This loss occurs only in the post tensioned members. The resultant stress can be found out as, \(\text{Total stress at top fibre} = \frac{P}{A} + \frac{M}{Z} - \frac{Pe}{Z}\), \(\text{Total stress at bottom fibre} = \frac{P}{A} - \frac{M}{Z} +\frac{Pe}{Z}\). Relaxation is assumed to mean the loss of stress in steel under nearly constant strain at constant temperature. It is weaker than static friction. Get Instant Access to 1000+ FREE Docs, Videos & Tests. {\bf{e}}}}{{\bf{Z}}}\), b= 300 mm, d = 900 mm, e = 350 mm, P = 700 kN, Where, Z =\(\frac{{\bf{I}}}{{\bf{y}}}\)=\(\frac{{\frac{{{\bf{b}}{{\bf{d}}^3}}}{{12}}}}{{\frac{{\bf{d}}}{2}}} = \frac{{{\bf{b}}{{\bf{d}}^2}}}{6} = \frac{{300 \times {{900}^2}}}{6} = 40.5 \times {10^6}{\bf{m}}{{\bf{m}}^3}\), Stress at top fibre at midspan due to prestress =\(\frac{{\bf{P}}}{{\bf{A}}} - \frac{{{\bf{P}}. ins.dataset.adChannel = cid; There are small frictional losses in the jacking equipment. 4. (AE 2013), 33. To modify the behavior of the member or structure in such a way to make it more suitable for its intended purpose. Tendon force is greatest at the jacking end, and decreases with distance from the end because of the friction which occurs during jacking between the tendon and the duct. electric poles are generally made of (AE-2008), 12. Besides giving Questions and answers for Test: Prestress Concrete - 2, EduRev gives you an ample number of Online tests for practice. Answer: Loss of prestressing in " pre-tensioned " members 1. two of the above, 3) If gravity pulls you down toward the center of Earth with a force of 500 N, how much upward force does the ground exert on you?a. Consider the statement (AE 2013) (d) friction. 2) Losses due to creep or relaxation of steel: Important PointsLosses in Post-tensioning, High-tensile bars threaded at ends are used in. 1. Prestress does not remain constant (reduces) with time. Wherever possible, the total chloride content of the concrete should be determined. III. How are they constructed? Loss of Prestress Tensile strength of high tensile steel is in the range of 1400 to 2000 MPa and if initially stressed up to 1400 MPa, there will be still large stress in the high tensile reinforcement after making deduction for loss of prestress. all surfaces have some roughness.b. It should be considered specifically when new concrete is cast against hardened concrete. On most of the previously constructed PCPs, one of the following prestressed orientations was employed: In order to avoid the danger of stress corrosion in pre-stresses concrete works, which of the following is not permitted to be used, The following loss does not occur in Pre tensioning of concrete. The consent submitted will only be used for data processing originating from this website. Score: 4.7/5 (32 votes) . moving object to change direction.d. ins.style.width = '100%'; This loss is generally of the order of 2 to 8% of the initial stress. If a concrete beam is subjected to an eccentric prestressing force of magnitude p, at an eccentricity e, the stress developed at the bottom fiber of the beam is (FC 2018), 44. Time-dependent losses such as shrinkage of concrete, creep and stands relaxation. So the loss in prestress occurs due to friction between the concrete surface and the tendon in the process of tensioning.The loss of friction is also accompanied by the wobble effect. the moon.c. Prestressed concrete is a system devised to provide sufficient precompression in the concrete beam by tensioned steel wires, cables, or rods that under working conditions the concrete has no tensile stresses or the tensile stresses are so low that no visible cracking occurs. (adsbygoogle = window.adsbygoogle || []).push({}); The system suitable for pre- tensioned prestressed concrete members is (FC 2018), 41. moving object to change speed.c. stationary object to start moving.b. In prestressed concrete stress block is recommended with appropriate safety factors. The beam supports a live load of 1.2 kN/m. For pre-stressed concrete, it is desirable to use (AE-1998), A) High strength concrete and high tensile strength steel, B) Low strength concrete and low tensile strength steel, C) High strength concrete and low tensile strength steel, D) High strength concrete and low tensile strength steel, SHOW ANSWER A) High strength concrete