Lateral Force Resisting Systems

How to Design for Earthquakes and Wind Storms

Introduction

Modern buildings are amazing creations that take a diverse team to conceptualize, design, and build, but we rarely stop to think deeply about the intricate engineering that keeps them standing even in the face of strong winds and earthquakes. Fortunately, this is a focus of structural engineers like co-elevate. Some loads follow gravity and are fairly easy to visualize. Other loads, like wind and seismic loads, are harder to visualize, both in how they load the structure and in how that load gets to the ground. co-elevate’s team is well versed in analyzing and designing the lateral systems that work hard to protect our buildings and occupants and safely pass these loads to the soil.

Structural Systems

Every building contains two primary structural systems: the gravity system and the lateral system. The lateral system is designed to resist horizontal forces like wind, seismic, and occupant movements and provide overall stability to the building. It is broken further into a diaphragm system and the wall/bracing system. The diaphragm system in most buildings is the floor or roof system. This system’s purpose is to collect lateral loads at each floor and pass them to the wall/bracing systems. Those wall/bracing systems then take those loads and pass them to the foundations and underlying soils. Different systems require different approaches, and we employ techniques from hand calculation to advanced finite element modelling to simulate and evaluate the system response under various load conditions.

Lateral Forces

Structural engineers are primarily concerned with two lateral forces: wind loads and seismic or earthquake loads, both of which are specified in our building codes. Although wind is often perceived as a gentle breeze, it can exert immense pressure on buildings during storms and attempt to topple them. The building resists this force through its exterior walls, which are connected and pass wind loads to the floor system, which in turn passes those loads to the wall/bracing elements and finally to the ground.

Earthquakes, unlike winds, load buildings indirectly through ground movements. As Sir Isaac Newton once said, an object at rest wants to remain at rest. As the earth displaces below a building, the inertia of the floors above grade counter this movement, leading to differential movements between floors that induce large forces throughout the building. If these are not resisted properly, severe damage, partial collapse, or even major collapses can occur.

Lateral Systems

Diaphragms

Diaphragms typically make use of a floor slab or roof deck in a building. Slabs and deck are already present for gravity loads, so we try to use them for multiple purposes where we can. Alternatives like horizontal trusses can be used, which is common in industrial settings or for particularly long spans. Diaphragms can be further broken down into flexible, rigid, or semi-rigid conditions, depending on the materials and spans involved.

A typical diaphragm is made up of 3 components: chords, collectors, and shear panels. A shear panel has the ability to pass shearing loads in plane, such as a slab or deck. However, these thin panels are not able to resist significant bending forces which arise at the same time. Chords of a system allow for a tension and compression couple to develop that resists the bending loads. The collectors (sometimes also called drags or struts) are used to bring loads to supporting members or to locally reinforce areas that need additional load transfer.

Diaphragms can quickly get complicated due to openings and layout changes and they have a lot of forces, subelements, and transfers to keep track of, but our team is skilled at reducing that down to simple, constructable details.

Bracing Systems

There are many forms of bracing, but they all stem back to one common concept: diagonal elements are good at passing loads from above to below. Forming a triangle with the brace creates a strong element that can pass the lateral load down. This also creates an overturning moment that is typically resisted by the columns flanking the braced bay. Brace systems tend to be very efficient structures that engineers favour, especially in low- to mid-rise structures.

Moment Frames

When a door, window, or open floor plan preclude the use of braces, there is still an option! Moment frames (also known as rigid frames) are formed by creating strong, robust connections between the beams and columns that convert the lateral shear into shear and bending in the columns and beams. These systems are much less efficient than braces, but they offer the advantage of allowing large openings, such as at storefronts and interior spaces.

Wall Systems

Concrete, masonry, and timber buildings often have repeating load bearing elements. While these resist gravity forces day to day, they can also be used to resist lateral forces as a shear wall. These walls cantilever from the foundation and use bending and shear to resist the lateral loads. Concrete and masonry shear walls use reinforcing steel to provide bending and shear resistance, while timber uses OSB or plywood sheathing for the shear resistance and built-up posts at the ends for bending resistance. Shear walls are very stiff, which makes them a good choice when a floor plan can accommodate their use.

Conclusions

Lateral systems are one of the hidden heroes of structural resilience. They work hand in hand with the vertical structural system to keep buildings standing in the face of major wind storms and the ground shifting beneath them. The loading and systems are not always intuitive, but that’s where the structural engineer gets involved. The structural engineers at co-elevate are focused on breaking down the unintuitive into easy-to-build, safe, and efficient systems that keep your buildings standing tall in the face of such events.