The demand for safer buildings that aren’t prone to topple in face of natural disaster continues to motivate engineers in the metal building industry. Jerry Hatch, P.E., offers Rural Builder readers an update on research projects that the Metal Building Manufacturers Association sponsors, specifically in the area of seismic testing. Hatch is manager of engineering development for NCI Building Systems, and also serves as chairman of the MBMA Technical Committee.
Q: For several years, work has been done to document how metal buildings react to seismic activity. What was the motivating factor behind this testing?
A: In the Northridge earthquake in 1994, many types of damage were observed that seismic engineers had not anticipated, and an effort began to understand why these things happened. The federal government, through FEMA, funded several research projects to study what was observed and prepare design procedures to preclude reoccurrence. These new procedures were first introduced into the International Building Code (IBC) 2000 version. Researchers have long been able to model building behavior after damage has occurred. The IBC2000 building code brought this into the mainstream of engineering practice. The industry’s goal has changed from designing to prevent any damage, to designing to prevent collapse for seismic loading.
At the time that IBC 2000 was introduced, few construction types had the necessary research to comply with modeling post-damage behavior for a large earthquake. Based on their insights, the developers of IBC2000 included approximations for many building types to fill the gap as industry professionals began to understand and implement the new requirements included in the metal building industry. The code writers’ expectation is that each industry would have performed the research to understand how their respective materials behave as damage occurs and to develop modeling techniques. The metal building industry has been performing related research since 2000. Once our research has reached a critical mass then we will make proposals for introduction of the latest research into the building codes. Other industries conducting research include laminated wood, concrete and masonry, and we would expect this research to continue over the decades as ways to improve modeling behavior for extreme earthquakes are learned.
How long has the project been ongoing?
A: MBMA has been sponsoring research since the IBC2000 building code was released in 2000, so at some level, this work has been ongoing for 15 years. This is the first building code that explicitly requires analysis past the elastic limit for when elements start bending, buckling and yielding.
Who are the principle parties involved?
A: Dr. Vahid Meimand [senior engineer, NBM Technologies, Inc.] has been involved in modeling the effects of earthquakes with regards to elements that cause buildings to collapse. MBMA has conducted research to understand when elements start buckling, yielding and bending. Dr. Chia Ming Uang at the University of California San Diego was the principal investigator for the testing effort. Dr. Lee Shoemaker [director of research and engineering, MBMA] is involved with the wind load provisions for low-rise and all-heights building.
Has the testing changed its focus since it began?
A: For decades, the metal building industry designed for linear elastic behavior, but the very latest code requirements dictate a change in that approach. Engineers are expected to understand what happens after the frame elements start bending, buckling and yielding during a seismic event. This is new technology to the engineering community. However, the frames will look very similar as to previous ones. Assessments, including shake table tests, have been made to understand how structures will behave once bending and buckling come into play. Dr. Vahid Meimand has been working to model this behavior, numerically, with a finite element program. Once he is able to model this behavior for the shake table tests, modeling of frames with other configuration will begin. Then, statistical analysis will be performed to demonstrate a margin of safety against collapse.
What beneficial changes do you see on the horizon?
A: Metal buildings are very resistant to the effects of earthquakes, in that they are low rise and light weight. Buildings that are most likely to see differences in resistance are the tilt wall and buildings clad with brick or masonry. Research demonstrating a margin of safety related to collapse is done on all metal buildings, which has never been done before. This research also paves the way for 3D design, which most agree to be the wave of the future for the building industry. 3D design has several benefits for building owners. The first benefit is that, prior to the building being built, the builder will be able to do geographical “walk through.” 2D design presents a variety of stability issues, and 3D design analysis is the only way to accurately design for these issues in an efficient way.
Have any products been developed as a result of this research?
A: Brad Fletcher (with Atlas Steel) introduced a new product that provides improved performance that will make HSS design more effective and easier. The product, ASTM 1085 HSS, covers cold-formed welded carbon steel HSS for welded or bolted construction. An important element is the development of a minimum value for yield stress. Previous products have had a different yield stress number that is dependent on factors including size and shape, but the minimum specified yield stress for the new product is 50ksi for all shapes and sizes, which simplifies the process. These advancements make the product lighter, less expensive, and stronger for the consumer.
Have the tests resulted in any recommendations for code changes?
A: The building codes currently have two different methods for numerically determining the effects of wind on buildings. Each method gives different answers. The first method is based on research of buildings up to 60 feet. The second method is based on buildings 200 feet and taller, but also claims to address shorter buildings. MBMA would like to see a transition between the methods that brings the two together, eliminating the conflicting code requirements with respect to wind loading.
The engineering industry continues to learn from experience, and its efforts to better understand the effects of the Northridge earthquake serves as a good example. FEMA-sponsored research will help to prevent the reoccurrence of collapsed buildings, as its methods have been updated each revision of the IBC codes. The next code version will be introduced in 2016. The metal building industry is very interested to stay on the forefront of current research and experience.
Research on wind and seismic elements is useful to manufacturers because it helps to build a more reliable product that fits the needs of manufacturers in an economical fashion.
What has the testing revealed so far?
A: With regards to seismic research, MBMA is working on areas of the moment frames that improve performance of the frame during an earthquake. Research will be done to understand when collapsing will occur. It has been difficult to research this topic, as we are not aware of metal buildings that have collapsed during an earthquake. Masonry has fallen off of a metal building and Dr. Justin Marshall at Auburn University is working with an NSF grant to give guidance on better masonry connections.
MBMA is currently mid-way through our research on moment frame behavior subjected to earthquakes, and so far, testing has shown that most damage occurs at the rafters adjacent to the column, column knee area and the column base plates. Each of these connection types can be thought of as releasing the energy imparted by the earthquake and in turn, this energy dissipation is associated with damage to the frame. We have performed a number of rafter buckling tests to better understand and model inelastic behavior in these three areas. The ultimate goal is to document what we have learned about frame behavior in earthquakes and establish margins of safety against collapse. So that while portions of a building might incur significant and costly damage, the building would not collapse, thereby increasing the structure’s safety. Once we have the opportunity to fully execute inelastic modeling, we’ll better be able to target specific areas, or elements, for further attention.
Despite the significant advances made in this field of study, there is still much to learn regarding wind and seismic effects on buildings. Research will be done on how environmental loading affects buildings while the building codes will continue to be updated based on research and experience.
Why is 2016 an important year for the research project in seismic testing?
A: 2016 is expected to be a big year for research because updates to IBC and ASCE7, with respect to seismic loading and behavior [were scheduled to be] introduced at the FEMA Building Seismic Safety Council colloquium in San Francisco on February 11. This will give MBMA insight on the rounds of updates to the 2016 codes for seismic loading. The codes will reflect the latest research that is being done in 2015 and 2016. Research in 2016 will also bring an incremental increase in the seismic and wind loading technology.
What other areas of testing has MBMA been involved in?
A: MBMA has been working to have the codes updated with better snow load information and John Keating, of NCI Building Systems, has led these efforts along with Scott Russell of NUCOR. Currently, ASCE7 has a snow load map of the entire country, and approximately 25 states supersede the ASCE7 snow load map with their own, so an effort is being made to consolidate this information. MBMA has been helping to orchestrate this project with the Applied Technology Council, the ASCE and Dr. Michael O’Rourke (of Rensselaer Polytechnic Institute) – who has authored most of the snow load provisions in ASCE. According to O’Rourke, more data collection is also needed to complete the snow load maps. RB