and high tensile strength steel, 3. Total loss of prestress is around 15-20%. var lo = new MutationObserver(window.ezaslEvent); 0 Nb. Loss due to the relaxation of strands is categorized as a long-term loss of pre-stress in a pre-stressed concrete member. A had a 4-storey building which was constructed by Engineer B and at the same time the contractor. The function of reinforcements is pre-stressed Concrete is to (2019 AE), (C) Provide sufficient torsion resistance, (D) Produce initial compressive stresses in concrete in the torsion zone, SHOW ANSWER (D) Produce initial compressive stresses in concrete in the torsion zone, 46. Let us discuss the types of losses in prestress given in the article below. Precast prestressed concrete used in the construction of dynamically loaded structures such as railway bridges and machine foundations and in the construction of long-span floors and roofs. Pages 324 This preview shows page 310 - 313 out of 324 pages. minimum clear spacing between groups of cables or ducts of grouped cables shall be greater, \(\frac{{\bf{P}}}{{\bf{A}}} - \frac{{{\bf{P}}. 3. The arch form is one of the earliest solutions to dealing with a material that has no tensile strength. To improve the resistance of the member to the dead and live loads. = 40h = 0.2 m Prestressing is done to reduce or eliminate the internal tensile stresses by applying an initial compressive axial force to the concrete. No loss due to elastic deformation if all the wires are simultaneously tensioned. Loss due to Anchorage Slip: In a post-tensioned member, the is loss of prestress due to the consequent reduction in the length of the tendon. Save my name, email, and website in this browser for the next time I comment. The gradual reduction of this introduced compressive stress in a prestressed member due to various reasons is called losses of prestress. In designing prestressed concrete flexural members, the breadth of compression flange is determined on the basis of (AE 2014), SHOW ANSWER b) ultimate flexural strength, 34. LOSS OF PRESTRESS FORCE CHAPTER 4 Prepared by: Suaidah Shima Binti Shahrin (2019695192) Nur Syamimi Binti Harun Prestressing is the process of introducing compressive stress to the concrete to counteract the tensile stresses resulting from an applied load. The high tensile steel tendon in a prestressed concrete member (FC 2018), b) Does not remain under a constant condition of either stress or strain, SHOW ANSWER b) Does not remain under a constant condition of either stress or strain, 43. Loss in prestress = Initial prestress - effective prestress. SHOW ANSWERc) will change if loading in the beam is changed, 29. High strength concrete for prestressing is required due to the following reasons: (i) Due to the large prestressing force to be applied by tendons, large bearing stresses get developed in concrete at the end by anchoring devices. pre-stressed concrete structure for long spans. These offer great technical advantages in comparison with other . (If pre-tensioning is asked then M40 will be the answer), The minimum grade of concrete that can be used for pre-tensioned beam system is. ins.className = 'adsbygoogle ezasloaded'; What is maximum bending moment that can be applied to the section WITHOUT causing tension at soffit of the beam? 5 AASHTO Lump-Sum Losses Total Loss Type of prestressing steel f'c = 4,000 psi f'c = 5,000 psi Pretensioning strand 45,000 psi Post-tensioning a wire or strand 32,000 psi 33,000 psi Bars 22,000 psi 23,000 psi a Losses due to friction are excluded. Wires of 5 mm or 7 mm are adopted. Force and Pressure Class 8 MCQs Questions with Answers. 3 alone . From the above factors, we can say thatLoss of pre-stress is not directly related to the grade of concrete. window.ezoSTPixelAdd(slotId, 'stat_source_id', 44); Circular prestressing is generally used in (AE-98), 4. Continue with Recommended Cookies, var cid = '7377982948'; In the field of bridge engineering, the introduction of prestressed concrete has aided the construction of long-span concrete bridges. Theloss due to anchorage does not occur in pre-tensioned members. 3. Manage Settings World's best MCQs website Civil Engineering, Consider the following factors :1. 