|Letter to San Francisco City Planning Regarding Seismic Safety
A letter from two noted experts in the field of structures and seismic safety:
Lloyd S. Cluff , Consulting Seismic Geologist
Abolhassan Astaneh, Ph.D., P.E., Professor, Steel Structures
January 30, 1998
Dear Mr. Maltzer:
This letter pertains to the Draft Environmental Impact Report (DEIR 96.544E) regarding the proposed expansion of Sutro Tower to accommodate new digital antennae (DTV). The DEIR is woefully inadequate in its treatment of the impact of an earthquake on the structure; in fact, there is only the sentence: No impacts from earth shaking are considered likely. The authors have 65 years of combined experience investigating damaging earthquakes worldwide, and know this to be a totally inaccurate statement.
Because Lloyd Cluff lives in the fall zone of Sutro Tower, on September 10, 1997, he wrote to Ms Gitelman in your Department expressing his concern. He enclosed copies of several of his photographs of towers destroyed in earthquakes, and recommended a dynamic analysis of the Tower. The Building Inspection Commission did not take jurisdiction of this issue in their meeting of January 21, 1998, determining that a dynamic analysis is beyond the applicable codes. However, to make an informed decision on the adequacy of the EIR, prior to issuing a permit for Sutro Towers planned installation of a 12-ton antenna, the Planning Department must have state-of-the-art information pertaining to the seismic stability of the Tower. The Tower will serve as the foundation upon which the antenna will be hung. It is our opinion that, absent a proper dynamic analysis demonstrating the seismic stability of the Tower, the EIR cannot be approved.
Although apparently few are aware of it, a dynamic analysis of Sutro Tower was performed by J. Lord and M. Zayed and presented at the American Society of Civil Engineers National Convention in San Francisco in 1973. The ground motions used in the analysis were recommended in 1971 by Professor of Seismology and former Director of the University of California, Berkeley, Seismographic Station, Bruce A. Bolt, Ph.D. We have copies of 1973 correspondence that show that the Planning Department at that time requested and received copies of the results of this analysis. The Department was interested in the seismic safety of Sutro Tower, to conform with State legislation mandating a Seismic Safety Element in all City Master Plans. The paper concluded that, the tower, as designed and constructed, should be expected to survive without collapse, given a 1906-type earthquake.
Now, 25 years later, Sutro Tower, Inc., wants to perform a major alteration to the Tower, but the City has refused to impose the same standard of care that was evident in 1973. Lloyd Cluff suggested the prudent course of action was to require Sutro Tower, Inc., to have knowledgeable scientists and engineers consider the new earthquake data and knowledge gathered during the past 25 years and conduct a full dynamic analysis. He also suggested the analysis be peer reviewed, and offered his assistance. His letter has not been acknowledged directly; however, through copies of pertinent correspondence, he has noted that the main concern caused by his letter has been over his use of the word full, because it doesn't appear in the code. One of the dictionary definitions of full is complete, entire, maximum, and it is a word commonly applied to dynamic analyses; indeed, Lord and Zayed used it in their 1973 paper.
Lloyd Cluff solicited the opinion of colleague Hassan Astaneh, professor of civil engineering at U.C. Berkeley, and a consultant on steel structures. Dr. Astaneh has been involved in the design of steel structures for more that 30 years, including a 400-meter-high guyed steel tower in Iran. He was the leader of the team that conducted the seismic evaluation of the San Francisco/Oakland Bay Bridge for Caltrans. He also has been involved in the seismic evaluation and retrofit design of other major steel bridges, including the Golden Gate, the Hayward/San Mateo, the Richmond/San Rafael, the Carquinez, and the Vincent Thomas bridges in the US, and the Auckland Harbor bridge in New Zealand. Dr. Astaneh agreed that a proper seismic safety evaluation is needed for Sutro Tower using state-of-the-art technology, and volunteered his help.
Following are some of the reasons why Dr. Astaneh believes a realistic seismic safety evaluation of Sutro Tower is needed:
At the time of the design and construction of the Tower, our knowledge of the characteristics of ground motions was relatively limited. The number of actual recordings of earthquake ground motions was small. Our knowledge of long-period ground motions (the type that will effect a tall structure such as Sutro Tower), as well as the vertical effects, was in its infancy. Today, because of the lessons from recent earthquakes and hundreds of strong-motion records, more realistic ground motions can be applied. Dr. Bolt's estimated ground motions were at the state of the art in 1971; they are known to be inadequate in 1998.
