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
Advantages of Using Tie Beam
- These beams do not convey any floor loads and act as length breaker for columns where the floor is unusually high.
- They carry axial compression.
- They transfer rafter’s load to the column.
- They prevent highly stressed slender longitudinal columns from buckling outward.
- They connect column footings or pile clips.
- They hold longitudinal bars in position while concrete is being placed.
- They keep the spread footings in their positions during seismic events.
- They serve as grade beams to redistribute vertical loads through moment and shear in the event of differential settlement.
- They serve as strip footings to support interior or exterior walls.
- 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
- The purpose of bar bending schedule is to determine the quantities of the reinforcement essential for the building.
- Bar Bending Schedule for footings is vital for gathering knowledge on the tie beam/strap beam reinforcement in the substructure.
- 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.
- 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.
- 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.
- 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:-
- Main Bars (Top bar, Bottom bar, Sidebar) are attached to the centre of one footing to the centre of another footing.
- 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
- 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.
- 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.
- 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:-
- 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.
- The earthquake detailing is also essential for the ties located in the middle portion.
- Before putting the ties in the rods of the columns, we need to clear the primary rods properly.
- 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.
- 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.
- 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
- 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.
- 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.
- The design of the tie beam will be governed by the amount of differential settlement of the foundation selected for the job at hand.
- 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?
- A steel braced frame columns are supported on Spread Footing with tie beam between the columns.
- 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.
- 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.
- It is a tension member, not compression so, the only function of the concrete is to protect the steel.
- 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.
- 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
- The tie beam will limit lateral movement in the footings due to any seismic activity or subgrade movement.
- 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
- 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.
- Columns may suffer damage due to overloading and natural disasters such as earthquakes and fires because of the limited strength and ductility of concrete.
- Failure of one or more columns may lead to the collapse of the structure.
- 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.
- However, either the large spacing or close spacing between ties results in lack of confinement of concrete core.
- 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.
- 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)
- Calculate the total length of the tie beam.
- Calculate the total dead load and live load as per the given section drawing.
- Based on load, calculate the maximum bending moment and shear force on the beam.
- 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.