysis should mean P delta analysis and if you do this, you can completely do away with moment magnification by slenderness calculations.
You can see video where I have explained this more clearly.
MINIMUM ECCENTRICITY IN COLUMNS: Cl 25.4 IS 456
This code clause is very rarely understood in the right way. There is a storey I have to tell you on how I stumbled up on this clause. I was working in an engineering firm years back and my senior manager in the company never allowed me to design columns in ETABS. And you know how difficult it is to design columns manually since the forces are interactive and the effective length calculations are challenging as per the point preceding this section in this blog. I wanted to know why ETABS is not matching the manual design. In the process I found out many reasons but this clause stands out and I have to mention this for sure as no one in my circle at that time when I discussed this point had appreciated this structural requirement of design of columns.
The code says ‘’ All columns shall be designed for minimum eccentricity, equal to the unsupported length of column/ 500 plus lateral dimensions/30, subject to a minimum of 20 mm.
Where bi-axial bending is considered, it is sufficient to ensure that eccentricity exceeds the minimum about one axis at a time’’
In this clause, the first part is understood by every civil engineer who has learnt design of columns. However, the second part of it can be understood if you really appreciate why eccentricity moment is considered. The reason for considering eccentricity is that, eccentricity is a result of site inability to exactly meet the beam column joint at the centreline. There will be some eccentricity. Now the code is saying that in both direction you don’t have to consider eccentricity simultaneously. This provision is not noticed by many. If you look any text book on structural design or even in SP16 which is a structural design aid for IS 456, do not consider this provision.
Please see the video where I have explained this with more practical moment values.
TORSION IN BEAMS Clause 41.1, IS 456
Torsion in beams is a less understood topic. Many blindly follows the code provision in 41.3.1 and 41.4.2. I have seen some structural engineering professors mandating design of torsion as per this clause. This is not always correct as releasing torsion can be economic at the same time doing exactly same magnification as this clause says. In reality, the thumb rule way of designing for torsion and releasing for torsion is doing the same thing. Both magnify the moments and shear. This means that design of torsion as per the equation is same as releasing torsion. However, point to note is that the code itself ask you to refer this clause for equilibrium torsion alone. For compatibility case, you can release torsion. Read clause 41.1 and the note under 41.1
I have a separate blog here for this topic which is comprehensive.
LOAD COMBINATION: REVERSAL OF STRESSES & STABILITY: TABLE 18 IS 456
For a general building structure, we consider Dead load, Live load, Wind load and Earth quake load. Some of these loads can act simultaneously and we have to consider its combined effects for stability and strength. The various load combinations are mentioned in IS 875 – PART 5 which we rarely refer. All the load combinations are also mentioned in Table 18 of IS 456 and we often refer only this table. If you go by this, you will usually end up with 26 factored load combinations and 18 unfactored load combinations. However, there is a combination that mostly young engineers miss to observe. 0.9DL + 1.5Eq. The question is why we need this combination when we already have 1.5DL+1.5Eq. 1.5 is more than 0.9, then why will we need this 0.9DL combination. If you refer the notes and sub notes as I mentioned in the initial section, you can see that there is a sub note that tells the significance of this load combination. It says that the combination 0.9DL+1.5Eq will be significant to check stability and stress reversal. The logic is that, the Dead load considered can be more than actual dead load that might occur due to changes or the fact that the Earth quake can occur during construction before the full DL is in place.
I have explained stress reversal in a webinar and you can see it here. If you have any questions on the stability part, contact me here Mention your need in the message box so that I understand that you need more information on this topic or any other. You can also use our forum to post your question or even comment in this blog section by logging in or comment in our structural YouTube channel.
All design codes are provisions that a civil engineer should adhere to. However, the engineer has some discretion to interpret it. Also, it is important to have the skill to read through the code and interpret it correctly using all the logic behind a confusing clause. If you are an entry level structural engineer or a structural or civil engineering student, discuss with seniors and master this art. You can ask your questions in our structural forum