Types of Structural Systems in Buildings

There are various types of structural systems in buildings. A structural or a Design engineer has to select the system that is economical and best suitable for the structure.

Any structure is made up of Structural elements (Beams, Columns, Slabs) and Non-structural elements (Doors, Partition Walls, Stairs).

The function of structural elements is to resist the loads acting on that structure and to transmit those to the ground.

For simplified analysis, Structural elements are classified into One-Dimensional (Beams, Columns, Trusses) and Two-Dimensional (Slabs, Plates) elements. These structural elements, put together, constitute Structural System.

Most common construction is Building (Residential, Commercial or Institutional). The Structural system and its Load transfer mechanism for a building are mentioned here in detail.

For convenience, we separate this load transfer mechanism into Gravity Load Transfer Mechanism and Lateral Load Transfer Mechanism, even though, both of these are complementary.

Lateral Load Transfer

Lateral loads like Wind load, Seismic load predominate in high rise buildings and hence govern the structural system.

Lateral Load Resisting system generally consists of one of the following:

Rigid Frame System

A combination of Columns and Beams. Beams and Columns are cast monolithically. This is most common among the various types of structural systems.

The beam-Column connection is made rigid and moment-resisting which helps in resisting the lateral loads. Hence, they are also called as the Moment-Resisting Frames.

The lateral stiffness of a rigid frame depends on the stiffness of the columns, beams and the connections if any.

The lateral deflection is high when compared with other lateral load resisting structural systems. As the height of the building increases, deflection gets increased.

It is effective for buildings of 15 to 20 stories.

Shear Wall System

It is one of the easiest and effective lateral load resisting systems.

Shear wall is a Solid Continuous Vertical Wall which extends over the full height of the building.

Shear walls are very stiff and they resist loads by bending like a vertical cantilever. They have high in-plane stiffness and strength.

Shear walls are usually Reinforced Concrete Structures. But the recent advancements in Structural Engineering made it possible to have Steel, Masonry and also Wooden Shear Walls.

It is effective for buildings up to 35 stories.

Wall – Frame System

This is a combination of Shear wall and Rigid Frames. Also called as Dual-systems.

This interaction is advantageous as the walls restrain frame deformations in the lower storeys, while the frames restrain the wall deformations in the upper storeys.

The deflection is much lesser than the Rigid frame and Shear wall alone. It is suitable for 40 to 50 stories.

Braced Frame System

This structural system consists of moment frames with specific bays provided with diagonal members (commonly steel) called the bracings which primarily resist the lateral loads on the structure.

In such a system the beams and columns are generally designed to take the vertical loads and the bracings are considered to resist all of the lateral loads

The lateral loads are transferred as axial tensile and compressive forces in the brace members.

Bracings are generally provided as Vertical bracing (Elevation bracing) throughout the height of the building which primarily resists the lateral loads and transfers it to the ground.

But bracings may also be provided in the horizontal plane also known as the Plan bracing which acts as the diaphragm to hold the structure.

However, floor slabs (RCC slabs) are generally sufficient for the diaphragm action and hence plan bracings are avoided.

There are various types of bracings used such as

• Diagonal bracing
• Cross (X bracing)
• V – bracing
• K – bracing
• Inverted V – bracing.

Cores

This structural system consists of a central shear core which is a network of walls interconnected with each other to form a rigid box-like structure.

Generally, lift wells are used as the core structure.

Such RCC cores provide greater stability to the structure resisting the lateral loads acting as a vertical cantilever.

Tubes

For tall and high rise buildings, use of a braced frame and structural walls alone may not be sufficient to control the overall lateral displacement as well as the forces.

In such cases, more rigid structural systems are required.

Tube structures are one such system where columns are closely spaced along the perimeter thereby increasing the number of columns in the perimeter forming a tube.

It looks like a hollow cantilevered structure. The interior columns can be reduced and can be designed to take only the gravity loads since the outer tube is highly stiff and resists all of the lateral loads.

Different types of tube structural systems are used such as

• Framed tube
• Tube in a tube (hull and core)
• Bundled tube

Gravity Load Transfer

Although a building is a 3-dimensional, we assume, analyse and design considering it as an assembly of 2-dimensional sub-systems lying primarily in the horizontal and vertical planes.

Hence, it is convenient to divide a building into the Floor or Horizontal System and Vertical or Framing System.

Floor Systems

The floor system resists the gravity loads acting on it and transmits these to the vertical system.

The floor serves as a horizontal diaphragm connecting together and stiffening various vertical frame elements.

Under lateral loads, the floor diaphragm behaves rigidly because of its high in-plane stiffness and effectively distributes the lateral load effects to the various vertical frame elements.

Wall – Slab System

It was a common technique in the early days, where a slab is directly supported on walls.

These walls are called Load Bearing Walls as they bear the load coming from slabs.

The slab can be One-way or Two-way and can be continuous or simply supported. There is no use of Beams and Columns and hence they can be ignored.

Load transfer is generally from Slabs to Walls and thereby to foundations.

The major drawback with this system is it cannot be used for Tall buildings i.e., it is only adopted for Low-rise Buildings.

Beam – Slab System

A majorly adopted among the various types of structural systems. It is best suited for both Low-rise and also Tall buildings.

The slabs are supported on Beams which are in turn supported on Columns. This construction technique is called Framed Construction.

Slabs can be One-way or Two-way and Beams are cast monolithically with the slab.

Gravity load from the slab is transferred to a beam which in turn gets transferred to columns and thereby foundations.

Beams are laid out in Grid pattern. Beams that are passing through Columns are called as Primary Beams, while the beams which are supported by other beams (not columns) are called Secondary Beams.

Ribbed Slab System

A special type of Beam – Slab system in which Slab is very thin (also called as Topping) and the Beams are very slender (called as Ribs) and are closely spaced.

Depth of ribs is nearly 3 to 4 times the thickness.

If a Two-way slab is adopted then such slabs are called as Waffle Slabs. This is most commonly used in large-span constructions and in this case, it rests directly on Columns (and the slab is to be made Solid instead of ribbed near columns).

Flat Slab System

More commonly seen in Commercial constructions and coming-of-age multi-storey buildings. It doesn’t have any beams.

The flat slabs are stiffened near the column supports by using Drop Panels and Column Capitals.

This is more suitable for higher loads and larger spans, because of its capacity in resisting shear and hogging moments near the supports.

Larger spans can be possible using Flat Slab system. Among the various floor systems, this has the highest dead load per unit area.

Vertical System

Vertical or Framing system resists the gravity loads and also the lateral loads from the floor system and transmits these to the foundations.

Columns

Columns are the most commonly adopted vertical system.

The size of the columns depends on the height and the loads acting on it and also the type of floor system, spacing of columns, number of storeys etc.

The columns are generally designed to resist axial compression combined with bending moments. These effects are severe in the lower storeys of the building i.e., the size of the column reduces as we go from bottom to top.

Columns along with Beams resist Lateral loads which depends on the rigidity of the joint.

Walls

This can be made of either masonry or reinforced concrete. Depending on the major function of these walls, it can be classified into Bearing Walls and Shear Walls.

Walls resisting Gravity Loads are called Bearing Walls and if walls resist Lateral Loads then it is called Shear Walls.