Lateral forces are horizontal forces that act perpendicular to a structure’s vertical axis. They are crucial in structural design because they can significantly impact a building’s stability and integrity if not properly accounted for.
These forces require specialised structural elements, such as shear walls and bracing systems, to transfer the load to the foundation and ensure the structure’s resistance.
Furthermore, most load sources that produce lateral forces also generate some vertical effects; therefore, it is of limited use to treat the horizontal-force impact in isolation.
Even where this may be valid as an investigative technique, it should always be borne in mind that lateral effects always occur in some combination with some vertical effects, including those due to gravity. Ultimately, it is the combined effects that structural engineers must understand and address.
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Table of Contents
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Sources of Lateral Forces
The principal sources of lateral forces in buildings are the following:
1. Wind Forces
Wind is moving air. Air is a fluid, and a general understanding of fluid mechanics helps understand the various effects of wind on buildings.
Our primary concern in this post is the impact of wind on the building’s lateral bracing system.
As a net effect, this force is an aggregate of the various effects of the fluid flow of the air around the stationary object (the building) on the ground surface.
2. Earthquake Forces
Earthquakes—or seismic activity as it is called—produce various disastrous effects, including tidal waves, massive ruptures along earth faults, and violent vibratory motions.
It is the last effect for which structural engineers/ civil engineers design lateral bracing systems for structural elements, dealing mostly with the horizontal aspect of ground motion.
3. Soil Pressure Forces
The problems associated with soil-restraining structures and the general behaviour of soils must be considered during the structural design of the foundation.
Wind, earthquake, and soil pressure forces on the building must eventually be resolved by the building foundations and supporting soils.
4. Structural Actions
The natural action of various structures in resisting gravity loads may result in some horizontal forces on the supports of the structure, even though the direction of the gravity load is vertical.
Common examples of such structural elements (structural members) are arches, gable roofs, cable structures, rigid frames, and pneumatic structures sustained by internal pressure.
5. Volume Change: Thermal, Moisture, and Shrinkage
The actions of thermal expansion and contraction, moisture swelling and shrinkage, and the initial shrinkage of concrete, mortar, and plaster are all sources of dimensional change in the volume of building materials.
Ideally, these effects are controlled through the use of expansion joints, deformable joint materials, or other means.
Furthermore, the potential forces that they represent must be well understood and accounted for in terms of structural resistance in some instances.
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Application of Wind and Earthquake Forces
To understand how a building system resists the lateral-load effects of wind forces and earthquakes, it is necessary to consider how these forces are applied and then visualise how they are transferred through the lateral resistive structural system and into the ground.
1. Wind Forces
The application of wind forces to a closed building is in the form of pressures applied normal (perpendicular) to the exterior surface of the building and surface shears applied to sides parallel to the wind direction.
In one design method, the total wind effect on the building is determined by considering the vertical profile, or silhouette, of the building as a single vertical plane surface perpendicular to the wind direction.
A direct horizontal pressure is assumed to act on this plane. The lateral resistive structure that responds to this loading consists of the following:
- Wall surface elements on the windward side: are assumed to take the total wind pressure and are typically designed to span vertically between the roof and floor structures.
- Roof and floor decks: considered as rigid planes (called diaphragms), receive the edge loading from the windward wall and distribute the load to the vertical bracing elements.
- Vertical frames or shear walls: acting as vertical cantilevers, receive the lateral loads from the horizontal diaphragms and transfer them to the building foundations.
The foundations anchor the vertical bracing elements and transfer the lateral loads to the ground.
2. Seismic Forces
The dead weight of the building construction generates seismic forces. When visualising the application of seismic forces, we examine each part of the building and consider its weight as a horizontal force.
The load propagation for the box-shaped building in the Figure below will be quite similar for both wind and seismic forces.
Propagation of wind force and basic functions of elements in a box building.
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Types of Lateral Forces: Resistive Systems
The following are the types of lateral resistive systems:
1. The Box System
This system consists of some combination of horizontal and vertical planar elements. Most buildings use roof and floor construction that qualifies as a horizontal diaphragm.
The box also uses vertical diaphragms (shear walls), although planes of trusses may also qualify due to their relative stiffness.
2. Internally Braced Frames
The typical assemblage of post-and-beam elements is not inherently stable under lateral loading unless the frame is braced in some manner.
