Everything you need to know about frame structures: materials used, types of static behavior, design techniques
A frame in construction, by definition composed of beams and columns, typically represents a fundamental element in the design of buildings and plays a crucial role in the stability and durability of a structure.
In this article, we will look at the most important aspects related to frame structures, identifying the elements that characterize them, the different types in relation to the different materials used, and the various categories of static behavior.
We will also show you with a video how simple and fast it is to model frame structures with a BIM structural design software.
What is a frame structure
A framed structure consists of several interconnected elements, each playing a crucial role in ensuring the overall stability and functionality of the system. The frame, defined as the structural element composed of two vertical columns and a rigidly connected horizontal beam, is repeated both horizontally and vertically. This arrangement allows for static continuity and efficient use of space with limited planimetric envelopes.
The columns – vertical inter-floor elements – can be aligned or positioned at a distance from each other based on the optimal span for the floor slabs, following a regular mesh of square, rectangular, or triangular shape. The beams, the horizontal plane elements or inclined structural members transfer loads onto columns
Frame structures mainly work under bending and shear, compression, and bending compression. Columns and beams are the key elements of this structure. The column, a vertical load-bearing element, transmits loads from the superstructure to the foundation, undergoing vertical and horizontal loads, normal stress, bending moment, or bending compression.
The beam, similar to the column but with predominantly larger dimensions, is geometrically defined as a solid generated by a flat figure moving in space, remaining orthogonal to the trajectories described by its axis. Beams can be primary or secondary, with different sections such as rectangular or I, T, L, C, H profiles, etc., in order to reduce weight and optimize material usage based on the stresses.
Beams can also have lightweight sections, such as box girders or lattice profiles. Box beams, suitable for large spans, consist of hollow closed sections with internal stiffening elements. Lattice beams, composed of vertical and diagonal rods, are ideal for axial forces and can be hinged at the nodes, providing a system of rods subjected to compression or tension, depending on their position in the lattice mesh.
Types of frame structures
The frame structure can be divided into different types based on both materials and static behavior. Let’s look at these differences together.
The materials generally used for frame structures are reinforced concrete, steel, and wood.
Reinforced concrete frames
Reinforced concrete frame structures for buildings, subject to horizontal actions, can ensure stability through the implementation of structural nodes in the bays. These nodes act as energy dissipaters, inserting additional reinforcement to prevent the expulsion of concrete during seismic events. Alternatively, reinforced concrete walls, called “partitions,” or stiffening cores, such as vertical compartments, can be used.
Columns and beams in reinforced concrete frame structures can take on different forms, such as square, U, T, L, or I. Beams can vary in height, thickness, and can be extradosed. The structures can be cast in place with steel rods positioned in formwork, ensuring good continuity of stress transmission through anchoring of the metal bars in the casts.
A faster solution is the use of block-formwork in which concrete is poured. In these solutions, the columns can be made with blocks that house the reinforcement, and the beams can be made from hollow blocks. For fully or semi-prefabricated structures, different configurations can be adopted, ensuring good flexibility of use and limited production and assembly costs.
Steel frame structures are used for civil and industrial buildings. In longitudinal frames, civil buildings have vertical structures, while industrial buildings have horizontal structures with longitudinal bracing. Buildings with longitudinal frames can incorporate internal or facade vertical bracing to increase flexural rigidity. Furthermore, for tall buildings in seismic areas, solutions can be designed where the vertical supports only resist vertical loads and rigid structures handle horizontal forces.
Metal frame structures involve the on-site assembly of profiles and sections through bolting or welding. These can be solid wall, cassette, or lattice structures. Beam-column connections can be made using various techniques, such as bolting, welding, or support on brackets. Structures with industrialized systems make construction more economical, allowing prefabrication and organization of transport and assembly.
Wooden frame structures can be made with solid wood or glued laminated timber. Connections can be made with bolts, nails, or adhesive joints. The spans of the beams and the size of the structural meshes vary according to the requirements, with wooden solutions offering design and assembly flexibility.
Connections between beams and columns can be made using angles, bolted or welded plates, bolts, or internal metal plates. Stiffening against horizontal actions can be achieved through cladding panels, floor panels, or metallic lattice bracing. Wooden frame structures can be particularly suitable for residential constructions.
Regarding static behavior, we can have the following categories.
Isostatic frames are statically determined structures, which means that it is possible to completely resolve all reactions and internal forces using equilibrium equations. These frames are characterized by a sufficient number of constraints to ensure the stability and determinacy of the system. Examples include rectangular and triangular frames, where the constraints at the junctions ensure a clear solvability of internal forces.
Hyperstatic frames are statically indeterminate structures, which means that the number of constraints and reactive forces is not sufficient to completely resolve all reactions and internal forces. These frames require the use of more advanced methods, such as the force method or the deformation method, to obtain a solution. Examples include frames with excess supports or restrained beams, which require a more sophisticated approach in determining internal forces.
