Stress Control by Deflecting & Debonding Tendons in PSC Design

If precast beams contain straight, fully bonded tendons, they can be easily detailed and manufactured. The main benefit of pre-stressing is that it can apply the dead toad of the unit to minimize the transmission of the tensile stresses in the concrete.

But dead load is lost in such members since this tension remains most critical at the ends of the beams, where the alleviating effect owing to dead load is zero. The methods of deflecting and debonding tendons are frequently applied in pre-tensioned beams to obtain a pre-stress distribution much like that is obtained by the draped profiles of post-tensioned systems. It maintains some of the dead-load benefits, which lead to fewer tendons in the beams or a slightly smaller depth of beam than would be feasible with straight, fully bonded tendons.

Stress Control by Deflecting Tendons: The method of deflecting some of the tendons upwards towards the ends of a beam at a proper position along the span transfers the important section at transfer to this position, where vital relieving stresses as beam dead-load bending moment is accessible. Based on the stress computations for the end regions of the beam, the design engineer set the number of tendons to be deflected and the position of the deflection point.

The deflection point normally remains in the neighborhood of the quarter-span position, where three-quarters of the mid-span value of dead-load moment is accessible to neutralize the tensile stress (top fibre) because of pre-stress at transfer.

The angle of deflection of these tendons are placed in such a manner that the effective eccentricity and the pre-stressing force of the tendons do not generate a tensile stress of more than N/mm2 at transfer at the important sections, a limit set in the Code.

This method of deflecting tendons is specifically effective where continuity for live loads should be set in the finished structure since by deflecting some of the tendons upwards towards the ends of a beam, some compressive stresses are produced in the top fibre at the ends. This is useful for withstanding tensile stresses occurred because of the hogging moments caused by the passage of live loads on the superstructure. It also minimizes the formation of compressive stress in the bottom fibre because of prestress and live loads or any other loads at the ends of the beam.

Another benefit of deflected tendons is that the Code allows the vertical component of the tendon force to be applied in withstanding the imposed shear force on the beams in areas which stay flexurally uncracked at the ultimate limit state. This component is also suitable for the flexurally cracked regions and for examining the maximum shear stress condition in the member. Due to some limited test evidence, however, the Code does not allow relief against shear in these later conditions. Actually, the shear resistance of any section is decided in both flexurally cracked and uncracked modes and the lower value is selected. The shear links are then designed to bear the rest of the enforced shear force. Because only the tendons which are situated within the web width are deflected, the strand pattern for the whole unit should be cautiously chosen so that sufficient strands are available for deflecting upwards towards the ends to meet the stress conditions during the length of the beam.

To get more details, go through the following link engineeringcivil.org



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Published By
Rajib Dey
www.constructioncost.co
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Benefits of floating column

Floating Column or Hanging Columns: The floating column belongs to a vertical member that is laid on a beam and it doesn’t deliver the load directly to the foundation. The floating column operates as a point load on the beam and this beam transmits the load to the columns situated under it.

The column may set out on the first or second or any other midmost floor as resting on a beam. Generally, columns are laid the foundation to deliver load from slabs and beams. But the floating column is laid on the beam.

It signifies that the beam providing support to the column performs as a foundation. That beam is known as a transfer beam. This is extensively applied in high storied buildings for both commercial and residential purpose. It facilitates to customize and rectify the plan of the top floors. The transfer beam that provides support to the floating column, reassigns the loads up to foundation. For this reason, it should have been designed with more reinforcement.

Floating Column in Buildings: In recent times, multi-storey buildings are developed for the purpose of residential, commercial, industrial etc., containing an open ground storey. To provide space for parking, the ground storey is reserved free devoid of any constructions, exclusive of the columns which move the building weight to the ground.

For a hotel or commercial building, usually, there are banquet halls, conference rooms, lobbies, show rooms or parking areas in lower floor, hence large alternate space is necessary for the transition of people or vehicles. The columns which are narrowly placed in the upper floors, should not be located in the lower floors. Hence, to get rid of this issue, floating column concept becomes vital.

