Building codes require that buildings be classified based on the risk to human life, health, and welfare associated with their damage or failure. Minimum design loads, maximum allowable story drift criteria, and lateral force resisting system limitations are derived based on this classification. Building codes in the U.S. generally reference the ASCE 7 provisions for appropriate building classification criteria.
The idea of designing different types of buildings to different seismic force levels based on their “risk” is not new. The Building Code utilized increased Importance Factors for schools and hospitals for many years to provide a greater degree of resilience in certain structures. In the early 2000’s the first edition of ASCE 7 utilized the term “Occupancy Category” to define a buildings classification. However, the term “occupancy” is primarily used with fire/life safety issues and only implicitly defined risks associated with structural failure of a building. Consequently, the 2010 version of ASCE 7-10, introduced the term “Risk Category” in lieu of “Occupancy Category” to distinguish between the two considerations. Per commentary section C1.5.1 in the ASCE 7-10:
“The Risk Categories in Table 1.5-1 are used to relate the criteria for maximum environmental loads or distortions specified in the ASCE 7 to the consequence of the loads being exceeded for the structure and its occupants.”
Table 1.5-1 the ASCE 7 defines four distinct Risk Categories:
Risk Category I
Structures that are normally unoccupied and would result in negligible risk to the public should they fail. These include structures such as barns and storage shelters.
Risk Category II
This category contains all buildings and structures not specifically classified as conforming to another category. The majority of structures such as residential, commercial, and industrial buildings are included in this category.
Risk Category III
This category includes buildings and structures that could pose a substantial risk to human life in case of damage or failure. Structures under this category include:
Careful assessment of the Risk Category for a new project is required prior to design. Minimum design loads for snow, ice, and seismic considerations are greatly influenced by the importance factors defined in Table 1.5-2 of the ASCE 7 for different Risk Categories:
Additionally, buildings located in regions with high seismicity are particularly sensitive to Risk Category classifications. Per Chapters 11 and 12 of the ASCE 7 Risk Category selection has major impacts on:
For this reason it is important to note that changing a buildings occupancy can result in significant changes to gravity (in snowy/icy regions) and lateral designs. Careful consideration must be given to projects involving existing structures whose occupancy change triggers a bump from a lower Risk Category level to a higher one. The existing lateral and gravity systems may require retrofits to accommodate stricter structural system limitations, increased load demands and stricter allowable drift criteria.
In addition to ASCE 7, individual states have further defined and clarified Risk Categories for different buildings and each state’s Building Code should be considered and referenced when determining a buildings Risk Category. It is also helpful to work with a design professional such as an Architect when determining number of occupants in complex buildings made up of multiple occupancies and where total number of occupants may require different Risk Categories. In fact different Risk Categories can be specified within the same building structure in special conditions.
The Nishkian team has years of experience with thousands of projects across all Risk Category types. Should you have any questions on an upcoming or current project, please do not hesitate to contact any of our offices.
By Chad Norvell, PE
Historically, tsunamis have been poorly understood by the public. Films often show tsunamis as towering tidal waves that cast deep shadows over tall buildings on the coast before violently crashing down. Video footage from the 2004 Indian Ocean tsunami showed the world what tsunamis really are—a wall of water that doesn’t necessarily tower over the coast, but that moves through with unstoppable force.
In this article, we will explore important tsunami basics, review the tsunami risk in Oregon, introduce changes to structural loading standards that now include tsunami loads, and discuss essential research findings out of Chile that affect our local understanding of tsunami risk.
Tsunami Causes & Terminology
In the Pacific Northwest, we are increasingly aware of the risk of a devastating Cascadia Subduction Zone earthquake. This potential future earthquake is likely to be associated with a large tsunami that will strike the coasts of Washington, Oregon, and Northern California, as well as echo around the Pacific basin, reaching as far as Japan and Australia. The figure below illustrates how a subduction zone earthquake triggers a tsunami.
When discussing tsunamis, it is important to understand the terminology used. What does it mean to say that in Pulicat, India, during the 2004 Indian Ocean tsunami, the maximum runup was 3.2 m and the inundation limit was 160 m? The figure below illustrates the primary tsunami measurements.
Source: U.S. Geological Survey Tsunami Terms
RUNUP ELEVATION: The difference between the elevation of maximum tsunami inundation and the reference sea level elevation.
INUNDATION DEPTH: The depth of the tsunami relative to grade level at the point of interest (e.g., where the structure is).
