This is one of the posts in a series of our posts that we discuss failures. Also check out the other posts in constructmagazine.com, where we discuss failures in different contexts of civil engineering.

A Brief Introduction of Earthquakes vs. Structures:

Earthquakes are very important aspect of most design in civil engineering, and for any type of structure. The fundamental earthquake principles as it applies to structures are of course the same, although there are specific items that apply to each structure. Here we will discuss earthquake related failures in buildings.

Earthquakes result from movements of large land masses, which in turn affect structures, by making them move too. Since the structure is initially static, any movement slow or not, will therefore introduce inertial force on that structure. Remember your physics class in high school and Newton’s law F=ma. Newton’s formula is not used in this form in earthquake design calculations but with some modification coefficients as necessary, but regardless, the logic stems from here. When the land mass under the structure moves with a certain acceleration there will be an inertial force generated within the structure. So, the earthquake force is as if someone pulls the carpet beneath your feet suddenly, which will introduce inertia force on your whole body which will be felt by every atom in your body. It is this ground acceleration that we civil engineers are interested in. In addition, this is in the form of a cyclic motion as the ground shakes back and forth. Mostly we are interested in the horizontal (we call it “lateral”) component of this ground acceleration, because structures are usually stable against vertical acceleration (such as the ever existing gravity of earth) however there are some situations that vertical ground motion can also become critical in design. How much acceleration will there be, at what shaking frequency is of greatest interest to us. The lateral acceleration can be so much that, it can be equal or even more than the value of g, the gravitational acceleration at some instant, as in strong earthquakes. And imagine that this continues in a cyclic way. This means significant demands on the structure. Lateral demand is highest at the foundation level (because it has to laterally support all higher levels) and it decreases as we go up. The total of this force at the very bottom of the structure is called the base shear. This lateral force must be countered and the earthquake energy entering the building must be absorbed.

After this short summary background about earthquakes and structures, we can now start talking about earthquake related items in buildings.

Earthquake Failures in Buildings:

Earthquakes in structures are resisted by LFRS – Lateral Force Resisting System of the structure. In buildings this can be done by

shear walls,

cross braces,

moment frame,

or the combination of these.

In addition, there are more advanced systems such as outrigger systems for very tall buildings.

The “regularity” of the structure is important. Regular means, the forces in the structure follow easy to flow, as straight as possible paths, when being transmitted from top to bottom. Irregular structures place extra demands on its members. Irregularities are among the top reasons of failures from earthquakes.

Earthquake energy entering the building must be dissipated in a controlled manner, otherwise, the remaining energy will simply break some members. This energy dissipation can be achieved by seismic base isolators, which are placed at the foundation level, seismic dampers, which are placed on certain members such on the braces. Many buildings however are built in the more traditional sense and they do not have such mechanisms installed. In this case, the building structure itself must dissipate all energy. Among the three main types of LFRS we listed above, shear walls provide strongest resistance against earthquakes and the make the structure the stiffest. Cross braces are usually used in structural steel buildings. Among the three, moment frame buildings are the most flexible structures because here all resistance we have against lateral forces is the moments created at beam column connections, while they rotate.

Strong column – weak beam principle is applied. Even if the beams fail, the columns should stand. While failing, the beams should fail from flexure, because this is the tension failure of rebar (ductile) which dissipates a lot of energy which is good, instead of shear failure, which is sudden and brittle with little deformation. Strong column – weak beam also has the advantage that, in this case a lot of beams can fail which means more energy dissipation, as opposed to only a few columns.

Short columns must be avoided. In a given floor, if one column’s free length is shorter than the others (which can be caused by a short wall near column, reducing its free unbraced length), this means it will be far more stiffer than the others against lateral movements during earthquakes. Stiffness attracts force. This means the short column will attract much higher force, which will lead to its collapse.

If there is a weak or soft story with respect to other floors, it will move the most during lateral shaking and this can lead to collapse of the entire structure.

