Performance+of+Precast+Structures+under+Seismic+Loading

=Performance of Precast Structures under Seismic Loading = = = //Tyler A. Poff, B.A.E., Penn State 2014 //

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Key Words
Precast, Seismic, Earthquake, Connections, Detailing

Introduction
Precast concrete is a structural system formed by using molds to form and cast a concrete member in a controlled environment. Once the concrete is cured, it is then shipped to the construction site and hoisted into place. By doing this, concrete is cured properly under the supervision of professionals and the speed of construction is increased. While precast has many advantages in construction, there are also many areas where it does not perform as well as other structural systems unless special care is taken. One major area where precast concrete does not perform as well as some other structural systems is with seismic loading.

Due to poorly behaving precast structures during earthquakes throughout the world, precast is viewed as a poor performing structure for resisting earthquakes. While historically, precast structures have not fared well against earthquakes, if proper care is taken precast structures can perform quite well in seismic regions. Many of the earthquake related precast failures throughout history have come from poor design, deficient diaphragm action, inadequate detailing, and/or deformation issues (Sauter).

Seismic Effects
When a building experiences forces during an earthquake, ground waves create inertial forces which take a building that is in equilibrium and move the base. These seismic forces go back to Newton's Second Law of Motion, where the force experienced (F) is equal to mass (M) multiplied by the acceleration (A) or better known as F=MA. For a building, the mass that experiences this acceleration is equal to the overall weight of the building. Thus in general, heavier structures such as precast experience increased seismic loads due to higher building weight. The acceleration that a building experiences during a an earthquake is measured with regard to the acceleration due to gravity (g), which takes place over a fraction of a second before the building experiences an acceleration in a different direction, suddenly changing the buildings movement [Figure 1]. During a reasonable earthquake, waves from ground vibrations create accelerations around 0.2g's in the building, which is approximately equivalent to turbulence on a plane. This acceleration applied to a building over time, exerts significant stresses on the building, and can weaken the structure leading to a reduction in seismic resistance (Arnold). media type="youtube" key="w8BewKziTpU" width="510" height="290" align="center"

Figure 1: Simulation of a structure under seismic loading. (Video Credit: KCRA News) Once seismic loads enter a building, the forces are then distributed throughout to the lateral resistance elements throughout the building by a rigid diaphragm, making connections in precast diaphragms extremely important so that the loads can be properly distributed to the elements. Once the loads enter the rigid diaphragms, elements take a percentage of the loads based on its relative stiffness compared to other lateral components within the structure. Since all the elements are tied into a rigid diaphragm, they will deflect the same amount causing the more rigid element to take more load. Therefore, if important members in the lateral design are connected incorrectly or damaged, it increases the chances of a complete collapse due to earthquakes ability to find the weak elements in the lateral system (Arnold).

Due to many different factors, the seismic effects due to the shaking the seismic forces in a building are often times amplified. One of these amplification factor is the quality of the soil supporting the structure. With soft soils, the amount of shaking experienced by a building can be amplified by up to 1.5 to 6 times the amount the is experienced by the rock below. Therefore, areas with soft soils, damage due to earthquakes tends to be more common. This phenomenon was prevalent when the 1906 San Francisco earthquakes was reviewed. Maps were developed to show the amount of damage that buildings experienced compared to the soil conditions showing that structures with soft soils experienced more damage (Arnold).

Seismic loading can also be amplified due to the natural period of the building and the period of the seismic waves. Since, all object have a rate the they move back and forth, a building will typically sway at a frequency of its' natural period. The period of a building varies mainly depending on the height of the building along with other factors such as the structural system, materials, and geometric proportions. The natural period of a building can range from 0.1 seconds for a one story to 7.0 seconds in a sixty story building as shown in Figure 2. Once a building undergoes seismic loading, its' natural period may also change due to structural damage (Arnold). With precast concrete structures, the structure may experience cracking due to the stresses causing the building to become less stiff, which increases the buildings period during vibrations.

The ground on which structures sit on also have a natural period at which it vibrates at during an earthquake. These values typically fall between 0.4 and 2 seconds, depending on the stiffness of the soil that is present on site. Harder ground such as rock experience lower natural periods than soft soils. When the period of the ground coincides with the same period of the structure causing resonance between the two. Thus amplifying the accelerations that the structure experiences during seismic activity. Often times, when the natural frequencies of a building and the ground are similar to one another the building will experience its' greatest amount of damage. Building response amplification was seen in the 1985 earthquake in Mexico City. Even though the building was over 250 miles away from the center of the earthquake, the soft soils caused buildings between 6 and 20 stories to resonate at similar periods as the ground causing significant damage. Therefore, in areas with soft soils, it is best to design short stiff buildings (Arnold).

