Stirrup Making Process: 5FAQs With Details Answers
A small detail may it be, stirrups are as important as any other construction elements. A missed out detail on stirrup design will cause a structure to fail. Here are some answers to common and frequently asked questions for you!
What Is Stirrup Construction
Stirrup is one of the fundamental part in a reinforced concrete design. In the design, stirrups are to hold the main reinforcement rebar together. For beams, these close loop are called stirrups. While when used in columns, they are called ties.
Stirrups and ties have the same purpose which is to hold the main reinforcement, preventing the reinforced concrete structure to buckle especially during strong winds and earthquake. Stirrups and ties are the perpendicular to the direction of the main reinforcement.
Stirrups and ties have important parts, these are the hooks and the bends. Hooks acts as the length extension to prevent the stirrups to split.
Hooks are designed for the resistance of buckling and during seismic activities. And most especially, hooks are to prevent the slippage of the rebar to keep the stirrups hold the main reinforcement tight. Bends are provided to keep the hooks in place and serves as an anchorage to the main reinforcement.
The most common bend angle of the stirrups are 90 degrees, 135 degrees and 180 degrees. Of which these bend angle provides sufficient resistance to seismic activities like earthquake and any other lateral forces.
What Is The Purpose Of Stirrups In Concrete
The most important purpose of the stirrups in a reinforced concrete element is to hold the main reinforcement tightly together. The stirrups and ties are provided to the structural element to prevent the column or beam to buckle.
The stirrups and ties are in adequate spacing to provide shear resistance to the beam and columns. These also hold tightly the main reinforcement during seismic activities and strong winds and also during the concrete pouring.
Stirrups and ties give stability to your reinforced concrete element which lessens the growth of the cracks. For short, because it holds the main reinforcement together, it preserves the strength of your structure.
How Are Stirrups Designed
To design a stirrups and ties for beams and columns, you consider the shear forces that acts towards and perpendicular to the axis of the beam of column.
Usually, these shear forces are the maximum point forces the supports of your beam which is why the stirrups are usually more near the support.
Providing the shear reinforcement means it is the resistance of the shear force that can cause stresses to your structure which commonly is seen by buckling and cracks.
The most stressed part of the beam that needs shear reinforcement are near the support, the beam supported by columns or girder and the concentrated load considering the distance of these to the support.
To design the stirrups, you must:
- Get the dimension of your beam and the compressive strength of the concrete and the yield strength of your steel. Know the loads your beams will carry.
- Compute and draw the Vu diagram and calculate the shear at a distance “d” from the support. The distance “d” is computed by the total length of the beam minus the bottom concrete cover, stirrup diameter and half the main reinforcement.
- Now, compare if stirrups are need for the beam by getting the Vc and locate where the stirrups are needed.
- Compute the maximum spacing according to the code and also compute the spacing required for the most part of the beam.
How Many Types Of Stirrups Are There
Stirrups type can be determined by its shape and the nature of its construction.
Types of Stirrups by Shape:
- Square Stirrup
- Rectangular Stirrup
- Circular Stirrup
- Triangular Stirrup
- Spiral Stirrup
- Diamond Stirrup
Types of Stirrups by nature of construction
- Single legged stirrup or also knows as the Open Stirrup
- Two or Double legged stirrup is a one of the Closed Stirrup Type
- Four legged stirrup is a another of the Closed Stirrup Type
- Six legged stirrup is a another of the Closed Stirrup Type
The most desirable type of stirrup is to hold the main reinforcement tightly to prevent it from slipping during the concrete pouring, the seismic activities and other matters like strong winds. It should provide resistance to shear and torsion.
How Do You Create Stirrups For Columns
There are many factors you have to consider in your stirrups and ties cutting length. These are the:
- The hook and bend length
- The bend angle
- The stirrup spacing
- The size of the stirrup diameter
For the hook and bend length, it depends on the bent angle.
- For 1 hook length = 9d or 75 mm
- For 45 degrees= 1d
- For 90 degrees= 2d
- For 135 degrees= 3d
Where in the d stands for the rebar diameter.
For the stirrup spacing, it shall be computed during the shear design which the spacing are closer or tighter near the support and farther as it comes to the middle.
For the entire stirrup cutting length, compute the perimeter of the beam (and deduct the concrete cover) + the hook length (multiplied with the number of bend) – the total bend length.
Why are the spacing in the middle greater compared to the spacing near the support
We must know first the purpose of the stirrups and ties and why they are necessary. The stirrups and ties are to hold the main reinforcement together and prevent the reinforced concrete structure from shearing and buckling.
The most critical point in the Shear Diagram is close and at the support and decreases to zero in the middle. The stirrups are shear reinforcement.
Therefore, stirrups are provided more near the support to resist the greater shear forces which is at the critical points, near the support.
What to do when stirrups are not required during my shear design
For instanced that the stirrups are not required, the code should prevail. The code which is to provide the minimum number of stirrups for every length should be followed to serve as the safety measures for any sudden changes in shear forces due to unexpected load.
Does the orientation of the stirrup and ties hooks matter during the installation
The alternating of the hook of the stirrups and ties during the installation is for concrete pouring purposes. Hooks are tied using a tie wire and if the hooks are placed in one direction it can obstruct or clog the aggregates as it flows through during the concrete pouring which can be corrected using the concrete vibrator.
So, the alternating of the hooks of the stirrups and ties doesn’t really matter or is better that the other. As long as the stirrup design are followed, especially with the hook and bend length and the spacing of the stirrups.
The stirrups are installed first in both ends of the beam, meeting these stirrups in the middle. Do not forget to tie the hooks together to provide tight holds with the hooks to the main reinforcement.
Cracks in beam are evident when there is an increase shear force on the element. These are mostly seen near the support such as columns or walls. These cracks can be avoided when the shear design is properly followed and are in the right spacing.
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