Earthquake-Resistant Precast Buildings - MC Magazine Winter 2001 - Concrete Publications

	MC Magazine Archive

Earthquake-Resistant Precast Buildings

Recent research and disasters indicate precast structures can remain intact at even the highest seismic zone levels when complying with new design requirements.

By S.K. Ghosh
Dr. Ghosh is president of S.K. Ghosh Associates Inc. and an international expert on the structural design of concrete and seismic design.

Until recently, precast concrete structures could be built in areas of high seismic activity such as California, only under an enabling provision of the ACI 318 Building Code Requirements for Structural Concrete. The provision would allow precast construction in a highly seismic area “if it is demonstrated by experimental evidence and analysis that the proposed system will have a strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure."

The enforcement of this qualitative requirement was, for obvious reasons, nonuniform. Approval to build precast e done to several precast concrete structures during recent earthquakes served as an impetus for the development of code-related provisions. As a result, earthquake-related design requirements for precast concrete structures have been evolving to meet the NEHRP Provisions.

The Northridge Earthquake of 1994
Several precast concrete parking structures located close to the epicenter of the 1994 Northridge, California, earthquake did not do well in the natural disaster. Investigations proved that the damage of the structures was not due to any deficiency inherent in precast concrete construction, but was instead caused by one of two other reasons.

First, in many buildings, some of the structural components (beams, columns, walls) are designed to resist the forces generated by earthquake ground motion, while other products are designed to carry gravity loads only. In such cases, it is vitally important that the gravity framing maintain its gravity-load-carrying capacity while it deforms with the earthquake-force-resisting system under the earthquake design criteria expected by the codes.

This “deformation compatibility” requirement was violated in a number of parking structures in areas affected by the Northridge earthquake. Following the quake, the deformation compatibility requirements were tightened in the 1997 Uniform Building Code, which currently forms the basis for most legal codes in the western United States. The requirements concerning reinforcement in gravity frame materials were also made more stringent in the 1995 edition of ACI 318 Building Code Requirements for Structural Concrete.

The second possibility was inadequate diaphragm (floor) action in many structures. Earthquake ground motion causes inertia forces at the various floor levels and the roof level of a building where the masses are concentrated. The inertia force imparted at a particular floor level is equal to the mass at that level times the response acceleration at that level, which could be several times the ground acceleration.

These inertia forces are transferred to the vertical elements of the lateral-force-resisting system (the frames and the walls) by the floor and roof slabs bending in-plane in diaphragm action. The edge portions of the slab perpendicular to the direction of inertia force act as chords resisting tension and compression stresses. The slab edges parallel to the inertia force transfer the force to parallel resisting elements (figure 1).

Buckled diaphragm chord reinforcement and development of cracks across the entire width of the diaphragm were the most common types of observed damage. In at least one parking structure, the lateral-force-resisting shear walls remained uncracked while precast concrete elements of the floor system collapsed, demonstrating that the diaphragm was the weakest element in the structural system.

Precast concrete parking structures were constructed throughout southern California with cast-in-place topping slabs, and the diaphragm reinforcement was placed within the topping slabs. Cracking of the topping slabs due to temperature effects and shrinkage led to the development of concentrated cracks in the diaphragm before the earthquake. The distributed reinforcement commonly used in the topping slabs did not have the strain capacity to bridge these cracks. Fracture of the web reinforcement led to significant decreases in the calculated shear strength of the diaphragm.

The experience from the Northridge earthquake directly led to changes in the diaphragm design provisions of the ACI 318-99 Building Code Requirements for Structural Concrete. Also, the design provisions for precast concrete structures in regions of high seismicity were adopted into the 1997 edition of the Uniform Building Code from the 1994 NEHRP Provisions, with modifications. The Structural Engineers Association of California (SEAOC) added major safeguards for precast concrete gravity systems.

The Kobe, Japan Earthquake of 1995
One positive element among all the devastation in Kobe was the performance of precast and prestressed concrete structures. Apartment buildings in Japan in the two-to-five-story height range are made of reinforced concrete bearing wall construction. Some of these buildings include precast concrete wall or panel units. Grouted splice sleeve connections are commonly used to tie the shearwalls to the foundation system, and to connect the stacked shear walls to each other.

During the inspection of three of these structures (figures 2, 3, 4), no damage was found in the precast structures themselves, and only minor spalling or cracking in the cast-in-place concrete was observed in a few areas where the location of the splice meets the foundation. These structures were ready for continued occupancy immediately after the earthquake, except in certain cases where soil subsidence adjacent to the structure might have cut off some of the utility services connected to the buildings.

Taller precast concrete structures are increasingly being built in Japan. These are likely to use frames, rather than bearing walls, in at least one direction. Such buildings were not found in the Kobe earthquake-damaged areas.

The Kocaeli, Turkey Earthquake of 1999
Various structural systems incorporating precast reinforced or prestressed elements have been used in the construction of buildings in Turkey. Some of these are patented systems adopted from Western European countries and others developed in Turkey. Three basic precast systems used in Turkey can be identified as frame systems, large-panel systems, and box or modular systems.

