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Steel Lintel on existing masonry: Use Cases and Benefits

Why installing a steel lintel enhances the strength and seismic response of existing masonry.

Installing a steel lintel on existing masonry is one of the most effective reinforcement measures to prevent walls overturning and to absorb horizontal forces during seismic events.

This article explores the types of steel lintels to choose from, how to install them, their impact on the overall seismic behavior of a masonry building, and how they reduce seismic vulnerability associated with improving existing masonry using a structural masonry analysis software.

Types of Lintel for Seismic Improvement of Masonry Buildings

Constructing a lintel (inter-floor or roof) is often necessary to ensure a box-like behavior of existing masonry buildings. Lintels can be built using various construction techniques. Each material has its pros and cons that can influence the overall behavior of the building and even compromise its stability during seismic events.
Until the 1990s, the most common intervention type for restoring historic masonry buildings was undoubtedly the construction of reinforced concrete lintels, placed at the top of the building. However, this widely used technique soon proved to be ineffective or even dangerous during seismic events and was gradually replaced by other intervention types. Experience showed that buildings that had undergone this type of intervention often suffered recognizable damages: heavily damaged masonry (in many cases completely disintegrated) and perfectly intact concrete lintels.

Why does this happen?

This results in the so-called “beam effect.” In practice, the rigid reinforced concrete lintel, compared to the poor quality of the underlying masonry, transfers the weight of the roof mainly to the corners of the building. As a result, the central portion of the masonry is left unloaded and, without the effect of vertical load, is more susceptible to overturning or disintegration phenomena. Thus, constructing highly stiffening elements on top of low-quality masonry can lead to the collapse of the masonry itself due to disintegration. The seismic force concentrates on elements with lower stiffness, causing severe damage to the masonry.

One solution to mitigate this phenomenon could be to improve the cohesion, strength, and anchorage of masonry panels, as well as the stiffness of the coping itself. To ensure that the masonry panel maintains its cohesion even in the event of an earthquake, interventions such as:

  • repointing of mortar joints;
  • reinforced concrete cladding;
  • carbon fibre reinforcement (FRP);
  • mortar injection;
  • reinforced stitches;
  • etc.

are necessary. To ensure proper verification of existing masonry and concrete lintels, it is always advisable to use a software with integrated FEM solver for calculation.

In summary, when planning to construct a concrete lintel, it is always advisable to:

  • improve the quality of existing masonry with consolidation interventions;
  • size the lintel appropriately, choosing a maximum height between 20 and 25 cm;
  • use concrete with low strength and low modulus of elasticity, preferably lightweight concrete;
  • connect the lintel to the underlying masonry using steel bars embedded in the masonry.

Other alternatives to concrete lintels may include constructing lintels in:

  • reinforced brick masonry, created by placing a steel reinforcement within a solid brick lintel. It can be constructed only if the underlying masonry reaches a thickness of at least three brick heads. This thickness is necessary to create the brick cladding around the reinforcement. If the masonry is at least 30/40 cm thick, it is an excellent alternative to concrete lintels. With lower stiffness, it distributes loads more evenly;
  • steel, suitable even for masonry with reduced thickness (30 cm), characterized by lightweight and limited invasiveness;
  • reinforced masonry with GFRP (Glass Fiber Reinforced Polymer) mesh, made with glass fibers, carbon, or other similarly resistant traction material.

Steel Lintel: What It Is and How It’s Constructed

Steel lintels can be built in two ways:

  • using two truss structures made with angles and metal plates, placed on the internal and external faces of the masonry and connected to each other with reinforced holes;
  • by using steel plates or profiles placed on the two faces of the masonry and connected to each other by passing bars.

The steel lintel should have a width equal to the thickness of the masonry and an adequate section. It can be made of structural steel or carbon steel. Each construction technology has its pros and cons.
In this context, the connection between the lintel and the masonry will be ensured by adhesion, meshing, and friction. Furthermore, for the top lintel, it is advisable to provide reinforcement of the upper masonry before installing the lintel and connect it to the underlying masonry by creating reinforced holes. Here are some construction details to be respected for their construction:

  • the minimum height of the coping must be equal to the height of the floor slab;
  • the width of the coping must be at least equal to that of the masonry;
  • for masonry with a thickness up to 30 cm, a maximum setback of the coping from the outer edge not exceeding 6 cm and 1/4 of the thickness is allowed;
  • the area of the longitudinal reinforcement of the coping must be at least 8 cm²;
  • the stirrups must have a minimum diameter of 6 mm and a maximum pitch of 25 cm.

Beneficial Effects on the Seismic Behavior of the Building

Steel lintels serve to connect masonry panels to each other, especially the upper ones, where the masonry is subjected to lower compression stress compared to the underlying walls, as they are subjected to lower vertical loads. The presence of an upper lintel improves interaction with the roofing structures and ensures a box-like behavior of the building. Additionally, it prevents out-of-plane overturning of the top-level wall if the coping is well connected to the underlying masonry and helps absorb the roof thrust in the case of a pushing cover.
Steel coping improves the transmission of horizontal actions (wind, seismic, horizontal component of the force due to roof thrust, etc.) from the horizontal elements to the masonry walls subjected to actions in-plane.

Roof's pushing action on existing masonry

Roof’s pushing action on existing masonry

Even at the roof level, the presence of a lintel allows for a bracing effect on the perimeter masonry walls, allowing for the redistribution of horizontal forces on the vertical elements.

For the steel lintel to absorb the horizontal thrust of the roof, it must have an adequate section and, consequently, adequate flexural inertia to prevent excessive flexural deformations in the horizontal plane that could lead to masonry overturning. The ability of the steel lintel to absorb the roof thrust also depends on the distance between two successive bracing walls to which the coping must transfer the action of the horizontal thrust.

If during sizing the roof thrust leads to an excessive section of the steel lintel, it is advisable to absorb this thrust by installing metal ties. In fact, the Technical Regulations specify that roofing structures must not exert thrust, and if present, it must be absorbed by suitable structural elements (ties, tie rods).

Therefore, if the intervention on the existing building also involves roof replacement, it is advisable to design the absorption of any thrust using ties of adequate section, reserving the steel lintel solely for the redistribution of horizontal forces generated by wind and seismic events.

Steel Lintel: Static Schemes for Sizing and Calculation

To size the minimum section of steel lintels, it is first necessary to define the static scheme for calculating stresses and displacements. The static scheme to be adopted will be a continuous beam scheme on multiple supports where each support represents the restraint action exerted by the bracing wall.
The uniformly distributed horizontal load acting on the continuous beam scheme will be generated by the following actions:

  • seismic action generated by the mass of the walls and the cover connected to the lintel;
  • wind action on the areas of influence of the walls connected to the lintel;
  • roof thrust in the case of a pushing roof and in the absence of ties.

By combining the actions in the appropriate way to obtain the load combinations at the Ultimate Limit State (ULS) and the Serviceability Limit State (SLS), using the partial combination coefficients prescribed by the Technical Regulations, it will be possible to calculate the stresses at the Ultimate Limit State and the displacements at the Serviceability Limit State to size the steel coping. To ensure correct calculation and compliance with regulations, it is always advisable to proceed using structural calculation software.