In urban areas, multi storey buildings are developed supported with floating columns at the ground floor for the different objectives. These buildings with floating columns are treated as secured under gravity loads and therefore are designed only for those loads. But these buildings are not suitable for earthquake loads and hence, these buildings are treated as insecure in seismic prone areas.

When the floating columns are arranged in buildings in seismic prone areas, the whole earthquake of the system is allocated with the column or the shear walls devoid of assessing any contribution from the floating columns.

Floating Column & Earthquake: The floating columns are useful for various projects specifically over the ground floor, where transfer girders are used with the purpose of providing more open space in the Ground Floor.

In the earthquake prone zones, the transfer girders which are applied should be designed and detailed correctly. If no lateral loads exist, the design and detailing work will not be complicated.

Concept of floating column primarily includes disrupting flow of transfer of EQ force.

• Floating columns must be designed as a normal compression member.
• At the time of designing transfer beam, it is designed as beam bearing all that load of column as a single point load.
• It should be remembered that EQ force developed should be reduced along the shortest path. It means load is dispersed between two intermediate columns which provide support to that beam.

High shear capacity beams/deep beams are utilized to provide support to the floating column. In some areas, the floating columns are inevitable. So, it is essential to alter code provisions for deep beams.

Article Source: engineeringcivil.org



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Published By
Rajib Dey
www.constructioncost.co
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How beams are categorized on the basis of different factors

In structural engineering, various structural members are titled on the basis of their behavior under applied load. If the primary mode of delivering the load occurs due to bending then the structural member is known as beam. It can be also said that a beam belongs to a structural component that has the ability to resist load initially by making resistance against bending.

Beams usually bear vertical gravitational forces but can also be applied for bearing horizontal loads (e.g., loads originate because of an earthquake or wind). The loads which beam bear are transmitted to columns, walls, or girders and which then transfer the force to the adjoining structural members and lastly to the ground.

Categorization of Beam: Beams are distinguished with their support condition, profile (shape of cross-section),geometry, equilibrium condition, and their material.

Categorization on the basis of supports: Categorization of beams on the basis of support condition is essential since the bending moment functioning on the beam is directly influenced by the support condition. The difference can be seen in the bending moment diagrams given below. For the equivalent length and loading the bending moment diagram fluctuates significantly with change in support condition.

Simply supported – Beams are supported on the ends which can rotate easily without any moment resistance.

Fixed – Beams are supported on both ends and constrained from rotation.

Over hanging – Simple beams which are expanded beyond its support on one end.

Double overhanging – Simple beams with both ends extending beyond its supports on both ends.

Continuous – Beams are expanded over in excess of two supports.

Cantilever – A projecting beam that is fixed only at one end.

Trussed – A beam is reinforced with the inclusion of a cable or rod to develop a truss.

Categorization on the basis of profile: The type and magnitude of internal stress created in the beam is directly reliant on the shape of the cross-section, thus categorization is necessary on the basis of profile.

Rectangular Beams: I-Beams, T-Beams, C-Beams, Other Cross-sections

Categorization on the basis of geometry: The type and magnitude of internal stress created in the beam is also directly reliant on the geometry of the beam, thus categorization is necessary on the basis of geometry.

Straight Beams, Curved Beams, Tapered Beams

Categorization based on indeterminacy: The design of beam is mainly created for bending moment and shear force. Assessment of these bending moments and shear force is called analysis. A beam is classified into following two categories on the basis of the type of analysis necessary to work out the reaction:

Statically determinate beams: equilibrium conditions tolerable to compute reactions.e.g. simply supported beams, cantilever beams, single and double overhanging beams etc.

Statically indeterminate beams: Deflections (Compatibility conditions) together with equilibrium equations should have been applied to determine the reactions.e.g. propped cantilever, continuous beams, fixed beams.

Categorization on the basis of the material: Various materials contain different advantages and drawbacks for being utilized as a building material for erecting the beams concerning cost and usage, as a result beams are also classified on the basis of the material used for their construction.

Concrete Beams, Steel Beams, Timber Beams



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Published By
Rajib Dey
www.constructioncost.co
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