22. ins.dataset.fullWidthResponsive = 'true'; 1343-1980, for pre-tensioned concrete is (AE 1998), 6. RMS CIVIL FACTORY 500 Nd. It is ITD's policy to ignore the effects of this . RMS CIVIL FACTORY Losses in Prestress Due to Creep of Concrete 6. Those which affect the ultimate moment capacity of a prestressed concrete beam would include. Consider m = 6. The total acid-soluble chloride content should be calculated from the mix proportions and the measured chloride contents of each of the constituents. However ,if a prestressed concrete member is cracked , it behaves in a manner similar to that of a reinforced concrete sections. Assume a prestress loss of 10% of the initial prestressing force. The cable is inserted into the duct. To use this online calculator for Loss of Prestress due to Slip, enter Area of tendon (A), Modulus of Elasticity of Steel Reinforcement (Es), Slip of anchorage () & Length of cable (L) and hit the calculate button. 2. For Post-tensioning minimum grade of concrete used is M-30. the moon is smaller than Earth.c. The change of prestress loss is highly related to the distribution of cracks. Upward pressure provided by the parabolic cable = 8Ph/L2 An abstract inherited property cannot be overridden in a derived class. High strength steel used in prestressed concrete can take how much maximum strain? For Pre-tensioning minimum grade of concrete used is M-40. Tendon force P x at the distance x from the jacking end can be expressed as . Fully pre-stressed concrete beams (AE 1997), a) Resists the full live load by pre-stress, b) Resists all the working loads by pre-stress, c) Resists a part of the load of pre-stress, SHOW ANSWER b) Resists all the working loads by pre-stress, 21. P/A = Pe/Z Spring keeps proper spacing for the wire. In a prestressed concrete beam, the loss is due to the following: Elastic shortening Shrinkage of concrete Creep of concrete Frictional loss Relaxation of steel Anchorage take-up Sanitary and Waste Mgmt. So, the percentage of loss of stress due to shrinkage in pre-tensioned members is 6 %. Introduction 1.1 Background The idea of prestressed concrete has been around since the latter decades of the 19th century, but its use was limited by the quality of the materials at the time. Types of Losses in Prestress 2. The increase in the resultant forces is due to a more or less constant lever arm between the forces, characterized by the properties of the composite section. Determine the final concrete stress (MPa) at the bottom fiber of the midspan section when the beam sustains an additional live load of 10 kN/m. sliding frictionc. A pre-stressed concrete induces artificially stresses in a structure before it is loaded. Prestressed Concrete Sources. It considers the actual behavior of the structureunderworking or service loads. forces of tension and compression change but lever arm remains unchanged, forces of tension and compression remains unchanged but lever arm changes with moment, both forces of tension and compression as well as lever arm changes, both forces of tension and compression as well as lever arm remains unchanged. Fully pre-stressed concrete beam means (AE 2010), c) working loads are completely resisted by prestressing force, SHOW ANSWER c) working loads are completely resisted by prestressing force, 30. 2. After stretching the bars to the required length, they are tightened using nuts against bearing plates provided at the end sections of the member. SHOW ANSWER d) because high strength is desirable. Loss due to shrinkage of concrete. The various limit state of collapse as per IS 1343: 2012are: The various limit state of serviceability as per IS 1343: 2012are: Reference: IS 1343 - 2012 (Page 42:, Clause 23), Loss of pre-stress is not directly related to, 1. ins.id = slotId + '-asloaded'; } is a force.b. two of the above, 2 ) Unlike friction, gravitya. 3. A = 46,400 mm2, I = 75.8 x 107 mm4, lifting frictionc. (2019 AE Nilgiri), 51. 