Unit III - Limit State Design for Bond, Anchorage Shear & Torsion

CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS

UNIT – III

LIMIT STATE DESIGN FOR BOND, ANCHORAGE SHEAR & TORSION 

Behaviour of RC members in bond and Anchorage



Design requirements as per current code



Behaviour of RC beams in shear and torsion



Design of RC members for combined bending shear and torsion

Compiled by: Mr. S. MANIKANDAN, Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College.

2


2 Marks [Questions & Answers] 01) Write down the value of design bond stress for M 30 grade concrete. Ans.: As per clause 26.2.1.1 of IS 456:2000, the value of design bond stress (bd) for M 30 grade concrete is 1.5 N/mm2. 02) Why is bond stress more in compression bars than that in tension bars? Ans.: The intensity of adhesion force at the adhesion force at the junction of steel and concrete is called the bond stress. The surface of contact of steel bar and concrete gets improved under compression whereas under tension it leads to separation. So the bond stress is more than that in tension bars. 03) Define: Flexural bond & Anchorage bond. Ans.: (i) Flexural bond: Flexural bond (bf) is one which arises from the change in tensile force carried by the bar, along its length, due to change in bending moment along the length of the member. Evidently, flexural bond is critical at points where the shear is significant. Since this occurs at a particular section, flexural bond stress is known as local bond stress. (ii) Anchorage bond: Anchorage bond (bd) is that which arises over the length of anchorage provided for a bar. It also arises near the end or cutoff point of a reinforcing bar. The anchorage bond resist the ‘pulling out’ of the bar if it is in tension or ‘pushing in’ of the bar if it is in compression. 04) Write about local bond and anchorage length? Ans.: (i) Local Bond: Local bond (or) Flexural bond at a point in a R.C.C. member is the rate of change of tension in the steel at the stipulated section. Adhesion and friction are the main components of local bond in mild steel smooth bars. (ii) Anchorage length: Anchorage length is defined as the length of bar necessary to develop the full strength of the bar. S.M.K./A.P./CIVIL/S.E.C.

UNIT III – LIMIT STATE DESIGN FOR BOND, ANCHORAGE SHEAR & TORSION

3

05) What do you understand by the term Anchorage? Ans.: With modern high bond bars the mechanism of reinforcement anchorage is due to (i) Adhesion of concrete and steel, (ii) Shear strength of concrete and (iii) Interlocking of ribs with concrete. 06) What is development length? (or) What do you understand by development length of bar? Ans.: The development length (Ld) is defined as the length of the bar required on either side of the section under consideration, to develop the required stress in steel at that section through bond. As per clause 26.2.1 the development length (Ld) is given by,  s Ld

=

 (0.87 fy) =

4 bd

4 bd

07) Differentiate shear failure and bending failure. Ans.: (i) Shear failure: Shear failure observed in reinforced concrete structures are diagonal tension failure, flexural shear failure, shear compression failure and shear bond failure. (ii) Bending failure: Flexure or bending failure is commonly encountered in structural elements of reinforced cement concrete. Example: Beams and Slabs, which are transversely loaded, Flexure usually occurs in combination with transverse shear and sometimes with axial compression or shear. 08) What do you mean by primary and secondary torsion? Ans.: (i) Primary torsion: Primary torsion (or) Equilibrium torsion is induced by an eccentric loading and equilibrium conditions alone suffice in determining the twisting moments. (ii) Secondary Torsion: Secondary torsion (or) Compatibility torsion is induced by the need for the member to undergo an angle of twist to maintain deformation compatibility, and the resulting twisting moment depends on the torsional stiffness of the member.

TWO MARK Q & A and BIG QUESTIONS

4


09) Write down the effect of torsion in RC beams? Ans.: Generally, the R.C. structures subjected to torsion of varying magnitude in addition with flexure and shear. Torsion moment occurs as a secondary effect in many structures. Example: (i) Peripheral beams in each floor of a multistory building are subjected to significant torsion loading in addition in addition to conventional flexure and shear. (ii) The beams supporting cantilever canopy slabs are always subjected to torsion. 10) How to overcome torsion on beams? Ans.: When torsion is present, different methods to overcome torsion is done by proper design as per Indian Standard Code. When torsion is prevent along with bending shear, is recommends the use of are equivalent shear for which the shear steels are calculated. Again Indian Standards when torsion is present as combined with bending, an equivalent bending moment is calculated and reinforcement for this equivalent bending moment is provided as longitudinal steel. 11) How is the reinforcement designed and provided in a rectangular beam when the equivalent torsion Mt, exceeds the design bending moment Mu? Ans.: In case of Mt exceeding Mu, as per clause 41.4.2.1 of IS 456: 2000, the longitudinal reinforcement should be provided on the flexural compression face. This is provided such that the beam can also withstand an equivalent moment Me2 being taken as acting in the opposite sense to the moment Mu. Me2 = (Mt – Mu) Transverse reinforcement comprises of two legged closed loops enclosing the corner longitudinal bars. The area of reinforcement required is given by the clause 41.4.3 of IS 456: 2000. 12) Define torsional shear. Ans.: Generally, the R.C. members subjected to torsional moment associated with shear force is known as torsional shear. (Tu / b) i.e.,

Torsional Shear stress, t = 1.6 bxd

S.M.K./A.P./CIVIL/S.E.C.


5

16 Mark [Questions] CE2306 (R 2008) - April / May 2015 (A.U. Chennai) – Q.P. Code: 71258

(Or)

CE2306 (R 2008) – November / December 2014 (A.U. Chennai) – Q.P. Code: 91243

(Or)

CE2306 (R 2008) – May / June 2014 (A.U. Chennai) – Q.P. Code: 51242

(Or) TWO MARK Q & A and BIG QUESTIONS

6



(Or)

CE2306 (R 2008) - May / June 2013 (A.U. Chennai) – Q.P. Code: 21210 (a) Derive the expression to determine the shear strength of RC section.

(16)

(Or) (b) An overhanging beam had 6 m span from support to support and 2 m hanging. The cross section of the beam is 300 mm X 500 mm and the design load applied through was 40 kN/m. 4 bars of 20 mm diameter plain bars are provided with 50 mm effective cover. What is the maximum bond stress developed and fine the anchorage length required for the overhanging portion.

(16)


(Or)



7

CE2306 (R 2008) - May / June 2012 (A.U. Chennai) – Q.P. Code: 10229

(Or)

CE1354 (R 2004) - April / May 2011 (A.U. Chennai) – Q.P. Code: 55232

(Or)

TWO MARK Q & A and BIG QUESTIONS

8


Compiled By:

Mr. S. MANIKANDAN M.E. (Structural Engg.) Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College, Arasampatti, Pudukkottai – 622507. Email: [email protected]


Unit III - Limit State Design for Bond, Anchorage Shear & Torsion

Recommend Documents