INUNDATION DISTANCE or INUNDATION LIMIT: The maximum horizontal distance inland inundated by the tsunami.
Oregon’s Tsunami Risk
According to the U.S. Geological Survey (USGS), Oregon has 25,000 residents, in addition to 55,000 tourists, who could be at direct tsunami risk (defined as being within the Tsunami Design Zone, or TDZ) along 300 miles of coastline subject to inundation. Two ports, a fuel depot hub, and $8.5 billion in essential facilities are located within this risk zone as well. The Oregon Department of Geology and Mineral Industries (DOGAMI) has produced a series of tsunami inundation maps covering the entire Oregon coastline, showing the areas at risk of inundation for both Cascadia Subduction Zone earthquakes [the left figure below] and Alaskan-Aleutian Subduction Zone earthquakes [the right figure below.]
Source: Oregon Department of Geology and Mineral Industries
Analysis by Oregon Public Broadcasting in 2015 showed that “about a third of schools, hospitals, police and fire stations along the Oregon coast are within a potential tsunami zone.” In Seaside, approximately 80% of residents live at elevations of 15 feet above sea level or lower, when DOGAMI estimates that even a small Cascadia Subduction Zone tsunami would have a wave height of over 20 feet. Further development on the Oregon coast will rely on structural designs that are tsunami-resistant.
New Tsunami Structural Load Standards in ASCE 7-16
The American Society of Civil Engineers (ASCE) publishes a document called “Minimum Design Loads for Buildings and Other Structures,” commonly referred to as ASCE 7. This consensus-based standard specifies the minimum required design loads for all the types of load commonly encountered in the structural design of buildings, including dead, live, wind, and seismic. ASCE 7 is incorporated by reference in the International Building Code (IBC), making its provisions law in much of the United States. The recently published 2016 edition of ASCE 7 (ASCE 7-16) includes a new chapter on tsunami loads, and new regulations on when buildings must be designed with consideration of tsunami loads.
In line with current practice for earthquake loads, ASCE 7-16 defines a Maximum Considered Tsunami (MCT) as the tsunami that has a 2% probability of exceedance in 50 years, or an average return period of 2,500 years. The runup elevation associated with the MCT is designated as the Tsunami Design Zone (TDZ), and hazard maps (like Oregon’s tsunami inundation maps) are based on that elevation. TDZ maps for Washington, Oregon, California, Alaska, and Hawaii are included in ASCE 7-16.
The provisions of ASCE 7-16 require consideration of tsunami loads only for Risk Category III and IV structures within the TDZ, generally meaning structures that could pose a great risk to human life if they failed (such as schools) or emergency services buildings (such as police and fire departments.). However, local jurisdictions have the option of designating threshold elevations under which even Risk Category II buildings (typical commercial and residential structures) would need to be designed to resist tsunami loads. In areas characterized by flat coastal planes (for example, Tillamook, Oregon), evacuation from the tsunami zone may be impossible, in which case vertical evacuation into relatively tall public or commercial buildings designed to resist tsunamis could save thousands of lives.
Designing structures to resist tsunamis requires consideration of four types of tsunami load:
ASCE 7-16 includes procedures for determining each of these loads.
Lessons Learned from Tsunami Research in Chile
Chile suffered significant tsunami damage associated with the M8.8 Maule earthquake in February 2010. Since then, observations from the tsunami, along with sophisticated research at the Universidad Técnica Federico Santa María and CIGIDEN (National Center for Investigation of Integrated Management of Natural Distasters) under Dr. Patricio Catalán, have yielded important and surprising lessons about tsunami behavior, two of which are summarized below.
The understanding of risk to our infrastructure from tsunamis is still not as mature as that of seismic and wind risk, but significant advances are being made, particularly via lessons learned from recent tsunamis like the 2004 Indian Ocean Tsunami, the 2010 Maule Tsunami in Chile, and the 2011 Tohoku Tsunami in Japan. These lessons are now being incorporated into the standards that structural engineers use to design buildings, providing us valuable tools for building more resilient communities in areas of tsunami risk. This is particularly important for us in the Pacific Northwest, where we have long coastlines and many communities at risk of inundation in a Cascadia Subduction Zone earthquake and tsunami.
Contact Nishkian Dean for more information or to discuss your project on the Oregon coast.
Chad Norvell, PE is a Project Engineer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
The bombing of the Alfred P. Murrah Federal Building on April 19, 1995 in Oklahoma City, Oklahoma marked a turning point. The years that followed, arguably the most damaging and shocking domestic terrorism event in our nation’s history, resulted in the tightening of standards for our government buildings. These standards are not isolated to security protocols, but to the requirements for the way these buildings are design.
