Tie Beams: Functions, Design, and Construction

What Is Tie Beam?

The beam which connects two or more columns or rafters in a roof or roof truss or in any height above floor level to make the whole structure more stiff and stable at the foundation level is called tie beam. Tie beams are mainly provided at roof truss and floor level and a plinth. They cannot carry any vertical load such as walls etc.

Tie beams sometimes work as the strap beam to take all stresses due to column eccentricity on the footing. Tie beams are worked as fixing members to reduce the unsupported length of the column.

Tie beams can also work to support walls or any other partitions among them. Tie beams are also worked as a tie element to protect the differential settlement between footings according to the strata.

Tie Beam Details

When the roof height is high than the normal height of the structure, then these types of beams are provided. They act as a length breaker in the columns. Tie Beam joins two or more columns to decrease their effective length and reduce their slenderness ratio.

Advantages of Using Tie Beam

1. These beams do not convey any floor loads and act as length breaker for columns where the floor is unusually high.
2. They carry axial compression.
3. They transfer rafter’s load to the column.
4. They prevent highly stressed slender longitudinal columns from buckling outward.
5. They connect column footings or pile clips.
6. They hold longitudinal bars in position while concrete is being placed.
7. They keep the spread footings in their positions during seismic events.
8. They serve as grade beams to redistribute vertical loads through moment and shear in the event of differential settlement.
9. They serve as strip footings to support interior or exterior walls.
10. They serve as grade beams over a pile foundation to link the pile-caps and support the structure above.

Bar Bending Schedule for Tie Beams

1. The purpose of bar bending schedule is to determine the quantities of the reinforcement essential for the building.
2. Bar Bending Schedule for footings is vital for gathering knowledge on the tie beam/strap beam reinforcement in the substructure.
3. Tie Beam stands for a beam that attaches the two footings in the substructure. If the two footings remain in the identical line, tie beam should be arranged.
4. Strap Beam alias inclined beam has the similarity with tie-beam, but it attaches two footings at a specific angle. Strap beam is placed if two footings remain at diverse levels.
5. Tie beam/ Strap beam is particularly situated among pile caps and shallow foundations. Their main objective is to force all shallow foundations or pile caps to roughly obtain the same settlements.
6. The horizontal bars which connect one footing to the other footing, belong to main bars and the vertical bars are known as stirrups. Stirrups facilitate framing the main bars in the exact position.

Tie Beam Reinforcement

The following points should be remembered to realise the reinforcement in tie beams:-

1. Main Bars (Top bar, Bottom bar, Sidebar) are attached to the centre of one footing to the centre of another footing.
2. Stirrups are provided from one face of the footing to another front of footing.

 Ties in Column

Proper use of column ties for making a perfect structural design

1. At first, straightening of the rods required for making ties maintaining exact diameters. The minimum length of the tie rods should not be under 10 mm.
2. On the basis of the measurements mentioned in the drawing, the rods should be cut with proper support and bend them cautiously; otherwise, it gets damages.
3. For maintaining proper specification as well as spacing of the ties, we need to follow the design engineers and follow the BNBC, ACI and ASTM codes.

 To withstand an earthquake, the following detailing must be maintained for the ties:-

1. The tie hooks should be bent by maintaining 135° angles. The head portion of the ties should be expanded up to 6 dB. For 10 mm rod, it should be 3 inches.
2. The earthquake detailing is also essential for the ties located in the middle portion.
3. Before putting the ties in the rods of the columns, we need to clear the primary rods properly.
4. After that, based on the design and spacing of the ties, the necessary numbers of ties are provided in the rods of the columns. In this time, the hooks of the ties should be bent and placed appropriately.
5. After putting all the ties, fasten them tightly with primary rods of the columns with the GI wire. We need to careful that the ties should not be displaced after getting fastened with wires.
6. For earthquake detailing, put the ties also in the joints of beams and columns. Sufficient numbers of ties in the joints of beams and columns are required.

 Tie Beam Design

1. Tie beam, grade beam, ground beam and plinth beam are not different, Tie beam is a beam used to tie two columns for resisting two movements by vertically and horizontally.
2. Tie beams may be at any level. If these are at plinth level, they are called plinth beams where it also helps in soil retaining of inner house area and also offers as support for walls.
3. The design of the tie beam will be governed by the amount of differential settlement of the foundation selected for the job at hand.
4. If delta is the differential settlement, then it distributes this moment among members of beam-column joint according to their flexural stiffness.

That is a way of proportioning the tie beam. Differential settlement consideration for tie-beam is a good design.

Why Are Use Concrete Tie Beam?

