A great deal of hubbub has surrounded the luxury real estate market in Downtown San Francisco in recent months. Newly completed is La Maison, a 28-unit luxury condominium building located at 241 10th Street between Howard and Folsom. Epitomizing comfort and sophistication, this 23,570 square-foot mixed-use residential structure offers a variety of one- and two-bedroom contemporary floorplans ranging from 518 square feet to 992 square feet, each individually custom designed to offer a unique living experience. Structural design was performed by Nishkian Monks. The structure is four stories of wood framing over one story of concrete construction (Type VA over Type IA). The wood framing consists of I-joist and glulam floor/roof framing with wood shear walls. The above grade post-tensioned concrete podium separates the residential units from the office/retail and parking spaces below. The structural foundation is an at-grade mat foundation.
The 28 residences include full upgrades with all available options, high-end appliances and finishes, custom closets, designer window coverings, custom designed interior kitchen, smart home technology throughout, including video security cameras by Nest, smart wiring and USB plugs, keyless entry and an on-demand home manager by Hello Alfred. But the amenities are what truly set this development apart. Residents enjoy a rooftop deck with unobstructed views, open-air outdoor dining area with grill and organic gardens, lobby lounge, and enclosed parking garage accessed on 10th Street. Additionally, La Maison puts SoMa or South of Market neighborhood, and the best of San Francisco at your door. It’s also incredibly connected with neighborhoods like Hayes Valley and The Mission, just a few walkable blocks away. And nearby transit hubs quickly connect you to the rest of the city and greater Bay Area.
Nishkian Monks proudly worked with developer/builder JS Sullivan Development and award-winning architect Alan Tse of TC Architectural Studio on this challenging urban infill project. To learn more about La Maison, click here. If you have any questions about an upcoming residential project, do not hesitate to contact any of our offices. We’d be happy to assist you.
Photo Collage Credit: Bruce Damonte
By Aerik Carlton
While working with private space industry clients on launch complex structures, we have found some notable structural challenges. This article is meant to share some of Nishkian Dean’s recent launch site design experience. Specifically, an atypical structural load consideration that was identified and requested to be evaluated by our client.
Recently, launch tower structures, designed by others, near one of our project sites had been found to present weld failures at circular structural member support connections. The design and construction team that completed the nearby tower believed the weld failures to be representative of “bad welds” during construction. This conclusion was drawn because there appeared to be random failure locations. The failed connections were rewelded, but the repaired welds were found to have failed at the same locations in a follow-up inspection.
The failure of these members was surprising, mostly because they had been designed for large pressures associated with launch activities and hurricane wind forces for these structures, which the structure had not experienced. The second weld failures also occurred in the absence of either launch activities or hurricane activity, and there were only low-wind-velocity days between weld repairs and the second failure. The determined cause was attributed to wind-induced flutter, or gallop, of the members that resulted during low-velocity-wind cyclical loading. The team’s solution was to install air foils for fluid flow disruption around the circular members that experienced weld failures.
Due to the large rocket thruster impingement loadings present during launch activities, circular structural members have been found to be advantageous. Circular cross-sections allow for the same structural section properties: moment of inertia, section modulus, etc. to be present regardless of the direction of load (i.e. as the rocket gains elevation and the angle of load from the engine thrust changes, the structural member maintains the same resistance without reduction due to the change in loading angle). Wind and rocket thrust loads can be reasonably reduced by 20-30% of the projected area due to the allowable shape factor for circular structural cross-section. Fluid shedding, due to the rounded shape, is what allows the reduction in projected area for pressure loadings. When we are talking about 8 psi (1152 psf) of thrust pressure on the structure, shape factor reduction to the projected area is significantly beneficial. The use of circular framing members presents a slightly different issue, however, when considering relatively light wind loading conditions as evident from the nearby tower weld failures our client highlighted.
Nishkian Dean was tasked with mitigating the same type of behavior for a new tower at a nearby launch site in conjunction with the structural design for the client’s new facilities; for crew to vehicle access and lightning protection. The client asked that we design these operations support structures to accommodate their current vehicles as well as allowing for larger, more powerful, and greater range vehicles planned for the future.
A circular cross-sectional shape can exhibit interesting behavior in calm, steady wind. The phenomena our neighboring tower experienced is called “Karman vortex street,” and it happens during steady state fluid flow (or steady non-gusting wind) around a cylinder. You can be forgiven for not having that term readily available for recall from your fluid dynamics course work. I was fully unaware of the term until researching a solution to our issue. Karman vortex street is when the laminar (steady, non-gusting) fluid flow (or calm steady wind) around the cross-section begins to “stick” to the back side (leeward side) of the member and results in an alternating, rhythmic pattern of pressure differences from either side of the shape. This alternating pattern of “stick” creates a turbulent flow condition behind the member, and similar to an airplane wing, induces lift on the member due to the pressure differences. Pairing the lifting force in each direction and the gravitation pull on the member’s self-weight with respect to time, we end up with a vortex-induced vibration (VIV) loading condition.
