Frame Structure vs Load Bearing Structure
I have been engaged in architectural design and structural calculations while working in the construction industry for a while, and those have a wide range of variations in terms of complexity and efficiency. Within my practical engineering experience and theoretical knowledge, I would like to share the differences between frame structures and load-bearing structures.
Frame Structures
Buildings that utilize frame structures are among the most common and frequently constructed in the 21st century. This type of structure relies on a framework or frame to bear the load, rather than relying on walls. An important factor in buildings using frame structures is the calculation of the strength of the frame.
As a historical reference, frame structures can be traced back to houses built using wooden or half-timbered frame systems. This type of building was commonly used in medieval Europe. In this type of building, the open spaces are usually covered with bricks, flat stones, or wooden panels. The construction of these wooden houses continued to develop and eventually reached the mainland of the United States, especially after the end of World War II.
During the 19th century, brick or masonry walls became the main load-bearing material, often supplemented with cast iron as a structural component included in the walls and building structure. With the advancement of technology and industrialization, steel and reinforced concrete became the most common materials in large-scale building structures. French architect Auguste Perret was the first to implement the concept of frame structures in his construction designs (1903). He aimed to eliminate as many non-structural elements as possible from his buildings, using reinforced concrete frames, and reducing traditional load-bearing walls by utilizing metal and glass materials as well as curtain walls as exterior cladding.
Various types of frame structures have been used in building construction. Frame structure models are then classified into two main types: rigid frame structures and braced frame structures. These structures are further divided into categories such as fixed-ended rigid frames and fixed-ended frames with pin supports. This is followed by tapered frame system structures and portal frame system structures.
Each frame structure model can be constructed using various materials such as reinforced concrete, steel, and wood. Frame structures consist of a combination of beams, columns, and slabs to resist lateral and gravitational loads. These structures are typically used to handle large moments resulting from the applied loads.
Rigid Structures (Rigid Frames)
Rigid frame structures, also known as moment frames, consist of linear elements such as beams and columns. The term “rigid” refers to the ability to withstand deformations. This type of structural model usually utilizes steel and reinforced concrete as the main materials. Rigid frames have a characteristic of having a minimal number of joints embedded within the frame and are typically statically indeterminate.
Rigid frame structures have good capabilities in resisting vertical and lateral loads, resulting in beam and column bending. The stiffness of the frame in rigid frame structures is largely influenced by the stiffness of the beam and column connections. These connections must be designed in such a way that they have adequate strength and stiffness, and the deformation can meet the safety factor limits.
Structural analysis methods such as the portal method (approximation), virtual work method, Castigliano’s theorem, force method, slope-deflection method, stiffness method, and matrix analysis can be used to solve the forces and moments occurring in the building structure and design the load reactions.
Rigid structures can be classified into two main types:
a. Fixed-Ended Rigid Frame Structure, where the supports of the rigid frame are fixed, as shown in the figure below:
b. Pinned-Ended Rigid Frame Structure, where the supports of this rigid frame type are pinned and are not considered as rigid frames if the support conditions are released.
- Frame Structure with Bracing Reinforcement
Bracing frame consists of beams and columns that are “clamped (pinned)” and connected with bracing to resist lateral loads. This type of frame is easy to analyze and construct. Lateral resistance is achieved through horizontal and vertical bracing.
There are many types of bracing that can be used, such as joint bracing, diagonal bracing, X bracing, K bracing or chevron, and also shear walls that resist lateral forces on the wall plane. This frame system provides more efficient resistance to seismic and wind forces. This structural model is far more effective than rigid frame systems.
a. Gabled frames are structural models that typically use a pitched roof. This frame system is used in areas with potential heavy rainfall and snow.
b. Portal frames, as shown in the picture below, are commonly used in industrial and logistics construction projects.
Load Bearing Structures
In load-bearing structures, the brick walls of the building are also utilized as the foundation and main component of the building frame to resist the applied loads. In a load-bearing structure, the loads are carried through the walls and transferred to the ground by the floors. The foundation is also made of bricks. This load-bearing structure type transfers the loads to the ground through walls without columns and beam frames. The use of load-bearing structures is the oldest method of building construction in history and is one of the most common construction models. In this type of structure, external loads such as earthquakes, wind, etc., are distributed through the walls.
The entire structure is supported by components such as brick or stone walls. However, one drawback of this structural model is its suspected lack of resistance to earthquakes. On the other hand, this type of building works well for relatively low-rise constructions.
Load-bearing structure models have limitations in their application to multi-story buildings. Such structures also require intensive materials due to higher dead loads. Load-bearing structures use less cement and steel compared to frame structures. The cost of repairs for buildings with this type of structure is relatively cheaper compared to frame structures.
One of the disadvantages of load-bearing structures is that they have a shorter lifespan compared to frame structures. Therefore, this structural model is rarely used in the construction of multi-story (high-rise) buildings.
Load-bearing structures have a smaller floor area due to thicker wall dimensions (prioritized). Therefore, the maximum efficiency in buildings using this structural model is achieved in the floor area. Structures with this model have significant constraints in changing the layout of walls. It is not easy to design the results. Room dimensions cannot be altered because the walls are installed as load-bearing structures. Structural elements such as cantilevers can be created or built but with short spans or lengths. Therefore, load-bearing structures also have limitations in modeling short cantilever spans.
Structures with load-bearing models can be said to have low flexibility. This is because the structural installation process for this model already includes the installation of walls as part of the overall structure. If there are future renovation or modification processes, changes will likely occur in all geometric elements of the building.
In collaboration with Engineering Space, we attempted to simulate the best options for analyzing load-bearing structure models. The Engineering Space crew suggested creating a building model with dimensions of 4 x 12 m, with a total building height of 7 m (including the roof).
The simple building was successfully modeled and entered the structural analysis stage, as seen in the collection of pictures above.
And as a comparison, Engineering Space also provided a modeling of a frame structure. Initially, the Engineering Space team decided to create a building frame with the same dimensions as the load-bearing structure model (4 x 12 m). However, after considering aesthetic aspects and technical capabilities, it was decided to use dimensions of 5 x 12 m for the frame structure model.
Next, I will attempt to provide a table of differences between the two structures we discussed in the following table:
In conclusion, structural models come in various types and variations. What matters most is how we can fulfill the goals and purposes of the building or infrastructure being constructed. It is crucial to base our decisions on the main variables considered in infrastructure development (discussed in the following link).
Resources link:
- Youtube of Engineering Space: https://www.youtube.com/@engineeringspace3170
- Simple frame structure modelling using steel material: https://www.youtube.com/watch?v=68wKtC_xHxM
- Beam RCC Modelling: https://www.youtube.com/watch?v=zS09Z1MYzkI