While, we are sure everyone can empathize with the extra effort and time it takes to pass through security check points while entering courthouses, police stations, and airports. Terrorism, unfortunately, has become a driving force in the added scrutiny we all face while traveling or entering government buildings. What most people do not see, however, is that most occupied government buildings are designed to resist the effects of explosive blast. And increasingly, architectural and engineering firms are tasked with designing buildings to minimize damage and loss of life from explosives.
The major consideration and design efforts by engineers for blast loads on buildings are focused on the exterior envelop. The window systems, exterior walls, the roof system, door systems, evaluating disproportionate progressive collapse risk, and even blast induced base shear have all become subject of blast engineering analysis. However, buildings are not the only structures that have entered this domain. Chemical processing facilities, bridges, and even rocket engine testing structures have all issued requirements to resist the effects of explosive load.
While most structural loads are defined statically, blast loading is dynamic. Unlike wind loads which are converted to static equivalent, the penalties of converting a blast load into a static equivalent can yield cost prohibitive designs. While the dynamic characterization in seismic design is like blast, the time durations very greatly. Seismic loads are defined in seconds while blast loads are measured in milliseconds.
Alfred P. Murrah Building taken April 20, 2015
Photo Credit: Tulsa District United States Army Corps of Engineers
Licensing Agreement: https://creativecommons.org/licenses/by/2.0/
Blast loads are typically defined by a peak positive pressure and associated impulse for design purposes. The negative pressure associated with suction after the passing of positive pressure waves is neglected in most building design criteria. Based upon the location of the structural element in relation to the blast origin, a simplified load shape is defined, and an equivalent duration is calculated. More complex blast time-histories can be developed from Computational Fluid Dynamics (CFD) analysis methods, however validation of CFD models can be difficult.
Single Degree of Freedom (SDOF) analysis methods are the industry standard, however Multi-Degree of Freedom (MDOF) and Finite Element Analysis (FEA) methods can, and have, also been used successfully. MDOF and FEA do not, in many blast cases, yield better or more correct results when compared with SDOF methods. The blast community has at-large, agreed upon SDOF as the standard, because of the quantity of necessary assumptions and the limited available empirical data related to modeling the complexities of detonated explosives.
SDOF evaluation is completed on an element by element basis, where the forcing function is defined by the blast time-history, a damping constant and stiffness function are defined, and the momentum effects of mass are all considered. From the SDOF results, the ductility, rotation, and demand to capacity ratios for shear and moment are derived, then compared with project specific performance limits.
Project specific requirements typically define an acceptable level of damage to defined structural element types. Damage levels are defined by accepted ductility, rotation, and demand to capacity ratio values. Because of the large forces involved in blast loads, it is typical for elements to be pushed into the plastic range, and large deformations can be expected.
Contact Nishkian Dean if your project has blast design considerations.
Aerik Carlton, is an Engineering Designer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
Biggs, J. M. (1964). Introduction to Structural Dynamics. McGraw-Hill, New York.
Unified Facilities Criteria (2008). UFC 3-340-02: Structures to Resist the Effects of Accidental Explosions. Change 2 dated 1 September 2014, Department of Defense.
By Dave Beh
Structural engineers design the primary structure to withstand seismic forces, as a minimum, as outlined by the design code. However, during an earthquake people can be injured and costly damage can result by falling non-structural components such as; kitchen hoods, bookcases or mechanical/electrical equipment. The code also requires seismic anchorage for certain non-structural components but these can sometimes get overlooked by designers/owners/plans examiners that simply don’t yet have the information or are unaware of the requirements.
Critical Lift Cranes are used to handle “critical” hardware. In the aerospace industry this could include high-value components, like spacecraft or satellites (in excess of $100 millions) or components that contain hazardous or highly toxic materials (like hypergolic fuels). This is an overview of some of the key considerations that go into specifying critical lift cranes. The procurement, installation and operation of critical lift cranes requires the definition of additional requirements above and beyond the national consensus standards (i.e. OSHA 1910.179, ASME B30 series, CMAA 70/74) typically specified for a standard commercial crane. It is imperative that these additional requirements be addressed in the initial procurement documentation prior to initiating a contract, since it will be difficult or impossible to incorporate them at a later date into a typical commercial crane without substantial modifications and significant cost. What follows is a summary of the more significant recommended requirements that must be specified for all critical lift cranes.
