By Robert A. Aman, PE, SE
Many buildings constructed today extend one or more stories below grade level to maximize building area and provide additional spaces for parking, storage, mechanical/electrical rooms, and sometimes even office space or living units. In many cases the soil excavations required for these below-grade structures are constrained by the site’s property lines and surrounding buildings, and therefore require that a vertical excavation cut is made within the property lines and construction boundaries. Since the soil excavations are vertical, temporary excavation shoring must be installed to prevent the soil from caving. In cases where there is available space adjacent to it, the excavation typically will be laid back on a slope at a 1 ½ horizontal to 1 vertical to prevent caving and to eliminate the need for excavation shoring. In situations in which this is not possible, shoring provides a means to safely accomplish the site excavation and greatly improve the utilization of the site development.
The design of excavation shoring can be complex and must take into account many factors, including depth of cuts, groundwater elevations, horizontal loads imposed by soil, resistance loads provided by soil at embedded shoring, and superimposed vertical loads from traffic, construction equipment, construction material storage, and adjacent structures. The vertical loads that occur adjacent to shoring walls usually result in additional horizontal loads applied to the shoring. The geotechnical engineer is responsible for providing all soil design parameters for the temporary shoring design, and works together with the foundation and structural engineer to accomplish this.
Excavation shoring, on most projects, is delegated to a specialty contractor who is also typically responsible for its design. In many jurisdictions, including the City of Portland, excavation shoring design drawings and calculations must be approved as part of the building permit submittal process. Site construction cannot begin without this information in hand, so it is important to understand the jurisdictional requirements before getting to this point so that the start of the project is not impacted.
When needed, there are several different shoring systems that can be utilized. The selection of the best system for each particular project considers the site soil conditions, adjacency with other elements, depth of excavation, and the experience of the specialty excavation contractor. Two common systems are soldier pile walls, and soil nail walls, which we go more in depth about below (pun intended):
Soldier Pile Walls
This method is fast to construct and typically consists of structural steel H-shaped piles that are inserted into a deep round hole filled with concrete that is spaced at regular intervals, usually in the 6- to 12-foot range. The concrete hole is typically 24” in diameter and the H-Pile is 10” to 14” wide/deep. Alternatively, the steel piles may be driven or vibrated into the ground without the use of any concrete. Where the excavation is located adjacent public property, the temporary shoring wall is typically located directly outside the property line, where permitted on a public right-of-way, and is used to apply a reinforced shotcrete wall that then serves as the permanent structural wall. Where shoring walls are located adjacent to a private property it can only be located over the property line with an easement from the property owner.
Between the soldier piles, 4×12 horizontal wood lagging is installed to retain the soil behind the wall. The lagging is installed in 3- to 4-foot increments as the vertical excavation cut proceeds downward. The soldier pile is typically embedded 10 to 12 feet below the bottom of the final excavation, and is designed to cantilever out of the ground. Where the excavation exceeds a range of 10 to 12 feet, the soldier piles may require a soil anchor/tieback or internal diagonal brace near the top of the wall for additional support. For deeper excavations, additional tiebacks are required as the depth increases further.
Soil anchor (or tiebacks) usually consist of a steel tendon or rod encased in a hole filled with a concrete grout mixture. The anchors are tensioned after installation is complete to fully engage the soil. Soil anchor tiebacks are installed at a downward angle and typically extend into the ground a minimum of 25 feet and may therefore encroach into the adjacent properties or the city’s right-of-way (in which case, permission from the property owner is required.) Encroachment into a public right-of-way is typically allowed for temporary construction.
Location of all existing utilities must be confirmed prior to the installation of the anchors. The City of Portland requires an additional encroachment permit, and states that the anchors are de-tensioned after the temporary wall is no longer needed. Additionally, the City requires that any shoring elements in the public right-of-way that are located within 5-feet below grade must be removed, including soldier piles and tiebacks. The use of internal shoring bracing eliminates encroachments, but the shoring is located where the building construction occurs and must not conflict with those activities.
Pros: fast to construct, can be used in deep excavations, flexible layout geometry, can be designed for large surcharge loads
Cons: in certain cases, may require additional soil anchors/tiebacks or support, tiebacks in adjacent property will require an easement, internal braced configurations can be obstructing.