To further explain and give a visual example of Karman vortex street, let’s take a look at the photo below. In the image, we can see an atmospheric example of Karman vortex street in cloud formations (located at the top of the photo) at the leeward (downwind) side of an island in the northern Sea of Japan. The photo has a wind directionality, moving from the bottom of the photo to the top, and the island is located at the bottom of the photo. As the clouds pass around the island, they swirl into eddies which alternate from the left and right sides of the island. This phenomenon is known as Karmon vortex street and was determined to be the cause of our client’s experience with weld failures.
Rishiri-to island, Sea of Japan off the northwest coast of Hokkaido, Japan
Taken: between April 19 and May 1, 2001
Photo Credit: NASA STS-100 Shuttle Mission https://spaceflight.nasa.gov/gallery/images/shuttle/sts-100/html/sts100-710-182.html
Licensing Agreement: https://www.nasa.gov/audience/formedia/features/Advertising_Guidelines.html
Determining which members in a structure will present VIV is the first step in designing for the issue. As our client saw with the neighboring launch tower, the structural members to exhibit VIV failures appeared to be random at first glance. To understand the nature of the failure, we needed to discover that we are dealing with a vibrational load that is time-dependent, and as such, we need to calculate the natural resonant frequency of our individual structural members. This process is very similar to what we, as structural engineers, do for seismic design when establishing our earthquake lateral loads. The load is dependent on the natural period of the structure. Turning to Roark’s Formulas for stress and strain, we obtain equations for the natural frequency of our members based upon support condition, length, material, and cross-sectional properties.
We can then establish the wind loading environment to see at what wind velocities moving around our individual structural member sizes will elicit vortex shedding frequencies in the vicinity of our structural members’ resonant frequencies. The tricky part here is establishing what makes sense for applicable wind velocities at the site and what boundary conditions can be set. Because hurricane activity was a design issue already being evaluated, the lower- to mid-velocities associated with hurricane wind were the upper boundary (assuming gusting would break up the required laminar flow required for Karman vortex street to occur long enough to gain a vibrational response). Evidence to the issues on the neighboring tower was given, as some of our tower’s members had resonant frequencies close to vortex shedding frequencies at wind velocities between 5 and 10 mph.
At this point we identify the members in the structure that are at risk of VIV. Using an equation for harmonic vortex shedding lifting force from Blevins (2001), we can establish a time-history load function. The load is then applied to a single degree of freedom model for the member, and we find the deflection versus time plot for the member during VIV that accounts for stiffness and damping. From the deflections and member properties, the reaction loads to the support connections are determined. And, from there, we evaluate our connections for cyclical loading.
In our design, we were able to simply lengthen our welds on the members that exhibited VIV to add the required weld capacity to overcome cyclical fatigue. However, there are many ways of mitigating the negative response to vortex conditions. Blevins (2001) outlines several options, including: helical strakes, shrouds, axial slats, streamlined fairings, splitters, ribbons, pivoting guiding vanes, and flat spoiler plates. The neighboring tower took advantage of the streamlined fairing option, while we implemented helical strakes on our tower topping fiber reinforced plastic (FRP) mast similar to those used on exhaust chimneys in other industrial applications.
Please contact Nishkian Dean if your project has vibration analysis considerations – we would be happy to share more of our expertise on this subject.
Aerik Carlton is an Engineering Designer with Nishkian Dean. Aerik can be reached at firstname.lastname@example.org.
AISC. American Institute of Steel Construction (2011). Steel Construction Manual. 14th Ed. AISC. Chicago, IL.
Biggs, J. M. (1964). Introduction to Structural Dynamics. McGraw-Hill. New York.
Blevins, R. D. (2001). Flow-Induced Vibration. 2nd Ed. Krieger Publishing Company. Malabar, FL.
Young, W. C., Budynas, R. G. (2002). Roark’s Formulas for Stress and Strain. 7th Ed. McGraw Hill. New York.
Welcome to 2018 and the first blog of the year from the Southern California arm of the Nishkian companies, Nishkian Chamberlain. We are excited to have the year underway with a number of new project designs starting, several major construction projects set to begin and significant positive signs for a great year!
