by Prestress refers to the stress applied to the structure in advance before the external load is applied to the component.
The prestress is generally opposite to the stress caused by the load.
There are many examples of the application of prestress in daily life.
The wooden basin (barrel) uses hoop to apply pre compression stress to the wood chip to offset the tensile stress generated by water.
Pre compression stress shall be applied by hand to offset the bending tensile stress generated by the book’s dead weight.
1.
Prestressed steel bars are used to apply pre compressive stress to concrete members.
Where prestressed concrete beams generate tensile stress under load, prestressed steel bars are used to apply compressive stress to them in advance to partially or completely offset the tensile stress generated by load.
The application of prestress can improve the crack resistance of concrete members; Ø The deflection caused by dead load shall be offset by the inverted arch generated by prestress; Ø Make full use of the tensile properties of high-strength steel and the compressive properties of high-strength concrete; Ø It can reduce the section size and the dead weight of the structure, so that it can be applied to large span and high-rise buildings.
2.
At present, the span of this simply supported beam with pre bending steel composite has reached 40m.
If the secondary pouring is applied to form a pre bent steel composite continuous beam, the advantages of this structure can be brought into full play, and the maximum span can reach 80m.
This new type of prestressed composite structure has light weight, high bearing capacity, small deflection, small structure height, and is easy to construct.
It has a good application prospect in urban overpasses.
It is envisaged to build a reinforced concrete bridge between the two mountains: erect temporary supports and lay precast concrete blocks; Ø Pre stress the precast concrete block with a row of jacks; Ø Pour concrete between jacks; Ø Remove the jack after reaching the necessary strength; Ø Re pour concrete at the position occupied by the original jack; Ø Dismantle the temporary support, and a prestressed bridge without prestressed reinforcement will be built.
(This is the same as the principle of carrying a book by hand) 3.
Prestress is an internal force.
Will the prestress applied to a thin plate destabilize it? When the compression bar is bent due to accidental eccentricity or accidental lateral force, the additional moment may make the member more and more bent, or even cause damage, which is called instability.
Thin plates or long thin rods under axial force may lose stability.
Prestress is an internal force.
When a prestressed thin plate is accidentally bent, the additional torque generated by prestress and the lateral pressure of the prestressed reinforcement on the concrete slab after bending are opposite to the direction of accidental bending, which will make the component straight.
It can be seen that prestress will not destabilize the compression bar, but also make the compression bar more stable.
It is not difficult to understand why we sometimes apply prestress to precast long columns and long piles.
4.
The concept of prestress has more general significance.
As mentioned above, prestress can “pull” the butt joint of the wooden basin that cannot be pulled, and also can make the concrete with low tensile strength seem to be able to bear a large “tensile stress”.
In fact, prestress is just a process of transforming the process of tension into the process of reducing the pre compression stress.
According to the same principle, pre tension stress can also be applied to make materials that cannot be compressed seem to be compressible.
The balloon membrane cannot be compressed.
If the membrane is first subjected to tensile stress by inflating and pressing, the structure can bear the load as long as the pre tensile stress is large enough to exceed the compressive stress of the membrane under the load.
The principle can be used to construct various inflatable structures.
Balloon Structure is an air supported inflatable structure with low air pressure.
Take out the roof of 1m2 at the top for analysis.
Generally, the self weight of the membrane material will not exceed q=100N/m2.
As long as the internal air pressure p ＞ q, the membrane can be lifted.
As you know, a standard atmospheric pressure is equivalent to about 10m water column pressure, that is, 100000N/m2.
It can be seen that as long as the indoor air pressure is about 1/1000 higher than the outdoor air pressure, the membrane can be lifted.
This differential pressure can be achieved only with an ordinary blower.
Generally, as long as this air supported inflatable structure uses a closed rotary door at the entrance, and uses a fan to continuously supplement air, maintaining a pressure difference of 150~300N/m2, it is sufficient to withstand the gravity of the covering layer, and maintain a certain degree of prestress in the membrane to ensure the overall stiffness of this air supported inflatable structure.
The indoor and outdoor pressure difference is only 1.5/1000~3/1000, which is an error that people cannot feel.
The trachea inflation structure, also known as the pneumatic inflation structure, has an air pressure of 1.5~2.0kN/m2 in the trachea.
The air pipe has a certain rigidity, bending resistance and compression resistance, and can be used as an arch ring to form a roof.
The pressure inside the tube of the gas filled structure is much higher than that of the air supported structure, but it is only about 1/1000 of the pressure of the automobile inner tube (about 2000kN/m2), which is easy to reach.
The air tube (bag) inflatable structure has good heat insulation performance, but its working pressure is slightly higher, and it also has higher requirements for material strength, air tightness and other quality, and its cost is higher than that of the air supported inflatable structure.
The inflatable structure is mixed with other structures, and the effect is also very good.
The inflatable structure is light in weight, low in cost, and easy to assemble and disassemble, especially suitable for markets, exhibitions, temporary performance venues, competition venues, etc.
with large spans..