Soil Nail Walls
Another common excavation shoring wall solution is a soil nail wall, which is purely an anchor tieback wall. A soil nail wall consists of steel bars installed in a drilled hole filled with a concrete grout mix spaced at approximately 5 feet on center. The rods are typically installed at a 15-degree downward angle and embedded in the 15-foot range. The nails are then covered with a 4”-thick reinforced shotcrete facing wall that retains the soil behind the wall. The walls are constructed from the top down in 3- to 6-feet-tall sections, depending on the soil type and its ability to withstand caving.
Soil nail walls may be advantageous where overhead construction requirements are tighter because they do not require drilling equipment or the installation of soldier piles. Smaller equipment is generally needed with this method, and no additional embedment of a vertical structural element is required. Embedment of soil nails is also much less than with tieback walls, which may reduce conflicts with adjacent underground obstructions or utilities. A soil nail wall will require a more specialized and experienced contractor, however. Like a traditional soldier pile wall, the structural wall for the building would be a shotcrete wall installed on the face of the soil nail wall. The design of the soil nails is typically provided by the geotechnical engineer, while the shotcrete facing wall is designed by a structural engineer.
Pros: do not require drilling equipment or the installation of soldier piles, needs smaller equipment, no additional embedment of a vertical structural element is required
Cons: limited depth of excavation, limited wall surcharge loads can be accommodated, requires a more specialized and experienced contractor
Basement excavations are a common necessity of many projects, and as you can see above, there are many options and avenues for selecting the right system for each individual project. Developing a strategy to address the excavation is an important part of the early design effort – it helps with creating a scheme that is readily buildable for a specific site and ensures that everything is in place for the permit application. The Nishkian firms are regularly involved in the design of many projects that incorporate basement excavations and are available to consult on your project needs.
Robert A. Aman, PE, SE is an Associate with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
A challenge of constructing larger and larger projects in dense urban environments is placing those buildings on sites with sub-optimal soil conditions. These sites may include soft compressible layers of native or fill materials, soils that may be subject to settlement during an earthquake due to liquefaction, sites that may be subject to lateral spreading during an earthquake, or conditions that require a high capacity foundation system.
DDC design and construction was performed by Farrell Design-Build Inc. for U.C. Berkeley’s Maxwell Family Field and Garage project in Berkeley, California. The site sits directly adjacent to the Cal Memorial Stadium, the Greek Theatre, and the Haas School of Business.
Traditionally, 2 options have been used to mitigate these conditions:
Both of these options have impacts on the project schedule and cost. Over-excavation requires heavy earthwork equipment, a large site for material storage and creates significant environmental conditions that must be addressed. Installing drilled piers or driven piles can be expensive, time consuming, and loud. Driven piles require traffic considerations and adequate storage, agreements with neighbors, and other environmental considerations.
A new term that has become more prevalent in soils reports and foundation design is Ground Improvement. This has become a generic term for a variety of methods that can be used to mitigate these soft soil sites without over-excavation or deep piers or piles. Ground improvement allows for a shallow foundation system to be used which will save costs and time.
Ground improvement comes in several forms, these include: deep soil mixing, drill displacement piers, and deep dynamic compaction. Deep soil mixing uses augers and other heavy equipment to pump grout and mix it into the existing soil. Deep soil mixing can be spread over a site to support a mat foundation, or can be closely spaced to support concentrated loads. These drilled elements can vary in diameter and depth and produce small amounts of spoils. Another type of deep soil mixing uses vertical blades to cut a trench in existing soil while mixing in a cement slurry. This is called cutter soil mixing with machinery that has blades that can cut through in situ soil up to 130 feet in depth. These improved trenches can be used as stiffen vertical support elements, retaining walls and to restrain liquefiable soil. Deep dynamic compaction uses rams or deep soil vibrators to consolidate and stiffen existing soil or existing soil with added aggregate. Adding grout to the existing soil increases the shear strength, lateral stiffness, and bearing capacity and allows for use of shallow foundation systems on top of the improved subsurface. Since each of these methods involves a specific type of specialized heavy machinery, the exact type of ground improvement will depend on the contractor selected. The result is that ground improvement is typically provided on a design-build basis.