With construction costs continuing to rise and rates beginning to follow this upward trend, Owner/Developers are beginning to look towards the many faces of building renovations in new projects. This includes full remodels of existing buildings, rehabilitation of historic structures, retrofits of structures due to ordinance mandates, adaptive reuse due to occupancy changes all falling under the umbrella of building renovations. This trend allows Owner/Developers to enhance an existing property and create new without actually building from the ground up. The path to project implementation is shorter without the significant capital investments that come with new projects and good margins can still be realized. That sounds like win, win, win!
Building renovations generate a number of structural challenges that must be considered. This starts with an understanding what the building is combined with the vision of what it is to be. Then the puzzle of determining how to get it done begins:
NOVA Academy Santa Ana, CA – Adaptive Re-Use
The challenges are many, but the end results are often beautiful. One such example is our award-winning project at Nova Academy. As we wrote in our July 2016 blog , this 1970’s office building went through an adaptive reuse turning an old office building in downtown Santa Ana into a vibrant learning center through a full building retrofit and renovation. The project, which utilized performance-based design and included full peer review, utilized viscous dampers to upgrade the pre-Northridge moment frame connection lateral system without requiring costly and time-consuming foundation upgrades.
NOVA Academy (Image courtesy of Berliner Architects) – Viscous Damper Brace
A significant tool in the Structural Engineer’s toolbox to efficiently manage building seismic retrofits and renovations is ASCE 41. In our May 2017 blog , we discussed the process this document allows to review, remodel and seismically retrofit existing buildings. Although often not specifically referenced in the Building Code, many jurisdictions allow it’s use given a thorough discussion of the purpose and methodology. This document is now being cited specifically in new City Ordinances in both Los Angeles and Santa Monica among other areas.
ASCE 41-13 Evaluation Process
Renovation projects are becoming more and more favorable as costs continue to rise and empty lots become fewer and fewer. Nishkian Chamberlain and the Nishkian Team have extensive experience with these types of development projects. Should you have any questions about an upcoming project, do not hesitate to contact one of our offices.
Pacific Amphitheater Entrance – Costa Mesa, CA
Nishkian Monks would like to introduce you to our newest team members: Justin Beschorner, Hannah Meyer, and Justin Jones.
A January 2018 update to our blog post from July 2017
Nishkian Dean previously reported on the URM Building Policy Committee back in July, which you can read here. This shorter post is intended to be a supplement to the original post, and to cover items not addressed in the previous article.
The URM Building Policy Committee had a final meeting on November 8, 2017 and completed a final draft of their report and recommendation in December, which the Portland City Council is likely to review in early 2018.
In summary, the Committee’s proposal is to require seismic strengthening of URM buildings using a tiered approached based on the building’s use and occupancy. The only exceptions to these recommended requirements are for one- and two-family homes and URM buildings that were previously seismically strengthened to an acceptable defined standard, as well as buildings serving religious functions or other buildings owned by non-profits that are not being used as schools. The exemption for Class 3 churches and other buildings used by non-profits would require that a placard noting the earthquake risk be placed at or near the entrances.
The Committee has defined 4 categories of URMs with differing levels of seismic strengthening requirements and corresponding time allotments to complete the requirements. The Classes range from 1 to 4, with Class 1 for Critical Buildings and Essential Facilities and Class 4 for Low-Occupancy structures. Class 2 is for Schools and High-Occupancy structures (such as churches and theaters), while Class 3 is for is the largest class of building and covers every other URM building not included in the other classes. For Class 3 buildings, the allotted time period to complete upgrades was reduced from 20 years to 15 years. Class 3 buildings make up over 80% of the URM building inventory.
Another recommendation by the Committee involves changing the current city code by modifying the thresholds for required seismic upgrades per Title 24.85. Title 24.85 requires owners to seismically retrofit their buildings when an owner spends approximately $43 per square foot on improvements within a two-year period. The Committee recommends lengthening the consideration period for cost-per-square-foot improvement periods from two to five years, and adding an upper limit to the total cost of improvements over a 15 year period to be twice the allowed five year costs. Title 24.85 also requires that the parapets be braced and the roof diaphragm be tied to the walls when more than 50% of the roof is replaced over a five-year period. The committee recommends that the trigger for roofing requirement upgrades happen over a fifteen-year period rather than a five-year period.
In our previous post, we calculated a considerable cost to implement the recommendations citywide. During the period for public comment, building owners expressed grave concern about the considerable costs. The Committee recommended that the City provide some funding mechanisms prior to implementing measures that would give funding assistance for owners, such as a property tax exemption through Oregon Senate Bill 311, and/or a few other sources laid out in the report.
These recommendations are on the docket to be considered by the Portland City Council in early 2018. At that point, the Council will need to decide if it will implement all or part of the recommendations. Possible outcomes may include an ordinance mandating seismic strengthening of URM buildings, the continuation of the status quo as required by Title 24.85, or the modification of Title 24.85 per the committee’s recommendation.
