Life should return to normal after the next earthquake strikes this medical office building and underground parking structure in southern California, thanks to PHMRF.
By Greg Snapper
It’s a sunny Monday morning just east of downtown Los Angeles – you turn the key in the ignition of your Toyota Prius and you’re off. You leave the driveway, head to the Interstate and within minutes you’re bumper-to-bumper with the SUV in front of you. An hour passes, but despite traffic, you manage to grab a prime parking spot at work. You walk into the lobby of your newly built office building, and as you say hello to fellow M.D.s, you happen to notice the flat-screen monitor at the receptionist’s desk begins to flicker, then her keyboard rattles, the blinds above the lobby windows crash to the ground and the building begins to sway – an unnerving feeling to say the least – as if the four-story giant just awoke from a long slumber, sprouted legs and began its own daily commute. A minute later, the temblor stops, the sway of the building ceases and you finally get your bearings in the lobby where plants and chairs are overturned; a fax machine lies wounded near the exit, and you walk through the lobby double doors feeling stunned. What you see next is quite surreal; you’re surprised to see your office building stands tall, seemingly unaffected. Suddenly, you wonder whether people are trapped inside the parking structure only feet from where you’re standing and only minutes ago where you parked, so you rush to help. But to your amazement, the structure seems to be whole, in one piece, and people emerge from the subterranean levels unhurt but just as stunned as you are to witness the destruction all around them.
This is not a script out of Hollywood, but it could indeed happen in the event an earthquake hits close to the White Memorial Medical Center’s (WMMC) new medical office building and 507-car underground parking structure in East Los Angeles. Outfitted with the specialized Precast Hybrid Moment Resistant Frame (PHMRF), the newest addition to the medical center was built to withstand massive earthquakes.
Building to Suit
Although high resistance to a catastrophic temblor is a key selling point for the hybrid frame, common construction issues like deadlines and community involvement influenced the owner’s final decision to go with precast concrete.
“We had a lot of people and equipment to move within two days,” explains Al Deininger, vice president of construction for WMMC. “Additionally, we needed to consider what the exterior and interior colors and shapes would look like to represent the predominantly Latino community we serve.”
The owner developed a design-advisory team to gather feedback from community members in the east of downtown Los Angeles area along with White Memorial employees to communicate new technical options for the new White Memorial campus as well as the design for the new medical office building.
“This created excitement,” Deininger says. “We are serving the people who make L.A. work; a population that is mostly working-class.”
Deininger says the project’s design/build contractor, Los Angeles-based Charles Pankow Builders Ltd., erected everything quickly and their methods were ideal for White Memorial’s construction needs.
“We were given the product we wanted all in the time frame we needed and within budget,” he says.
The hybrid frame isn’t about budgets. It’s about exceeding the standards of its unique design, which is relatively simple in theory. Consider that the various construction elements used are at many builders’ fingertips: Precast beams and columns, fiber-reinforced grout, reinforcing steel (grouted), post-tensioning steel (ungrouted) and de-bonded rebar are the bulk of PHMRF.
Simply put, the technology can be likened to a rubber band stretching in two opposite directions.
The hybrid frame design provides elasticity in response to dynamic loading caused by an earthquake, and the effects are like a flat rubber band held at both ends and stretched. The rubber band will stretch but not break, and then return to its former state. A hybrid frame building reacts much the same way, resisting the lateral forces of the temblor and reverting to a static state after the action stops. The combination of post-tensioning steel and reinforcing steel, in proper proportions, pushes the building back to its original position after an earthquake.
The standard steel frame design absorbs energy through inelastic behavior, more like bending a paperclip back and forth. Like the paperclip, once bent, the steel frame does not revert to its original position after the earthquake ceases. The result is a building that ends up leaning like the Tower of Piza.
The rubber band-like hybrid design uses a precast column/beam connection with both elastic and inelastic action at the joint. The specialized joint and unique design concepts involve a shift away from current designs, which require structures to absorb seismic energy through inelastic response of the framing members. The post-elastic performance lies in the connection rather than a structural member. The hybrid frame isolates and separates the strength and energy absorptive components within the joint.
The hybrid frame installed at the medical office building includes five bays, made up of six columns and five beams per level. For erection, the beams rest on small platforms temporarily supported off of each of the six columns. The gap between the end of the beam and side of the colum is grouted, forming a joint in which all of the earthquake motion is absorbed. The frame was installed on two sides of the building parallel to one another, which in the event of a temblor would resist the lateral forces.
