Strategically Preparing RCC and Prestressed Concrete for GATE Civil Engineering
This article on RCC and Prestressed Concrete has been written by Rohit Sachdeva. He graduated from Delhi College of Engineering (now DTU) in 2012 (a gold medalist in his batch) in Civil Engineering branch. Then he appeared in Civil Engineering (CE) paper in GATE 2017 and secured an All India Rank (AIR) of 93.
In this blog, I will be discussing a very important subject of Civil Engineering: RCC and PSC Design. Despite being a very vast topic, its weightage in recent years in GATE has been medium. On the other hand, studying this topic in detail is most important for all domains: structural, geotechnical, transportation and water-resource engineers, for both design and field related job.
However, I will be focusing how to prepare this subject from GATE point of view, and which topics to target so that unnecessary time is not wasted in studying this subject in too much detail. Many students tend to skip this subject because of its vastness; but if the targeted approach is followed, then questions from this topic can be answered with almost 100% accuracy.
A rough breakup of questions of various topics in last 30 years in GATE is as follows:
|Sl No||Topic||No. of Questions|
|1 mark||2 marks|
|1||Fundamentals and Workmanship||34||6|
|2||Singly Reinforced Beams||7||18|
|3||Doubly Reinforced Beams||1||2|
|6||Bond & Development Length||5||2|
|10||Limit State of Serviceability||1||1|
|12||Prestressed Concrete (PSC)||7||22|
An analysis of last 5 years of GATE papers reflects that RCC and PSC carry 4-8 marks. The most important reference material for this subject is Indian Standard Code IS 456:2000. There are a lot of formulas directly from IS 456 (particularly from Sl No. 5-8 from which questions are asked) which need to be learned. It is therefore advised to keep IS 456 handy for any clarification of symbols in the formulas, and make a formula sheet for future revision. The textbook(s) to be referred are mentioned in a separate comprehensive blog which you should refer.
Time required for preparation
12-14 days (if you have 8-10 month of preparation) with 4 hours daily
7 days (if you have 4-5 months of preparation) with 7-8 hours daily
In this blog, I will try to put detailed hints/approach to solve the questions so that it becomes very easy to cover the entire subject. Let’s start our topic-wise discussion (most important concepts are bold & italics):
1. FUNDAMENTALS AND WORKMENSHIP (1/2 DAY)
This is the backbone of Concrete Design & carries maximum weightage. It is very important from point of view of interviews also.
Characteristic strength & target mean strength, confidence limits, characteristic load, Young’s modulus of concrete (of 3 types), creep coefficient; Types of steel reinforcement (ordinary, hot-rolled and cold-treated & their stress-strain graph); Limit State Design: Permissible stresses in Concrete (0.45fck) and Steel (0.87fy), types of limit states (collapse & serviceability), load combinations, FOS (workmanship) of Steel vs Concrete.
As we can see, this small section carries a lot of weightage; not much numerical practice is required but concepts should be crystal clear.
** NOTE: From this onwards, each topic will be discussed from both WSM and LSM approach. It is advisable to draw the linear strain & stress diagram (for WSM) or linear strain & parabolic stress diagram (for LSM) in any question. It becomes easier to solve.
2. SINGLY REINFORCED BEAMS (2 DAYS)
Working Stress Method (WSM): Modular ratio, calculation of actual depth of N.A. (equate moment of area) & critical depth of N.A. (equate permissible stresses in steel & concrete), Under-reinforced, Balanced & Over-reinforced sections, calculation of Moment of Resistance (MOR) in these 3 cases (calculate C or T depending on case and multiply with lever arm). Practice total 5 numericals to find MOR (given section properties) or stresses (given load).
Limit State Method (LSM): Calculation of actual depth of N.A. (C=T) and critical depth of N.A. (as per IS 456), Design formula for MOR (either under-reinforced or balanced section). Practice 3 numericals to find MOR.
3. DOUBLY REINFORCED BEAMS (1 DAY)
WSM: Calculation of actual depth of N.A. changes from above case (due to stress in compression steel). Draw the stress diagram always to find MOR. Practice 2 numericals to find MOR.
LSM: Most probably question will not be asked. Calculation of actual depth of N.A. is based on C (concrete+steel)=T again. Practice 1 numerical to find MOR just to get an idea.
4. FLANGED (T or L) BEAMS (1 DAY)
Effective width of flange (isolated or beam/slab monolithic), actual depth of N.A. (either in flange or web), calculation of MOR.
WSM: Assume it in flange first, if wrong then web; stress diagram is linear. Practice 2 numericals of MOR.
LSM: Since stress diagram is not linear, it gets complicated. Assume it in flange, if correct find MOR; if wrong, skip the question.
5. SHEAR (1/2 DAY)
Minimum shear reinforcement, the spacing of stirrups (vertical & inclined both), shear contribution by bent-up bars, maximum spacing of stirrups. These formulas should be noted in the formula sheet.
6. BOND & DEVELOPMENT LENGTH (1/2 DAY)
Bond stress calculation, increase in permissible bond stress in HYSD bars and in compression, development length (in terms of bond stress), development length at supports.
7. TORSION (1/2 DAY)
Equivalent shear force & bending moment due to torsion, the spacing of shear reinforcement when torsion is present, side face reinforcement.
8. COLUMNS (2 DAYS)
This is one topic from which a question is highly probable.
WSM: Effective length of columns, Load carrying capacity of short column*; long column* (use of Cr), min. & max. % of steel, min. no. & dia. of main bars, max. spacing b/w main bars, dia. and spacing guidelines of transverse reinforcement (ties).
LSM: Minimum eccentricity in column (& corresponding load carrying capacity*), load carrying capacity of column with no moment*.
*There are different (similar looking) formulas in all these cases and it should be very carefully checked which one to apply. For circular column with helical ties, multiply with 1.05.
9. FOOTINGS (1/2 DAY)
Calculation of size of footing, calculation of depth based on bending moment, one-way shear and punching shear.
10. LIMIT STATE OF SERVICEABILITY (1/2 DAY)
Questions from this topic are now being asked more commonly (write in formula sheet).
Effective span of beam with different supports, deflection (span/250; span/350 or 20mm), span to depth ratios for cantilever, simply-supported and continuous beams, slenderness limit for lateral stability (clear distance b/w lateral restrains), reinforcement in beam (min. & max. tension; compression), slab (min. reinforcement; max. dia; max. distance b/w main & distribution bars)
11. SLABS (1/2 DAY)
Topics mentioned in (10), 2-way slab provisions for extent of tension reinforcement (at continuous & discontinuous edge) from IS 456.
12. PRESTRESSED CONCRETE (3 DAYS)
Very important topic, and mostly a 2-mark subjective (NAT) question will be asked.
Basics of prestressing, pre-tensioning & post-tensioning, load balancing method, straight/bent/parabolic cables, stresses at transfer and final stages, stress concept method (P-line), strength concept method (C-line), cracking moment, Loss in Prestress (VERY IMPORTANT!).
All types of losses are important and should be given equal importance. Some are conceptual (like elastic shortening) and some are formula-based (like shrinkage of concrete). Practice 5-10 numericals of prestressing properly.