Even though standards have been developed to determine seismic loads, buildings often times are exposed to forces greater than those that are designed for. Even though countless structures have experienced forces that are greater than what the were designed for, many survived with minimal damage to the building. This is due to force analysis not being an inexact science which often times is extremely conservative. Due to this, the building is typically stronger than the calculated design strengths. This happens due to not including, components such as partitions, in analysis, as well as engineers often times assuming that building materials are actually weaker than they are in reality.Even with brittle materials such as precast concrete, reinforcing is added to add ductility to the structure. Thus creating the ability for the building to deform and take larger loads before complete failure occurs. Therefore, for a structure to fail during seismic activity, typically it was detailed incorrectly, or extremely under-designed (Arnold).

Code Provisions
The 2002 version of ACI 318 was the first edition to include code provisions for designing precast structures in high-seismic areas. Therefore, precast structures could only be designed assuming that the structure acted as a monolithic poured concrete structure in terms of strength and toughness. As precast structures became more popular throughout the United States, the Precast Seismic Structural Systems (PRESSS) Program was developed to establish recommendations for design engineers to use when developing precast structures is seismic regions. Through multiple research teams, advancement was made for these structures to give designers direction in developing precast structures within seismic zones. From the advancements, the American Concrete Institute (ACI) has adopted codes for designing precast structures in earthquake prone areas. These codes can be found in Chapter 21 of the current code, ACI 318-11 (Cleland 2014).

Even though codes are beginning to be adopted into ACI 318 for precast structures in seismic regions, they are still vague. Many of the codes found in Chapter 21 referring to precast structures say that the structures should follow the same design codes as a cast-in-place concrete system. If the precast structures are not meeting the codes set out for cast-in-place structures in ACI 318, they must satisfy requirements set out in ACI 374. These requirements include: (a) details and (b) materials used in tests specimens shall represent those used in the structure, and the design procedure used to proportion the test specimens shall define the mechanism used to resist gravity and earthquake effects and shall establish acceptance values for sustaining that mechanism (ACI 318 2001).

Structural Detailing
Detailing of precast structures within high seismic regions is of utmost importance. Within precast structures, it is important to have structural redundancy and joints with continuity to redistribute stresses and keep the structure intact. These issues along with failed connections, detailing inefficiencies and shoddy workmanship have led to many precast building failures. Through research, it has been found that a well-design and detailed precast structure can perform admirably during large earthquakes (Sauter).

When designing a precast structure for a seismic zone, the most important detail is the connection between the floor diaphragm. To be able to redistribute the loads to the lateral force resisting elements, the diaphragm needs to act as a monolithic member. Therefore, the connections between adjacent precast members must tie all of the elements together to provide the redistribution. While there are many ways of doing this,the most efficient way of tying the members together is by using a cast-in-place reinforced concrete topping [Figure 3]. In the event where a cast-in-place slab adds to much weight mechanical fasteners can be used but tend to have brittle failures due to the high concentration of stresses in the members. It is preferred to pour a continuous cast-in-place strip with overlapped reinforcement between the two precast members [Figure 3](Sauter).

Due to the use of multiple member shapes being continuously used in precast construction, it has led to joint details being over-simplified causing member to be simply supported. Due to the high stresses in connections weak points are created causing precast structures to perform poorly in seismic regions because of the lack of redundancy and and continuity between the structure (Sauter).

One type of connection found in precast structure is a "dry" connection. These connections are completed on site by connecting an embedded steel member in the precast to another member by welding or bolting. While many types of mechanical connectors have been developed, it had been found that many dry connections fail during earthquakes. This is due to the high stresses that are developed which create weak points in the structure. On top of creating these weak points, it has been found that the workmanship on connections done in the field are often times brittle. Therefore, dry connections are not typically the best option when designing a precast structure in a seismic region (Sauter).

Due to the issues with dry connections, wet connections are prefer is seismic zones. Within precast structures, the two main types of wet connections include cast-in-place reinforced concrete, and post-tensioned joints. Studies have shown that these types of connections have performed excellently during earthquakes due to their monolithic behavior during cyclic loading. These connection give precast connections the continuity and redundancy desired in a precast structure during an earthquake (Sauter).