Load bearing walls may get crushed. Wall failures can be in or out of plane.

Shear wall failures are such as excessive bending, shear, which shows in the form of diagonal cracks, buckling if the wall is not adequately braced by slabs at floors.

Shear walls must definitely continue all the way down to the foundations because they attract a lot of lateral force and not transmitting this to the foundation directly will cause too much demands within the structure otherwise. Columns should also continue all the way to the foundation level, although in this case there might be some exceptions depending on the project.

Sometimes two separate shear walls near each other are tied (coupled) with each other by short beams, called coupling beams to create a much stronger resistance against lateral forces. The detail of these beams must be done properly otherwise the coupling action, which the design counts on, may be eliminated.

Slabs in a building do not just carry people and furniture over them. They have another primary duty which is even more critical. They also serve as diaphragms, meaning, they serve as horizontal connections between the vertical elements such as columns and shear walls (which are the lateral force resisting elements) to brace them and also to make them work together during lateral shaking. This way, a much more resistant box action can be formed against lateral loads. Slabs achieve this by their stiffness and through special reinforcing details within them that run as connections between vertical members. If this is not properly made, diaphragm action may not be fully developed which can lead to overall failures.

Imagine a flexible table, with 4 legs attached to a shaking plate below. This is a plate that can move horizontally back and forth. Now shake the plate horizontally back and forth. The table top will deflect symmetrically. Now assume you placed extra legs on one side of the table just for the purposes of this experiment. Shake the plate again. You will notice that while still deflecting horizontally, the top of table will tend to rotate as well, putting the entire table in torsion. You have caused overall torsion this time because the lateral stiffness provided by legs didn’t coincide with the center of mass of the table (remember F=ma), where the inertial shaking force acted on average. Similarly, since all we have are the columns and shear walls that provide lateral resistance in a building, their center of stiffness, which practically means their geometric center, must coincide with the center of mass of the floor. Otherwise the building will experience full torsion which can lead to overall failure.

Due to the reason explained in the previous paragraph, the overall torsion risk, it is better to place the lateral force resisting elements, towards the perimeter as much as possible, to increase the building cross section’s moment of inertia, and in turn stiffness against torsion.

If a building has basement, this can act like a “socket” which increases resistance during lateral shaking. To achieve this action, the basement floors must be able to perform like box frame, with the foundation system, basement walls and basement floor slabs (and especially the ground floor slab, which takes on much higher lateral load than any other slab). Any weak links in this box action can lead to overall failures.

The extent of column damages are very important. In first phase, only the concrete cover around rebar falls of. In the next phase there is also some deformation in the rebar. In the final phase, there is a lot of deformation in the rebar and the concrete core inside the ties (horizontal rebar that are placed in small intervals to hold the vertical bars together and laterally brace concrete core) is crushed, which means total damage of the column. During post earthquake damage assessment of a building, if the building is safe to enter, columns are the first places to look at.

The slabs around the shear walls must be properly attached with reinforcing to shear walls otherwise during lateral movements they may just get torn away form the wall. Also the more continuous the slab around the shear wall, the better for transmitting lateral forces with diaphragm action.

For foundation related failures see our post failures in geotechnical engineering in constructmagazine.com.

Non structural damages can occur such as:

• Collapse of ceilings, partition walls,
• Falling of fixtures, shelves
• Facade system, glasses, windows, doors can fall, get damaged
• Plumbing, HVAC systems may get damaged, elevators stop functioning

At this point, you may also want to check out our post an overview of earthquake analysis methods, in constructmagazine.com.

In this Construct Magazine post, we tried to only “scratch the surface” by providing an introductory summary about earthquake resisting mechanisms and failures in buildings by giving some common examples together, which can help creating at least an initial big picture inn one’s mind, and can serve as starting point for further exploration. The author often referred to his published book “How to Construct: Introduction to Civil Engineering, Structures and Construction” during writing this article.

By: Ahmet Tuter

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