Prestressed Concrete
Practice QuestionsMarks 1
1
A post-tensioned concrete member of span 15 m and cross-section of 450 mm $$\times$$ 450 mm is prestressed with three steel tendons, each of cross-sectional area 200 mm2. The tendons are tensioned one after another to a stress of 1500 MPa. All the tendons are straight and located at 125 mm from the bottom of the member. Assume the prestress to be the same in all tendons and the modular ratio to be 6. The average loss of prestress due to elastic deformation of concrete, considering all three tendons is
GATE CE 2022 Set 2
2
The creep strains are
GATE CE 2013
3
Which one of the following is categorized as a long-term loss of prestress in a prestressed concrete member?
GATE CE 2012
4
As per Indian standard code of practice for prestressed concrete $$(IS:1343-1980)$$ the minimum grades of concrete to be used for post-tensioned and pre-tensioned structural elements are respectively
GATE CE 2005
5
$$IS:$$ $$1343 – 1980$$ limits the minimum characteristic strength of pre-stressed concrete for post tensioned works and pretension work as
GATE CE 2005
6
Let $$jd$$ be the lever arm between the resultant tensile force $$T$$ in the reinforcing prestressing steel and the resultant compressive force $$C$$ in concrete of a reinforced / prestressed concrete beam. When subjected to increase in the external bending moment, which one of the following statements is true in reinforced and prestressed concrete beams?
In reinforced concrete beams
GATE CE 1993
7
Let $$jd$$ be the lever arm between the resultant tensile force $$T$$ in the reinforcing prestressing steel and the resultant compressive force $$C$$ in concrete of a reinforced / prestressed concrete beam. When subjected to increase in the external bending moment, which one of the following statements is true in reinforced and prestressed concrete beams?
In prestressed concrete beams
GATE CE 1993
8
The loss of prestress due to elastic shortening of concrete is least in
GATE CE 1992
9
A uniformly distributed load intensity, $$w$$ acting on a simply supported prestressed concrete beam of span $$L,$$ producing a bending moment $$M,$$ at mid-span is to be balanced by a parabolic tendon with zero eccentricity at ends and eccentricity $$e,$$ at mid-span. The prestressing force required depends on
GATE CE 1992
Marks 2
1
An inverted T-shaped concrete beam (B1) in the figure, with centroidal axis X – X, is subjected to an effective prestressing force of 1000 kN acting at the bottom kern point of the beam cross-section. Also, consider an identical concrete beam (B2) with the same grade of concrete but without any prestressing force.
The additional cracking moment (in kN.m) that can be carried by beam B1 in comparison to beam B2 is ________ (rounded off to the nearest integer).
GATE CE 2024 Set 1
2
A simply supported rectangular concrete beam of span $$8$$ $$m$$ has to be prestressed with a force of $$1600$$ $$kN.$$ The tendon is of parabolic profile having zero eccentricity at the supports. The beam has to carry an external uniformly distributed load of intensity $$30$$ $$kN/m.$$ Neglecting the self - weight of the beam, the maximum dip (in meters, up to two decimal places) of the tendon at the mid-span to balance the external load should be _______________
GATE CE 2017 Set 2
3
A pre-tensioned rectangular concrete beam $$150$$ $$mm$$ wide and $$300$$ $$mm$$ depth is prestressed with three straight tendones, each having a cross-sectional area of $$50$$ $$m{m^2},$$ to an initial stress of $$1200$$ $$N/m{m^2}.$$ The tendons are located at $$100$$ $$mm$$ from the soffit of the beam. If the modular ratio is $$6,$$ the loss of prestressing force (in $$kN,$$ up to one decimal place) due to the elastic deformation of concrete only is ________
GATE CE 2017 Set 1
4
In a pre-stressed concrete beam section shown in the figure, the net loss is $$10\% $$ and the final prestressing force applied at $$X$$ is $$750$$ $$kN.$$ The initial fiber stresses (in $$N/m{m^2}$$ ) at the top and bottom of the beam were:
GATE CE 2015 Set 2
5
A rectangular concrete beam $$250$$ $$mm$$ wide and $$600$$ $$mm$$ deep is pre-stressed by means of $$16$$ high tensile wires, each of $$7$$ $$mm$$ diameter, located at $$200$$ $$mm$$ from the bottom face of the beam at a given section. If the effective pre-stress in the wires is $$700$$ $$MPa,$$ what is the maximum sagging bending moment (in $$kN$$-$$m$$) (correct to $$1$$-decimal place) due to live load that this section of the beam can with stand without causing tensile stress at the bottom face of the beam? Neglect the effect of dead load of beam.