Isolation Systems
Through testing, it has been determined that isolation systems are an effective way of reducing the seismic loads that lateral members experience during an earthquake [Figure 4]. These systems include but are not limited to friction dampers, rubber bearings, and plastic bumpers. By using an isolation system in a building, many benefits are experienced, including: reduced seismic forces, reduced lateral drift, reduced structural/nonstructural damage, and higher safety factors against collapse. On top of all of the aforementioned benefits, these systems have the ability to develop safer, more economical buildings for earthquake design. By including isolation within the connection of the lateral force resisting system to the floor diaphragm, the elements are allowed a certain amount of slip dissipating energy in the process through friction (Lostra 2013).

Multiple tests have been done on precast structures using ductile jointed connections with favorable results. One of the many tests that have been performed took place at the University of California, San Diego. In the test, a five-story scaled building with and isolation system was placed under cyclic loading to gauge its performance. After evaluation, it was found that the building performed positively, causing only minor damages to the precast members and the joints. Another test that the University of California, San Diego performed occurred in 2004 on six-story buildings on campus. During testing, a model with rigid connection was compared to a similar model using energy dissipating connections between the lateral force resisting elements and floor diaphragms. By including the energy dissipating elements, it was found that accelerations, displacements, and overturning moment were all decreased due to the friction in the connections (Lostra 2013).

While isolation systems are typically only applied at the base level of structures, is has been proven that these systems can also be implemented at other levels throughout buildings as well. An example of an isolation system being applied at levels other than the base occurred in the 1989 three-story building at 185 Berry Street. The original building only had three levels but two levels were later added using an isolation system so the shear loads at the base would not be increased (Lostra 2013).

As more research and testing is done on isolation systems applied at different location throughout structures it is expected that design codes will develop for use of these systems. While many codes have not been developed yet, resting has proven that including isolation systems in structures with high seismic activity is a great way of dissipating the energy and lowering the forces experienced by the lateral force resisting elements (Lostra 2013).

On January 17, 1994, an earthquake of 6.7 magnitude hit in Los Angeles, California causing many fatalities and collapsing one of California State Northridge's precast parking structures to collapse [Figure 5]. After investigation by Dames & Moore Inc. it was determined that the collapse most likely occurred due to a faulty connection. Through the investigation if was also found that the structure experienced displacements at each level that were far too large for the non-ductile columns to withstand. Through the analysis done by Dames & Moore Inc. it was found that the parking garage did not meet code due to its lack on continuity and poor ductility (Donovan 2009).

//1999 Kocaeli-Golcuk & Duzce-Bolu Turkey Earthquake//
On August 17, 199, an earthquake of 7.4 magnitude hit in Western Turkey causing causing many casualties and causing severe damage to many buildings. Of the buildings, the precast structures were one of the worst performing structural systems. Many of the precast structures were found to fail for one of two reasons. The first reason that many of the buildings experience serious damage was due to poor connections between structural elements. The second reason was due to improper diaphragm action due to precast members not being tied together to create a rigid diaphragm to distribute the lateral loads. Due to the detailing errors, many of these structure collapse due the seismic activity (Tennant 2011).

//[|2012 Emilia Romagna Earthquake]//
On May 29, 2012, the second of two earthquakes, of nearly 6.0 magnitude, hit the Emilia Romagna region of Italy leaving many homeless, and causing multiple casualties in the process. During the two earthquakes, many buildings experienced failures including many precast industrial structures. Many of these structures failed to inadequate connection strengths. Due to the low strength, it made the connections susceptible to failure during the two seismic events. The biggest cause for failure during the earthquakes was do to columns rotating in opposite directions causing beams to slide off of the connection points and collapse. Another main area of failure during the earthquake occurred with the shear keys, due to inadequate detailing in the reinforcement causing little redundancy in creating a monolithic structure (Loannou 2012).

Lessons Learned
While it has long been thought that precast structure was unable to perform well during seismic activity, through research and testing it has been determined that they can perform equivalent to a cast-in-place concrete structure. While many codes still have not been developed, research teams are working on research to further the code for precast structures in seismic zones while creating guides for designers to follow. It was found that many of the structures in the past have failed due to improper detailing, poor workmanship, and lack of continuity/redundancy within the structure. Precast buildings that did not have these issues have performed admirably due to them acting as a monolithic structure.