Nishkian Engineers have utilized ground improvement techniques on several recent projects to provide less invasive and more cost-effective foundation solutions. One recent project is The Encore residential development in Redwood City, CA. This 6-story building of concrete and wood frame construction does not have huge foundation loads. However, approximately one third of the building footprint had a subgrade layer of soft material that had a high potential for liquefaction settlement. Ground improvement of this select area was a cost- and time-effective solution to mitigate these conditions in lieu of other, more costly options.
Nishkian worked with Regis Builders, the general contractor, and Farrell Design Build, the ground improvement contractor, to develop the system to support this building. Farrell quickly mobilized their equipment on the prepared site and utilized Drill Displacement Columns (http://www.farrellinc.com/services/foundation-systems/auger-cast-column-drill-displacement-column) up to 30 feet in length to provide support in compression for the foundation and ground floor slab in the soft zones. Farrell also installed displacement ground anchors for tensile resistance under lateral elements. After this quick process the shallow spread footing foundation system was excavated and installed.
Another relevant project is the Maxwell Family Field and Garage which sits directly adjacent to the California Memorial Stadium on the University of California Berkeley campus. Long ago, the site was once a creek bed. During the development of the campus, the creek was turned into a set of large culverts, and filled in to provide a flat surface. This type of loose fill makes building a seismically safe structure more difficult. Similar to the challenges of building on bay mud in San Francisco, the ground could liquefy during an earthquake, resulting in amplified forces on the structure. This condition is exacerbated by the presence of the Hayward fault, which runs just a few hundred feet away from the site. Although there are many ways to improve the soil, the best option for the Maxwell Family Field and Garage project was Drill Displacement Columns (DDC). DDC design and construction was performed by Farrell Design-Build Inc. as well.
Ground improvement installation by Farrell Design-Build Inc. for the Maxwell Family Field and Garage project at the University of California Berkeley campus.
In a previous blog post, building noise and vibration mitigation was discussed as it pertains to tenant improvements (TI) in existing buildings and how the building code sometimes falls short concerning client parameters. As described in the previous post, this is often the case with fitness clubs that move into mixed-use spaces below residential or offices that are sensitive to sound and building vibrations, but the need for vibration mitigation goes well beyond fitness clubs.
The previous blog post examines how performing a finite element analysis of an existing floor system can determine its natural frequency and the natural frequency of a modified, stiffer system. The American Institute of Steel Construction (AISC) has previously put forth a “Design Guide” to design and account for vibrations in new buildings of typical framing. The Design Guide provides for determining perceived floor accelerations that change based on the natural frequency of the floor system. It is of particular note to avoid systems with frequencies that would match those of the space occupied to avoid resonance, where the amplitude of the motions would become very large. These accelerations are compared against recommended peak floor accelerations for human comfort which is dependent on the type of occupancy; offices and residences have a lower threshold than shopping malls and gymnasiums.
However, another increasingly prevalent challenge is the need to design for truck loading on ground floors that serve as drive aisles or emergency access. Conditions can occur where a heavier truck loading is adjacent to retail, office, or residential spaces, or at times, below these spaces either during construction or the lifetime of the structure. Special considerations must then be made to account for the excess vibration that may be encountered as a result of these potentially larger forced vibrations and to design for a higher level of vibration serviceability.
Owners of new buildings typically have two main concerns when considering the effect of adjacent parking or trucking; the transmission of noise and vibration into the sensitive adjacent tenant areas, whether retail, residential, mixed-use, etc. Careful measures and criterion must be developed to mitigate the noise and vibration from the loaded areas from propagating into the 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. Nishkian Chamberlain works with the acoustic/vibration consultant to determine a course of action to be taken and works toward providing a solution to achieve the desired performance.
Nishkian Chamberlain engineers provide building owners, property management organizations, and tenants with a level of confidence that their tenants will be able to cohabitate in a comfortable environment. Should you have any questions about an upcoming or ongoing project, do not hesitate to contact any of our offices. You can also send an email directly to Craig Chamberlin at email@example.com.
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.