Frame Systems. For many years precast in Turkey was restricted to one-story framed structures for industrial buildings, in which various systems were used. In general, the main structure consisted of manufactured concrete and columns or subassemblages and beams. With the exception of one system, beams were joined to the columns or subassemblages by pin connections. The connections were detailed so that they could carry shear, but no significant moment transfer was possible. Such precast industrial buildings, by and large, did not perform adequately in the Kocaeli earthquake. Several instances of structural collapse were observed.

Large-Panel Systems. Precast large-panel systems have been and are used mainly for residential buildings. Large-panel systems consist of large concrete panels connected in the vertical and horizontal directions to enclose appropriately sized spaces for the rooms of the buildings. The panels form the main structural system, which consists of vertical wall and horizontal floor panels. The floor panels transfer the gravity loads and lateral loads (if properly connected) to the wall panels. At least two of these buildings were found to have performed more than adequately in the Kocaeli earthquake amid a lot of devastation (figures 5, 6).

Box or Modular Systems. Box or modular systems are closely related to large-panel systems. In this method of construction, precast boxes are cast as integral units in the plants or individual components are assembled with connections to provide integral behavior. To make a multi-story building, room-size precast box units are stacked and connected by proper devices. The walls provide resistance to lateral forces in both directions.

The greatest advantage of this system is that all wall and floor finishing and piping can be made in a precast concrete plant. The system was used in Turkey in a few housing projects in the early 1980s but has not been used since.

UBC & NEHRP Provisions for Precast
The 1994 NEHRP Provisions presented two alternatives for the design of precast lateral-force-resisting systems (figure 7). One choice is emulation of monolithic reinforced precast construction. The other alternative is the use of the unique properties contained in precast concrete products interconnected predominantly by "dry" connections (jointed precast).

A “wet” connection uses any of the splicing methods required by ACI 318 to connect precast or precast and cast-in-place components, and uses cast-in-place concrete or grout to fill the splicing closure. A “dry” connection is a connection between precast or precast and cast-in-place elements that does not qualify as a wet connection.

Design procedures for the second alternative (jointed precast) were included in an appendix to the chapter on concrete in the 1994 NEHRP Provisions. These procedures were intended for information and trial design because existing knowledge made it premature to propose codifiable provisions based on information available at that time.

1997 UBC
The Ad Hoc Committee on Precast Concrete of the SEAOC Seismology Committee used the 1994 NEHRP requirements for precast concrete lateral-force-resisting systems as a starting point for their work in developing a code change for the 1997 UBC. However, the committee decided to limit their scope to frames (excluding panel systems) and the monolithic emulation option only, primarily due to time constraints.

elastic while designated portions of structural elements (plastic hinges) undergo inelastic deformations (associated with damage) under the design basis ground motion. Prescriptive requirements are given for precast frame systems with strong connections. Such requirements for precast wall systems with strong connections are not included.

The 1994 NEHRP Provisions also addressed the emulation of monolithic construction using ductile connections, covering both frame and wall systems, where the connections have adequate nonlinear response characteristics, and it is not necessary to ensure plastic hinges remote from the connections. Usually, experimental verification is required to ensure that a connection has the necessary nonlinear response characteristics.

The designer is required to consider the likely deformations of any proposed precast structure, vis-à-vis those of the same structure in monolithic reinforced concrete, before claiming that the precast form emulates monolithic construction. The 1997 UBC does not directly address ductile connections.

1997 NEHRP Provisions and 2000 UBC
1997 UBC. The 1997 UBC provisions concerning the design of precast concrete structures in regions of high seismicity were adopted into the 1997 edition of the NEHRP Provisions. The first edition of the International Building Code, which is expected to replace the existing model codes as the basis of the building codes of most legal jurisdictions, based its seismic design provisions on the NEHRP Provisions. The design provisions for precast concrete structures exposed to high seismic risk are included.

2000 NEHRP Provisions. The 2000 NEHRP Provisions is in the final stages of development. The Concrete Subcommittee of the BSSC’s 2000 Provisions Update Committee has greatly expanded the design provisions for precast structures exposed to significant seismic risk. The scope of these provisions is illustrated in figure 9. It should be apparent that virtually all viable options of precast concrete construction have now been considered.

Much has been accomplished in the building codes arena to enable the satisfactory design of precast concrete structures exposed to high seismic risk. The 2000 NEHRP Provisions represents a culmination of efforts that have been underway since the late 1980s. Recent earthquakes have provided new information regarding the performance of precast concrete structures and lessons learned have been codified. With the 2000 International Building Code, precast concrete buildings can be designed with the necessary seismic detailing and features to ensure adequate performance. Whether this will lead to more precast buildings being specified in seismically active parts of the country remains to be seen.

Technical subcommittees for the 2003 NEHRP Provisions are now about to be formed. It is possible to participate in the process by contacting the Building Seismic Safety Council, which is part of the National Institute of Building Sciences in Washington, D.C. Simply call 800-480-2520 or 202-289-7800, or log onto their web site at www.bssconline.org.

Back to MC Magazine Archive