GATE CE 2013
6
A concrete beam prestressed with a parabolic tendon is shown in the sketch. The eccentricity of the tendon is measured from the centroid of the cross-section. The applied prestressing force at service is $$1620$$ $$kN.$$ The uniformly distributed load of $$45$$ $$kN/m$$ includes the self-weight. The stress (in $$N/m{m^2}$$) in the bottom fibre at mid-span is
GATE CE 2012
7
A rectangular concrete beam of width $$120$$ $$mm$$ and depth $$200$$ $$mm$$ is prestressed by pretensioning to a force of $$150$$ $$kN$$ at an eccentricity of $$20$$ $$mm.$$ The cross sectional area of the prestressing steel is $$187.5\,\,m{m^2}.$$ Take modulus of elasticity of steel and concrete as $$2.1 \times {10^5}\,\,MPa$$ and $$3.0 \times {10^4}\,\,MPa$$ respectively. The percentage loss of stress in the prestressing steel due to elastic deformation of concrete is
GATE CE 2009
8
A pre-tensioned concrete member of section $$200\,\,mm \times 250\,\,mm$$ contains tendons of area $$500\,\,m{m^2}$$ at the centre of gravity of the section. The prestress in tendons is $$1000\,\,N/m{m^2}.$$. Assuming modular ratio as $$10,$$ the stress $$\left( {N/m{m^2}} \right)$$ in concrete is
GATE CE 2008
9
A concrete beam of rectangular cross-section of size $$120$$ $$mm$$ (width) and $$200$$ $$mm$$ (depth) is prestressed by a straight tendon to an effective force of $$150$$ $$kN$$ at an eccentricity of $$20$$ $$mm$$ (below the centroidal axis in the depth direction). The stresses at the top and bottom fibres of the section are
GATE CE 2007
10
A concrete beam of rectangular cross section of $$200\,\,mm \times 400\,\,mm$$ is pre-stressed with a force $$400$$ $$kN$$ at eccentricity $$100$$ $$mm.$$ the maximum compressive stress in the concrete is
GATE CE 2005
11
A simply supported prestressed concrete beam is $$6$$ $$m$$ long and $$300$$ $$mm$$ wide. Its gross depth is $$600$$ $$mm.$$ It is prestressed by horizontal cable tendons at a uniform eccentricity of $$100$$ $$mm.$$ The prestressing tensile force in the cable tendons is $$1000$$ $$kN$$. Neglect the self weight of beam. The maximum normal compressive stress in the beam at transfer is
GATE CE 2004
12
A concrete column carries an axial load of $$450$$ $$kN$$ and a bending moment of $$60$$ $$kN$$-$$m$$ at its base. An isolated footing of size $$2m \times 3m$$ side along the plane of the bending moment is provided under the column. Centers of gravity of column and footing coincide. The net maximum and the minimum pressures in $$kN/{m^2}$$ on soil under the footing are respectively.
GATE CE 2003
13
A prestressed concrete rectangular beam of size $$300\,\,mm \times 900\,\,mm$$ is prestressed with an intial prestressing force of $$700$$ $$kN$$ at an eccentricity of $$350$$ $$mm$$ at midspan. Stress at top due to prestress alone, in $$N/m{m^2}$$ is
GATE CE 1997
14
A prestressed concrete rectangular beam of size $$300\,\,mm \times 900\,\,mm$$ is prestressed with an intial prestressing force of $$700$$ $$kN$$ at an eccentricity of $$350$$ $$mm$$ at midspan. Stress at top due to prestress alone, in $$N/m{m^2}$$ is
GATE CE 1994
15
A prestressed concrete beam has a cross-section with the following properties: Area $$A = 46,000\,\,m{m^2},$$ $${\rm I} = 75.8 \times {10^7}\,\,m{m^4},$$ $${Y_{bottom}} = 244\,\,mm,$$ $${Y_{top}} = 156\,\,mm.$$ It is subjected to a prestressing force at an eccentricity $$'e'$$ so as to have a zero stress at the top fibre. The value of $$'e'$$ given by
GATE CE 1991
16
A rectangular simply supported prestressed concrete beam of span $$L$$ is subjected to a prestressing force of $$P$$ acting centrally at end sections and the prestress tendons are parabolically draped with maximum eccentricity of $${e_{\max }}$$ at the mid span section. The uniformly distributed upward load $$(W)$$ on the beam due to prestressing will be:
GATE CE 1990
Marks 5
1
A $$10m$$ long prestressing bed is used to cast $$4$$ (pretensioned) prestressed concrete beams of $$2.3$$ $$m$$ each. A schematic representation of the bed is given in the following figure. The continuous prestressing reinforced is pulled at the end $$'Y'$$ of the bed through a distance of $$20$$ $$mm$$ to introduce the required prestress, before the concrete is cast. After the concrete has hardened, the prestressing reinforcement is cut at points $$A,B,C,D$$ and $$E.$$ Assume that the prestress is introduced without eccentricity, what is the loss in prestress on account of elastic deformation of concrete. Assume $${E_s} = 200000\,\,N/m{m^2},$$ reinforcement is $$500\,\,m{m^2},$$ size of beams $$ = 200\,\,mm\,\left( b \right) \times 400\,\,mm\,\left( h \right).$$
GATE CE 2002
2
A simply supported rectangular prestressed concrete beam $$200$$ $$mm$$ wide and $$400$$ $$mm$$ deep has an effective span of $$12$$ $$m.$$ The prestressing cable has a triangular profile with zero eccentricity at ends and $$70$$ $$mm$$ at the midspan as shown in the figure below. The effective prestress is $$800$$ $$kN$$ after all losses. Determine the value of point load, the beam can support at the midspan if the pressure line passes through the upper kern of the section. The weight density of the material of the beam can be taken to be $$25$$ $$kN/{m^3}$$
GATE CE 2000
3
A beam with a rectangular cross section of size $$250$$ $$mm$$ wide and $$350$$ $$mm$$ deep is prestressed by a force of $$400$$ $$kN$$ using $$8$$ numbers $$7$$ $$mm$$ $$\phi $$ steel cables located at an eccentricity of $$75$$ $$mm.$$ Determine the loss of prestress due to creep of concrete. Grade of concrete is $$M40;$$ Coefficient of creep is $$2;$$ stress at transfer is $$80\% $$, Modulus of elasticity of steel $$\left( {{E_s}} \right)$$ is $$2.0 \times {10^5}\,\,MPa.$$
GATE CE 1999
4
The cross-section of a pretensioned pre-stressed concrete beam is shown in Figure. The reinforcement is placed concentrically. If the stress in steel at transfer is $$1000$$ $$MPa,$$ compute the stress in steel immediately after transfer. The modular ratio is $$6.$$
GATE CE 1998