Matthew Stuart, P.E., S.E., P.Eng.

Course Outline

The focus of this one hour online course is the design of reinforced concrete masonry lintels (commonly referred to as bond beams) and precast reinforced concrete lintels. Information is also provided on the how loads supported by lintels are evaluated in light of the arching action of masonry.

This course includes a multiple choice quiz at the end.

At the conclusion of this course, the student will:

• Understand the mechanisms of masonry walls that affect the impact of both uniform and concentrated loads on supporting lintels.
• Become familiar with standard details associated with the construction of new lintel supported openings in existing masonry walls.
• Understand both allowable stress design associated with analysis masonry bond beams and strength design associated with the analysis of precast concrete lintels.

Vertical loads carried by lintels typically include; (1) distributed loads from the dead weight of the lintel and the masonry wall above the lintel, any floor and or roof dead and live loads supported by the masonry and (2) concentrated loads from floor beams, roof joists and other members which frame directly into the wall. Depending on the construction of the wall and nature of the framing that is supported, distributed loads acting on a lintel can be further separated into four load types:

1. uniform
2. triangular
3. concentrated
4. partial uniform

In some instances, the masonry wall will distribute loads so that they do not act on the lintel. This is called arching action of masonry and is based on the amount of masonry that is above the opening over which the lintel spans. The impact of distributed and concentrated loads on lintels is affected by arching action. Arching action can be assumed if the following criteria are met; (1) the masonry is laid in running not stacked bond, (2) sufficient wall height above the lintel exists to form a 45 degree triangle with at least 8 inches of wall height occulting above the top of the arch and (3) minimum end bearing is maintained. For this last criterion it is important to recognize that arching action results horizontal thrust forces at the base of the arch. This thrust must be accounted for in order for arching action to occur. Therefore it is not recommended that arching action be assumed above openings that occur next to corners of a building or at locations where the adjacent block at the bottom of the arch is discontinuous.

As already indicated, the design loads applied to a lintel depend on whether arching action is present or not. In the case of the weight of wall supported by a lintel, arching will cause only the weight of wall within the triangular area below the top or apex of the arch to impact the lintel. The triangular load has a base equal to the effective span length of the lintel and a height as shown below:

Where,

L = 4 x Wall Thickness + Width of Beam
w = P / L

The load is then resolved onto the lintel as a uniform load with a maximum length equal to four times the wall thickness plus the width of bearing. The magnitude of the load per unit length is computed by dividing the concentrated load by this same length: w=P/L. An example of the impact of a concentrated load offset from a lintel opening is shown below:

In some cases a series of concentrated loads may be considered as uniform on a lintel. The criteria as to whether a series of concentrated loads can be assumed as an equivalent uniform load on the lintel is a function of the spacing of the loads. If this criterion is met the equivalent uniform loading can be neglected if the bearing elevation of the beams occurs above any arching action that is present. Otherwise, each concentrated load must be resolved into an equivalent uniform load independently.

As indicated above, in some cases a series of concentrated roof or floor loads on a wall laid in running bond may be considered as an equivalent uniform load. This condition applies to relatively light loads spaced closely together such as floor joists or roof rafters in residential or other similar construction. Concentrated loads of these types may be considered as uniform as shown below:

In general these types of concentrated loads can be considered as uniformly distributed if the total height of masonry between the top of the lintel and the bearing elevation of the joists is at least 1/3 the center-to-center spacing of the loads.

Heavier concentrated loads, such as that which may be encountered in industrial and commercial buildings can also be considered to act as equivalent distributed loads as shown below:

Where, SL is less or equal to 4'-0"

In general, uniform loading can be assumed whenever the spacing of the loads is less than 4 feet and the wall height above the lintel is greater than 1/2 the load spacing. Therefore heavy loads spaced more than 4 feet apart in general should be considered as individual concentrated loads and distributed to the lintel as equivalent uniform load independently.

Concentrated loads over stack bond masonry are not transferred or distributed across vertical joints. An example of this condition is shown below. Loads should not be assumed to be transmitted across vertical joints even if joint reinforcement is used in the wall construction.

It is also common for an engineer to be faced with the prospect of designing and detailing a lintel for a new opening to be cut in an existing wall. Examples of steel lintels that can be installed before the demolition of a new opening occurs are shown below:

Small Masonry Opening in Existing Wall

Large Masonry Opening in Existing Wall

In some cases, however, it is necessary to use either a masonry or precast lintel for a new opening in an existing wall. For this situation it is necessary to take advantage of the arching action of the masonry wall above the opening. The arching action allows for the complete removal of the masonry, as shown below, in order to erect the lintel. The masonry above the lintel is simply in-filled above the lintel after the new opening is complete.

New Masonry or Concrete Lintel in Existing Wall

The following ASD methods and parameters apply to unit masonry lintels. The ASD method compares the design stress produced in a member by applied loads to allowable stresses permitted by the Code. In ASD, the masonry is assumed to resist the compressive forces. The tensile strength of masonry units, mortar and grout is neglected. All tensile stresses therefore are assumed to be resisted by the reinforcing steel. The equations governing ASD are listed below:

Strength design is a method of analysis that compares factored loads to the design strength of the member. Precast concrete lintels are typically designed using this method. This method allows for the load, which produces failure, to be predicted. This method also allows for the failure mode to be controlled so that ductile rather than compressive failure occurs first. Strength design flexural compression, tension and shear are determined in accordance with principles established by the Code. The tensile strength of masonry is neglected and the resulting nominal strengths are computed using the governing equations listed below:

The allowable flexural stress for masonry lintels, Fb, is equal to 1/3 of f’m. The allowable shear stress for masonry lintels is equal to the square root of f’m. For steel, in ASD, the allowable stress is 24,000 psi for grade 60 reinforcing bars. To summarize:

Reinforced concrete masonry strength design reduction factors for flexure and shear are based on ACI 530 and are 0.8 and 0.6, respectively. Precast concrete strength reduction factors are based on ACI 318 and are 0.9 for flexure and 0.85 for shear.

The effective span length of a lintel is defined as the clear span plus the depth of the member but not greater than the distance measured between the support centers. ACI 530 states that end bearing should not be less than 4 inches. As an integral part of a wall, lintels are typically considered as laterally supported. Lintel deflection is limited to the effective span divided by 600 or .3” when used to support unreinforced masonry per ACI 530. The commentary of this same Code waives the L/600 criterion if the supported wall is considered reinforced masonry.

The effective compressive width, b, of a lintel should be taken as the nominal width less 3/8”. For example, you should use 7-5/8” as the actual width of an 8” CMU block. The effective depth, d, is also taken as the nominal depth less 3/8”. The depth of cover and half the diameter of the reinforcing bar should also be subtracted from this depth. Limitations on reinforcing bars as placed in masonry bond beams is shown in the detail below:

With a 1 1/4” face shell and a minimum concrete cover of 3/4”, typically 2” of cover should be assumed for all reinforcement.

Course Summary

The design of masonry and concrete lintels requires an understanding of the load path mechanisms of unit masonry walls. The natural arching action of running bond masonry walls often provides an alternate load path for both uniform and concentrated loads located above a lintel. Concrete masonry lintels are most often designed using the allowable stress design or service stress method of analysis. The design precast concrete lintels must comply with the ultimate strength design provision of ACI 318.

Please use the information you read in this course to answer the quiz questions.

Related Links

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.