Shear wall panels may be used to achieve this bracing, in which case the system functions as a box.
It is also possible, however, to use diagonal frame members to achieve trussed panels, X-bracing, knee braces, and other similar structures.
The term braced frame is used to refer to a truss-braced frame.
3. Rigid Frames
Although commonly used, the term’ rigid frame’ is a misnomer, as this technique often produces the most flexible lateral resistive system.
4. Externally Braced Structure
Building systems lacking internal stability may be reinforced with external elements, such as guys, struts, and buttresses.
5. Self-Stabilising Elements and Systems
Retaining walls, flagpoles, pyramids, tripods, and so on, in which the basic form of the structure achieves stability, are examples of self-stabilising elements and systems.
These may be independently stabilised or may serve to brace an attached unstable system.
Please note:
A key property of bracing systems is their relative stiffness or resistance to deformation. This is particularly important for evaluating energy capacity, which is especially critical for investigating the response to earthquake effects.
A box system with vertical diaphragms of concrete or masonry is very rigid, whereas wood frames and moment-resistive steel frames are usually quite flexible.
Stiffness affects the fundamental period of the structure and thus influences the percentage of the dead weight assumed as a horizontal force.
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Lateral Resistance of Ordinary Construction
Even when buildings are built without consideration for lateral forces in their design, they will still have some inherent capacity for lateral-force resistance.
It is beneficial to have an understanding of the limits and capabilities of ordinary construction as a starting point for considering designs that enhance lateral force resistance.
Wood Frame Construction
Wood structures can be categorised broadly as light wood frame or heavy timber. Buildings with light wood frames often lack sufficient walls—either interior or exterior—that can serve as shear walls.
However, these types of construction have the following changes:
- Lack of Adequate Diaphragm Surfacing: The desired surfacing materials may not be strong enough for use in the lateral resistive system.
- Lack of Continuity of Framing: Because it is often not required for gravity loads, horizontal framing may not be aligned in rows for use in the lateral resistive system. Any discontinuity in the regular order of framing can present a problem, usually solvable but requiring some attention for the structural design.
- Lack of Adequate Solid Walls to Serve as Shear Walls: This may be due to inadequate building planning, with insufficient walls at certain locations or in a particular direction. Walls may also simply be too broken up into short lengths by doors or windows. For multistory buildings, there may be a problem where upper-level walls do not occur above walls in lower levels.
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Structural Masonry
Masonry structures offer numerous advantages for developing resistance to wind loads. As built today, they have great potential for lateral shear resistance.
They are also quite stiff and heavy, offering considerable resistance to lateral deformation and providing significant resistance to horizontal and uplift effects of wind.
For regions of high risk for earthquakes, the only masonry structural construction used is reinforced masonry.
Structural masonry walls have considerable potential for utilisation as shear walls. However, some concerns must be addressed:
- Increased Load: Due to their weight, stiffness, and brittle nature, masonry walls must be designed for higher levels of lateral seismic force.
- Limited Stress Capacity: The unit strength and mortar strength must be adequate for the required stress resistance. Both vertical and horizontal reinforcement are required. Voids usually needed to be filled for shear wall and foundation uses.
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General Building Design Problems
Some of the major issues that should be kept in mind in the early planning stages are the following:
- Need for a Lateral Bracing System: In some cases, due to the building’s form, size, or the decision to use a particular type of construction, choices may be limited. In other situations, there may be several options, each with different required features, such as column alignment and the incorporation of solid walls. The particular system to be used should be established early.
- Implications of Architectural Design Decisions: When certain features are desired, it is essential to understand that they may have consequences in terms of design problems with the lateral bracing system.
- Design Styles Not Developed for Lateral Loads: In many cases, popular architectural design styles or features are initially developed in regions where windstorms or seismic effects are not of concern. When these are imported to other areas with high risk for lateral effects, a mismatch often occurs.
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Final Thought on Lateral Forces
A structural engineer must ensure that they control lateral forces to obtain a building that will be safe and economically viable for human use.
For tall buildings, it is important to control wind and earthquake forces to prevent possible failure and cause deaths and property damage.
As discussed above, these forces require specialised structural elements, such as shear walls and bracing systems, to transfer the load to the foundation and ensure the structure’s resistance.
Shear walls must be properly designed in order to make the building stable.
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