A stable and safe structure is necessary to prevent the loss of life and property. Structures are constructed with the help of different members like beams, columns, slabs, etc. by connecting with each other. These structures are capable of bearing lateral as well as vertical loads.
The intersection of the connected points of the different members is known as a joint. There are different types of joints in the structure provided, according to different purposes and requirements. Examples of joints are pin joints, fixed joints, rigid joints, etc.
Some frames are divided according to the types of joints. Moment frames and braced frames are two of them, which are divided according to the types of joints. In this article, I will discuss about moment frame vs braced frame.
Moment frame vs Braced frame
What is a moment frame?
In the moment frame, each and every connection is nothing but a moment connection. The moment connection is one type of connection where the beam and column joint have zero rotation.
All the members, i.e., beam and column, are connected with each other with rigid connection as shown in the figure above in the moment frame.
These types of frames are constructed to bear vertical as well as lateral loads on the frame structure. The vertical load may be a point load or a UDL load. The lateral load may be due to an earthquake or wind load.
Advantages of moment frames
- Moment frames are more ductile and can withstand more damage during earthquakes.
- Moment frames can resist both vertical and lateral loads effectively.
- These frames are more flexible and allow more open space over the floor of the structure.
- Moment frame structures can be constructed for high-rise buildings.
- It does not require regular maintenance like a braced connection.
Disadvantages of moment frames
- Moment frames require more time and become more expensive to build
- It is difficult to repair the structure after damage
- It is not more effective as a braced frame to resist lateral loads due to wind or earthquakes.
- The components of the structure are difficult or impossible to rearrange and reuse.
What is a braced frame?
In a braced frame, each and every joint is a pin joint, as shown in the figure above. But what is a pin joint? The joints that make the joints of the beam and column to rotate freely. Here, you can say that the bending moment at that point is zero.
So, to provide lateral stability of the frame, we have to provide some extra members called bracings. Hence, this type of frame is known as a braced frame. I hope now you are clear about the moment frame vs braced frame.
Advantages of braced frames
- Braced frames are less expensive to build than moment frames.
- These types of frame structures are easier to repair after damage
- Brace frames are more effective to resisting lateral loads like wind loads and earthquake loads.
- They have stiff connections.
Disadvantages of braced frames
- Brace frames are less ductile, so they have to face more damage during earthquakes.
- It can only resist lateral loads like wind loads, earthquake loads, etc.
- They are less flexible and allow less open space on the floor of the structure.
Where to use moment frames and braced frames?
While designing the structure, one of the most frequently asked questions about the moment frame and the braced frame is when to use a moment frame and when to use a braced frame.
First of all, simply you have to know that the moment frame is mostly capable to resist moments which are generated throughout the mid-span of the beam.
But, the braced frame is mostly capable to resist the moment generated at the end of the beam i.e. at the support of the frame structure.
Now, whenever we apply a concentrated or uniformly carrying load to the moment frame, a bending moment is generated throughout the beam.
Here, the maximum bending moment will appear at the mid-span than the support and sagging moments is generated, as shown in the figure below.
Similarly, whenever we apply lateral load on the braced frame structure the bending moment is generated at the end of the beam.
Here the maximum bending moment will appear at the end support, as shown in the figure below.
By the above condition, we can conclude that if we have mostly UDL, gravity load, dead load, or live load on the frame structure, then we can use a moment frame. i.e., just provide a moment connection in all the beam-column junctions.
But Instead of these loads, if we have mostly lateral loads on the frame structure. Then we can use a braced frame. i.e., just provide pin-jointed connections at all the beam-column junctions.
In the case of a moment connection, if you provide the lateral load, the deflection will be much larger. But if we use bracing instead of a moment connection, we will have a very small lateral deflection.
Moment frame vs Braced frame
|Feature||Moment Frame||Braced Frame|
|Joint type||They have rigid joints between the beam and column||They have pin joints between the beam and the column.|
|Ductility||They are more ductile to resist damage during seismic loads.||Braced frames are less ductile to resist damage during seismic loads.|
|Cost||They are more expensive while construction||They are less expensive during construction.|
|Flexibility||They are more flexible to provide open space over the floor||They are less flexible to provide open space over the floor|
|Wind resistance||They are less effective to bear wind loads.||They are more effective to bear wind loads.|
|Seismic resistance||They are less effective to bear the seismic load||They are less effective to bear seismic load|
|Application||Moment frames are mostly used for residential and commercial buildings, etc.||Moment frames are mostly used for steel structures like bridges, culverts, etc.|