Joe Sanders, regional manager for Pankow, says the key to the joint technology is in the predetermined crack location of the hybrid frame.
“If you were constructing a cast-in-place building, the crack location is not predetermined. The building, to a large degree, destroys itself resisting the earthquake,” he says. “The hybrid frame has a much higher tolerance for movement and restoration to its original location without suffering debilitating damage to the point where the building might have to be torn down or significantly retrofitted.”
Building in Earthquake Country
Prescriptive limits and building code requirements set by the International Code Council, an entity dedicated to public safety through the development of building safety codes, can limit builders’ freedoms in regards to structural and architectural design along with product use. Fortunately, there is now a way to improve upon the constraints built into the code and owners can buy better performance at the same cost. Suzanne Nakaki, principal with the Nakaki Bashaw Group, the structural engineer of record on the project, explains the importance of the project in the Los Angeles area and how builders should be willing to take a chance.
“In the hybrid frame that was used at White Memorial, it is the focus on the behavior of the beam column interface that helps minimize earthquake damage, but in addition allows the architect creativity,” Nakaki says. “Constraints built into the code don’t apply to the hybrid frame and engineers and contractors shouldn’t be afraid to use this technology to enable themselves to think outside the box. While there is an investment required to learn any new technology, it pays off quickly for this high performance product.”
Making Things Simpler
Although the frame is a high-performance product, the application of the hybrid frame has become simpler with each installation, says Al Fink, president of Mid-State Precast in Corcoran, Calif. Mid-State has been involved with eight hybrid frame projects since the late 1990s.
“We managed to iron out all problems in prior projects; there’s always a learning curve,” Fink says. “This was a pretty simple job because we’ve become very familiar with the construction techniques, and it’s become routine.”
The increased ease of installation runs parallel with several other benefits. Sanders says the speed and economics of using precast concrete should be a huge attraction for the construction industry.
“Over the course of a few weeks, you can erect a building and not be dependent on issues such as labor availability or bad weather conditions at the exact time you need to put the building up,” he says. “This application is unique in the fact that it can be done by a wide variety of precasters.”
Sanders says the precast hybrid frame was ideal for the project. An introduction to the precast industry will help open the market and create popularity for this different building type and ultimately become more competitive.
“A building the size of White Memorial and where it is located, the soil conditions, the economic factors and the owner’s desire for open space made the hybrid frame the right solution.”
This “right solution” will have a chance to prove itself in the next big L.A. earthquake. The only uncertainties of that event is when it will strike and how severely. Will tenants go back to work in the medical office building that day? Will business run without missing a beat?
Hopefully that major earthquake will never happen. But if it does, the hybrid frame will be ready, and could help avert a major catastrophe.
Testing, Approval and the Public Domain
The hybrid frame was developed through testing in the early 1990s and funded primarily by construction company Charles Pankow Builders Ltd. and the National Institute of Standards and Technology (NIST). John Stanton, professor at the University of Washington, and Dean Stephen, past-president of Pankow, were the driving forces behind initiating the development of the hybrid frame.
At the conclusion of three tests in the 1990s by NIST, the University of Washington and the Precast/Prestressed Concrete Institute (PCI), Pankow received International Code Council approval to utilize the system, but that approval required Pankow’s involvement in the manufacturing and inspection of the product. However, PHMRF was purposely placed in the public domain so as to serve the general public and not solely Charles Pankow Builders Ltd.
“The technology was developed and kept nonproprietary from our perspective for the purpose of allowing other people to pick it up and apply it,” Joe Sanders, regional manager of Pankow, says. “It started out that way because we were the only ones who had approval to do it. I’m not aware that at this time that anyone else has the approval to do it, but there’s nothing preventing them from doing so.”
Project: White Memorial Medical Center medical office building and parking structure
Project Owner: White Memorial Medical Center, Los Angeles
Structural Engineer: The Nakaki Bashaw Group, Irvine, Calif.
Contractor: Charles Pankow Builders Ltd., Los Angeles
Architects: Barrio Planners (Building) and HNA Pacific (Parking), Los Angeles
Precast Manufacturer: Mid-State Precast, Corcoran, Calif.