First it is very difficult to determine a specific relationship between bolt tension and torque. Tension is the application of force that causes stretching whereas torque causes twisting and tightening of the bolt and is an indirect indication of tension. In other words, tension is the stretch or elongation of a bolt that provides the clamping force of a joint where torque is a measure of the twisting force required to spin the nut up along the threads of a bolt.
High-strength bolts are designed to stretch slightly, and this elongation is what clamps the joint being connected together. Torque is best viewed as a very indirect indication of tension, as many factors can affect this relationship, such as, temperature, tolerance, surface texture, rust, oil, debris, thread series and material type just to name a few. This variability can be on the order of +/- 40% or more. The relationship between Torque and tension based on the following formula:
These are simple questions that require a complex answer. Reshoring is the process of utilizing multiple levels of shores below the story being cast to distribute the applied construction loads to multiple stories. Concrete is heavy and without a sufficient number of levels to support the weight the slabs can become overloaded.
Planned tenant improvements (TI) and a review of building code requirements were discussed in a previous blog post, but… what happens when structural requirements of a new tenant space may need considerations different from what the “Building Code” specifies for strength and stiffness? We commonly experience specific client parameters beyond what the Building Code addresses for our fitness club clients who are commonly moving into new mixed-use spaces below residences or into repurposed, previously designed, office space. While there are alterations that we often think of as standard structural tenant improvement modifications, such as new openings for staircases, or new MEP units for ventilation, some of the upgrades to the existing structure require investigation beyond typical Building Code issues.
Owners of new and existing mixed-use buildings typically have two main concerns when considering leasing space to a new fitness club tenant, the transmission of noise and vibration into sensitive adjacent tenant areas. The comfort of office and residential tenants, which typically share tenancy in the mixed–use building development, is a great concern. Careful measures and criterion must be developed to mitigate that the noise and vibrations from the fitness club tenant from propagating into more sensitive areas of the structure and disturbing the other building tenants. In collaboration with an acoustic/vibration consultant, recommendations for the comfort level of all the building tenants will typically determine what treatments need to be made, but the structure itself must be prepared to receive the treatment.
The planning and design process for private or public building construction is a critical component for a successful project. Every building project faces its own unique set of challenges, including finances, site location, schedule, public approval, environmental impacts, owner satisfaction, and meeting building code requirements. While the decision or need to construct a building typically determines its use and function, the size, shape, height, construction materials, and structural systems utilized tend to develop during the process. The building code plays a role in defining and shaping the building’s aspects by requiring adherence to a method of classification. The current 2012 International Building Code (IBC) requires that all buildings and structures, both existing and new, be classified under two categories:
The most recent building Code changes in the 2012 International Building Code included what seem to be increases in the design wind speeds used throughout the country. Is it getting windier? The answer is no. But there have been changes in the determination and application of wind speed and their use in designing buildings and their components.
By Ken Oliphant, MSCE, PE, SE
The Nishkian firms are often consulted at the onset of a renovation, tenant improvement, or building addition or following unexpected building damage caused by wind, earthquake, flood, fire, or vehicle strike. In these cases, our Client – the architect, contractor, developer, insurance company, or building owner want to know the structural implications of the building addition, alteration, or repair and what upgrades, if any, the building code and building official will require. Questions that often arise include:
- Will the entire structure need to be analyzed?
- What forces will the building be required to resist?
- Are there any code-triggered upgrades?
- Is a complete seismic upgrade required?
These questions are of critical importance to our clients as they play a pivotal role in shaping both the project scope and budget.
Bolts, washers and other types of fasteners might be small, but they are a fundamental part of a structure. That is why having the right corrosion protection for the bolts that hold together a structure and knowing the environment it is exposed to is crucial to the safety and strength of the structure.
There are many types of metal high-strength carbon steel fastener assemblies available offered with different coatings, each with its own advantages and disadvantages. Some coatings are highly resistant to chipping, high heat, or certain chemicals. The specifications that cover the performance of coatings are covered by various ASTM (American Society for Testing and Materials) committees who investigate and review what fastener requirements currently are and specify how there are to manufactured, applied and used. These committees continue to develop coating standards specifically for metal fasteners.
During the design process, it’s essential to consider the anticipated structural load of a project.
Loads are commonly understood as forces that cause stresses, deformations, or accelerations. These loads are applied to a structure or its components that cause stress or displacement.
There are many types of structural loads that you need to account for during the design process. And some – like live loads – present specific challenges that require a deeper understanding to conceptualize.