Fixed node frames
Fixed node frames are characterized by rigid junctions between beams and columns. This means that the nodes cannot rotate and that the frame can resist deformations without allowing significant displacements at the junctions. These frames are often used in situations where greater rigidity and deformation resistance are required.
Movable node frames
Movable node frames are characterized by flexible junctions between beams and columns. This means that the nodes can rotate, allowing greater flexibility in structural behavior. These frames are often used in situations where greater adaptability to deformations is required or to handle variable loads.
Primary components of a structural framework: columns and beams
Reinforced concrete columns and beams
In the design, calculation, and construction of reinforced concrete columns and beams, it is essential to comply with the regulations and current building code requirements.
The reinforced concrete column, to simplify the construction process, often assumes a square or rectangular shape, rarely circular. The arrangement of longitudinal iron rods is fundamental and can vary depending on the stresses in the element. The morphology and arrangement of the rods are influenced by the load and constraint conditions. The longitudinal reinforcement, placed to resist tensile stresses, is complemented by transverse reinforcement consisting of small-diameter stirrups to prevent expansions and lateral deflections.
The columns on the upper floors, with reduced loads, may have smaller dimensions but require a higher percentage of iron to resist stresses such as wind pressure. The cuts in the internal and perimeter columns are made to balance the loads in each section, ensuring a baricentric resultant.
Reinforced concrete columns can be cast in place or prefabricated in the workshop. The rods and stirrups of prefabricated columns are generally of smaller diameter compared to those of cast-in-place columns.
As for the beams, these exploit the mechanical characteristics of concrete to resist compressive stresses, while steel, in the form of steel bars, manages tensile actions. For spans up to 8-10 meters, full-section reinforced concrete beams can be used; for spans greater than 12 meters, a double system of main and secondary beams is recommended. For even larger spans, pre-stressed reinforced concrete beams can be used.
To ensure safety in case of fire, the steel reinforcement of columns and beams must be properly covered with at least 2 cm of concrete (cover), increased to 4 cm in aggressive environments.
The choice of the optimal dimensions of the beams depends on various factors, including the base/height ratio and the orientation of the supported, cantilevered or bracketed beam.
Steel columns and beams
Steel columns can take on different shapes, including circular, square or rectangular profiles, characterized by a lower vulnerability to instability effects due to slenderness. Alternatively, it is possible to create a column by combining semi-finished products which, through connections, constitute composite elements. The type of connection between the composite elements influences the structural behavior: bolting and riveting require movable elements and pre-drilling, while welding, thanks to metal fusion, avoids the need for drilling, thus obtaining monolithic elements. For the calculation of steel structures, you can use a steel connection design software.
The load-bearing capacity of a steel column is influenced by its slenderness, which in turn guides the choice of the cross-section. Circular tubular profiles offer the maximum inertia to lateral bending, while square sections show good behavior under concentrated loads. However, the use of these profiles is limited due to the difficulties and costs associated with assembly with other structural elements. Open sections are subject to bending and, for critical loads, also to torsion. IPE and HEA sections are the most commonly used.
The steel beam can be designed as a solid wall (with single or composite section profiles), as a box or lattice. “Double T” profiles are commonly used as load-bearing beams, while other types such as “C”, “L”, “T”, etc., are used to form composite or lattice beams. The height of the simple “double T” beam depends on the bending moment acting on the beam. In case of excessive height of a single profile, it is possible to use spaced twin elements and connected separately to the column or coupled to each other.
The connection between the various parts of a composite steel column occurs 40-50 cm above the floor level. The two column trunks can have the same section or different sections, and the connection can be made by welding or bolting.
The solid wall beam is mainly used to cover large spans with high loads, such as road and railway bridges.
Lattice beams, composed of elements arranged in a grid, are suitable for covering intermediate spans and are characterized by high strength and ductility. The choice between continuous or non-continuous beams depends on the joint conditions. The connection between the components of the lattice beam can be made with bolts or welds.
The verification of steel beams includes the verification of stresses, deformations, and local instabilities.
Wooden columns and beams
Wooden columns, often made with T or I-shaped box sections or by coupling elements, show excellent resistance to both compression and bending. However, excessively slender and inadequately braced columns in their length can be subject to instability phenomena, with possible lateral deflection due to concentrated loads. These phenomena can be accentuated by the presence of knots, deviations of the wood fibers, low material rigidity, or eccentric loads.
Simple wooden beams can have conical or rectangular shapes, depending on the wood profile.
Laminated wood columns and beams are made up of successive layers of selected and glued wood strips (lamellae). These elements boast high mechanical strength, are resistant to hygro-thermal variations, and show low vulnerability to attacks by parasites and fire.
Through the use of glued laminated wood or new composite materials, it is possible to give beams different shapes, such as tapered, angled, longitudinally curved, or lattice beams, according to specific structural requirements.
The sizing criterion for a solid wood beam will be different from that of a laminated wood beam, as the choice of material influences the behavior, characterized by different elasticity and deformability of the various parts.
How to design a framed structure: video
Below is a demonstration video that allows you to understand how to operate with a structural design software in the modeling and calculation of the framed structure.