1. A steel braced frame columns are supported on Spread Footing with tie beam between the columns.
2. To resist vertical and horizontal reaction from Frame by designing a tie beam one has to consider that has a regular concrete beam with axial load.
3. That axial load acts as compression or tension so for axial compression its like axial column capacity. For axial tension required, Steel Area should also be checked.
4. It is a tension member, not compression so, the only function of the concrete is to protect the steel.
5. After checking the reinforcement for strength, making sure you have sufficient lap at required splices the amount of strain in the tie and how it affects the performance of the frame.
6. Tie members don’t have to be wrapped with concrete, but they do have to be protected. Concrete, always prefer to use couplers rather than lapped bars.

Two Additional Thoughts

1. The tie beam will limit lateral movement in the footings due to any seismic activity or subgrade movement.
2. If the beam is extended to other adjacent footings, it can become a grade beam where it can pick up additional dead load if needed, to resist overturning from the braced frame.

#1. Tie Beam Reinforcement Details

1. Reinforced concrete (RC) is widely used for construction all over the world. Columns transfer the loads from beams and slabs to the foundation. Columns support high compressive forces in mega structures such as long-span structures and tall buildings.
2. Columns may suffer damage due to overloading and natural disasters such as earthquakes and fires because of the limited strength and ductility of concrete.
3. Failure of one or more columns may lead to the collapse of the structure.
4. Both longitudinal and lateral reinforcements are essential for RC columns. While the concrete core is subjected to radial compression in the horizontal direction, the confining volume is subjected to tension.
5. However, either the large spacing or close spacing between ties results in lack of confinement of concrete core.
6. While the low volumetric ratio of ties reduces the confinement of concrete core, the high volumetric rate of ties defects concrete continuity. It creates a weak plane between the core and the concrete cover besides creating construction problems due to the congestion of column cage with reinforcement.
7. The shortage of confinement offered by ties was the reason for using materials such as Expanded Metal Mesh (EMM), Welded Wire Mesh (WMM) and Fiber Reinforced Polymer (FRP) to confine the concrete core.

#2. Quantity of Reinforcement in Tie Beam (Calculation)

1. Calculate the total length of the tie beam.
2. Calculate the total dead load and live load as per the given section drawing.
3. Based on load, calculate the maximum bending moment and shear force on the beam.
4. Weight of 1m of steel=d²/162 (where d is diameter) kg, From this Formula, how much weight of Reinforcement required can be calculated as per the Area required.

FAQ: Tie Beams

What is a tie beam?

A tie beam is a horizontal beam that connects two or more columns or rafters to enhance the stability and stiffness of a structure, particularly at the foundation level. They are mainly used at roof trusses, floor levels, and plinth levels and do not carry vertical loads such as walls.

Where are tie beams typically used?

Tie beams are typically used in roof trusses, at floor levels, and at plinth levels in structures. They are also used between columns to reduce the unsupported length of the columns and to help manage differential settlement between footings.

What are the main functions of tie beams?

The main functions of tie beams include connecting columns to reduce their effective length, acting as length breakers in tall structures, transferring loads from rafters to columns, preventing buckling of slender columns, and maintaining the position of spread footings during seismic events.

What is the difference between a tie beam and a grade beam?

While both tie beams and grade beams connect columns, a grade beam is usually placed at ground level or below and can support walls and other loads, whereas a tie beam is generally above ground and does not support vertical loads. Tie beams focus on connecting columns to reduce their effective length and prevent buckling.

How is the reinforcement in tie beams designed?

Reinforcement in tie beams involves the use of main bars (top bar, bottom bar, sidebar) running from the center of one footing to another. Stirrups are placed from one face of the footing to the other to maintain the position of the main bars. Proper detailing and adherence to codes like BNBC, ACI, and ASTM are crucial.

What are the advantages of using tie beams?

Advantages of using tie beams include reducing the effective length of columns, preventing column buckling, transferring rafter loads to columns, maintaining the position of footings during seismic events, and redistributing vertical loads in the case of differential settlement.

Can tie beams help in earthquake-resistant design?

Yes, tie beams play a crucial role in earthquake-resistant design by preventing lateral movement of footings and maintaining structural integrity during seismic events. Proper detailing, including the use of 135° bent tie hooks and appropriate spacing, is essential for earthquake resistance.

What materials are used for tie beam reinforcement?

Reinforcement materials for tie beams typically include steel bars for the main reinforcement and stirrups. Expanded Metal Mesh (EMM), Welded Wire Mesh (WMM), and Fiber Reinforced Polymer (FRP) can also be used to confine the concrete core and enhance the strength and ductility of the tie beams.

How do tie beams help in differential settlement?

Tie beams help manage differential settlement by distributing the moment caused by settlement among the members of the beam-column joint according to their flexural stiffness. This helps ensure that all shallow foundations or pile caps settle uniformly.

What is the purpose of bar bending schedules for tie beams?

The purpose of bar bending schedules for tie beams is to determine the quantities of reinforcement required for the structure. This schedule helps in planning and ensuring that the correct amount and placement of reinforcement bars are used to achieve the desired structural performance.