40 mm or maximum size of cables or bar of nominal maximum size of aggregate plus 5 mm. and the stress variation in the nonprestressed reinforcement is about 130 Nlmm2 (18.9 ksi). The gradual reduction of this introduced compressive stress in a prestressed member due to various reasons is called losses of prestress. School Savitribai Phule Pune University; Course Title CIVIL 123; Uploaded By MagistrateOctopusPerson341. If a simply supported concrete beam, prestressed with a force of 2500 kN, is designed by load balancing concept for an effective span of 10 m and to carry a total load Of 40 kN/m, the central dip of the cable profile should be, Let the dip of the cable be h. volume, 4 ) Jody has a mass of 50 kilograms. ins.style.minWidth = container.attributes.ezaw.value + 'px'; I. It is prestressed with 9 numbers of straight 7mm diameter wires at 0.8 times the ultimate strength of 1570 N/mm2. II. After five years, the building developed cracks and its stairway eventually gave way and collapsed due to poor quality of materials used in the construction, resulting to injuries to some lessees. so M40 will not be the answer. Approximate Prestress Loss Values For Post-Tensioning . Basically the load-carrying mechanism is comprised of a constant force with a changing lever-arm as in case of prestressed concrete sections and a changing force with a constant lever arm prevailing in reinforced concrete sections. 31. An example of data being processed may be a unique identifier stored in a cookie. rolling friction, 1 ) Gravity always causes objects toa. An abstract method is implicitly a virtual method. See Page 1 . Final Gain = -9.96 ksi. {\bf{e}}}}{{\bf{Z}}}\), \(\frac{{\frac{{{\bf{b}}{{\bf{d}}^3}}}{{12}}}}{{\frac{{\bf{d}}}{2}}} = \frac{{{\bf{b}}{{\bf{d}}^2}}}{6} = \frac{{300 \times {{900}^2}}}{6} = 40.5 \times {10^6}{\bf{m}}{{\bf{m}}^3}\), \(\frac{{700 \times {{10}^3}}}{{300 \times 900}}- \frac{{700 \times {{10}^3} \times 350}}{{40.5 \times {{10}^6}}}\), \(\text{P} = 4 \times \frac{\pi}{4}\times 10^2\times 500\), \( \frac{157\times 10^3}{150000} - \frac{M}{15\times 10^6} +\frac{157\times 10^3\times 100}{15\times 10^6} = 0\), \(\frac{{{\Delta f_p}}}{f} \times 100 = \frac{{60}}{{1000}} \times 100 = 6\%\), \(M = \frac{{\left( {w - P} \right){L^2}}}{8}\), \(\sigma = \frac{P}{A} + \frac{{My}}{I} - \frac{{Pey}}{I}\), \(\frac{{0.0001}}{{{{\log }_{10}}\left( {t + 2} \right)}}\), \(\frac{{0.0002}}{{{{\log }_{10}}\left( {t + 2} \right)}}\), \(\frac{{0.0003}}{{{{\log }_{10}}\left( {t + 2} \right)}}\), reductions of the prestressing force are termed as the losses in prestressing, loss due to friction does not occur in pre-tensioned members. The box has no friction when it is stationary.d. This loss is generally of the order of 2 to 8% of the initial stress. 1 alone Who should the lessees sue for damages? d, 7) Which pair of forces in question 5 produces a net force of zero?a. loss due to anchorage does not occur in pre-tensioned members. c. Earth already orbits the sun. Earth already orbits the sun.d. Figure 14 shows the 3-D time-dependent prestress loss of the loaded beam measured by the BOTDA sensors in the smart steel strand at the normal service limit state. 50 mm or 3 times diameter of cables or nominal maximum size of aggregate plus 5 mm. Consequently, the required pavement thickness for a given load is significantly less than that required for conventional concrete pavement. after transfer of the prestress force to the girder (loss due to prestress combined with gain due to the member self weight). It is similar to the creep of concrete. The spacing of stirrups in a pre-stressed beam should not exceed (AE 2015), c) 0.75 times the effective depth of the beam, d) 0.3 times the effective depth of the beam, SHOW ANSWER c) 0.75 times the effective depth of the beam, 18. In pre-tensioned work, the cover of concrete measured from the outside of the prestressing tendon shall be at least (AE 1998), 5. The maximum cube strength of concrete used for a pre-stressed member is (AE 2013), 16. IV. The clear cover to cables in a PSC post-tensioned girder should not be less than (2019 AE Nilgiri), March 4, 2022March 4, 2022 Question is Select the incorrect statement., Options are (A) The loss of prestress is more in pre-tensioning system than in post-tensioning system., (B) Pretensioning system has greater certainty about its durability., (C) For heavy loads and large spans in buildings or bridges, post-tensioning system is cheaper than pretensioning system, (D) none of the above, (E) , Leave your . Loss due to Relaxation: Relaxation is assumed to mean the loss of stress in steel under nearly constant strain at a constant temperature. is less affected by gravity.d.
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