Nishkian Dean will continue to monitor the development of this important issue affecting many of our clients, so stay tuned.
A new mixed-use development is underway at 1395 22nd Street in San Francisco. The location will offer residents easy access to one of San Francisco’s Caltrain stations. The project will include two structures: an eight-story residential building over a below grade parking garage adjacent to a three-story industrial building. The northern residential building will contain over 250 rental units and the industrial space will be used for Production Distribution and Repair (PDR) by the City of San Francisco. The vertical load-carrying system for these buildings consists of post-tensioned concrete slabs supported by reinforced concrete columns and concrete shear walls. The façade uses various building materials to appear as individual residential buildings along the hill. The two buildings are supported by deep foundations and are separated by a seismic joint.
The site, perched on the east side of Potrero Hill, creates an interesting construction condition. The floor plates increase in plan as the building ascends the face of the hill. Where columns and walls intersect the slope, tiebacks will be used to tie the structure into the hillside. These tiebacks were peer-reviewed as part of the entire foundation system by a peer-review panel selected by the City of San Francisco.
A serpentine stair at the north side of the construction site will connect Missouri Street at the top of Potrero Hill to Texas Street at the bottom. The stair covers an elevation gain of approximately 85 feet and allows access to the residential building on multiple landings.
Align Real Estate, Perry Architects, Min|Day, Fletcher Studio, BUILD Group, and Nishkian Menninger are collaborating on this 250-unit, transit-oriented, mixed-use residential/PDR buildings and public stair project. Construction for the main structural system is scheduled to be completed by the end of 2018.
Renderings courtesy of Min | Day and Perry Architects
With joy and gratitude we wish you a wonderful holiday season!
Set to be one of the newest mixed-use residential developments in Los Angeles, CA’s Koreatown, 700 Manhattan is well underway in construction and just completed the basement structure and is beginning to rise above grade. The seven-story, mixed-use-complex will contain 160 residential units, more than 10,000 square feet of ground-floor commercial space, and several amenities including a dog run, and pool/spa deck. Located between Manhattan Place and Western Avenue and 7th Street, the building contains two levels of below grade parking, ground floor retail, parking spaces, and second floor amenity space. Then floors 3 through 7 contain residential units.
Development of the 700 Manhattan mixed-use complex is being performed as a tremendous collaborative effort between Jamison Properties, GMP Architects, Nishkian Chamberlain Structural Engineers and Wilshire Construction.
The structure consists of a cast-in-place concrete system up to the third-floor podium. Floor slabs consist of flat plate, two-way concrete slabs spanning between concrete columns and/or bearing walls. The lateral force resisting system in the building consists of special reinforced concrete shear walls up to the third-floor podium. For levels 3 to 7, the building is of wood framed construction with plywood shear walls as the lateral force resisting system.
Residential development continues in Koreatown and throughout Los Angeles. We appreciate the opportunity to be part of the team on this significant development. The Nishkian firms have extensive experience with multi-family construction development projects. Should you have any questions about an upcoming project, do not hesitate to contact one of our offices.
The recently completed new commercial building, The Palisade, illustrates the changing tide in the fastest growing and highest density of residential neighborhoods on the west end of Bozeman. Nishkian Monks participated in the project as the structural engineer of record, working directly with Bitnar Architects and general contractor Langlas & Associates. Developed by Paine Group, Inc., The Palisade is a 6,600-square-foot commercial building located at 630 Boardwalk Avenue. The structure is located at a gently sloping site. Above grade, the exterior and interior walls are of light-gage metal stud construction with thin set brick veneer at the exterior walls. The roof framing is accomplished with pre-engineered open web steel trusses. The building is founded on conventional concrete strip and spread footings with a slab-on-grade at the ground level.
New tenants Lone Peak Physical Therapy, New Wave Float Therapy, and The Bar Method have moved in. The Palisade was conceived to support Ferguson Farm, the new 19-acre, B2 Zone mixed-use infill development on the north side of Huffine Lane between Cottonwood Road and Ferguson Avenue.
In addition to providing the design of the structural system and construction administration services for The Palisade, Nishkian Monks also served as the primary special inspection agency for this project to help ensure an elevated level of quality throughout the construction process. The Palisade received the 2017 Excellence in Design “Merit” Award by the Montana Chapter of the American Institute of Architects. The award was presented to Thomas Bitnar, FAIA, CKA, LEED AP BD+C at the 2017 AIA Montana Fall Conference in Missoula this fall. A big thank you and congratulations to Thomas Bitnar, the project team, and everyone else involved!
Photo Credit: Zakara Photography
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.