Although the results of Lord and Zayed's 1973 dynamic analysis indicated the structure would survive a damaging earthquake, the analysis was used merely to test the Tower against strong ground shaking, not for actual design. The Tower was designed using the 1969 Building Code. Since then, the technology of modeling steel structures, especially with latticed members, has advanced considerably. The main advances have been in the areas of modeling of connections and the behavior of latticed members, and the capability to model local gaps and nonlinearities. Such modeling techniques are necessary if one wants to incorporate the possibility of uplifting of a leg or rocking of the foundation, which could be detrimental to the seismic stability of Sutro Tower. To analyze the Tower properly, one has to use the current technology.
Prior to the 1985 Mexico City earthquake, steel structures were considered almost invincible; there was no report of serious or widespread damage to steel structures. However, during the past 12 years, damage to and collapses of steel structures during the Mexico City, Armenia, Loma Prieta, Kobe, and Northridge earthquakes have changed this notion dramatically. These actual events and the laboratory testing that followed have indicated that the existing stock of steel structures, particularly structures as complex and important as Sutro Tower, may need reevaluation and substantial seismic retrofit. Almost all the structures that were damaged or collapsed satisfied the provisions of the governing building code.
The behavior of steel members having large cross sections in recent earthquakes has been relatively brittle, especially when constrained, resulting in fracture and catastrophic failure. Such members are similar to those used in Sutro Tower and other structures during the 1970s. To ensure such conditions do not exist here, a thorough seismic evaluation is necessary, particularly due to the three-legged nature of the Tower, which renders it a very determinate system that has no secondary stress path or possibility of redistribution. Loss of any leg will result in instability and collapse. In the current bridge codes and the 1997 Uniform Building Code, there are provisions to address this issue.
The realistic capacity of existing members and connections of the Tower should be established and used in the seismic safety evaluation instead of the nominal capacity from the specifications. Sutro Tower has withstood the fog and corrosive atmosphere of Mount Sutro for more than 25 years, and the actual condition of the Tower materials is in question and important to assess. The possibility of hair-line cracks due to metal fatigue under wind conditions has not and should be investigated; this can be critical in some welded connections.
We have read that Kline Towers, builders of the facility, redesigned Sutro Tower to the Building Code standards in February 1995, and that Sutro Tower, Inc., has charitably performed a seismic upgrade of the Tower in 1997. They both imply the Tower is safe. We would argue that it is impossible to perform a proper seismic upgrade in the absence of a proper seismic analysis. Indeed, the plans for their seismic upgrade, which have become available only recently, are dated 1995 and clearly are part of the DTV project. According to a November 6, 1997, letter from Yan Yan Chew, the Building Departments permit was indeed a structural strengthening permit for anticipating future antenna loading. More significantly, the plans are for upgrading for wind loads, not seismic loads. It is sometimes argued that accounting for one hazard covers another, but in the case of Sutro Tower, where we have experienced winds in excess of 100 miles per hour, even their design wind load of 70 miles per hour is found lacking.
At the January 21, 1998, meeting of the Building Inspection Commission, it was stated that the Federal Communications Commission requires that the consortium, which includes KRON, KPIX, KGO, and KTVU, be prepared to broadcast from DTV antennae by November 1998. The community's safety concerns were going to cause San Francisco to lose out on high-definition television! Actually, the consortium has voluntarily agreed to go on-line with DTV 6 months earlier than the actual deadline, which we understand to be May 1999. In addition, in our experience, a dynamic analysis of a structure the size of Sutro Tower could be performed in approximately 1500 to 2000 hours. If this analysis had been prudently required at the time of the first permit application in May, it would have been completed by now. If no problems had been found, the new antennae would probably be installed today.
Also in our experience, a dynamic analysis of the type appropriate for Sutro Tower would cost in the neighborhood of $200,000—a small price when contrasted with the revenue that would be lost if the Tower was rendered nonfunctional. From a purely financial point of view, one would think Sutro Tower, Inc., would want to protect their investment. Of course, if weaknesses were noted, the seismic design and retrofit would take additional time and money. They may be afraid of the results of a dynamic analysis, but we fear the consequences of not having those results.
On October 13, 1997, Nancy Hogan, President of the Twin Peaks Improvement Association, wrote to Ms Joan McQuarrie, Chief Building Inspector, Department of Building Inspection, wherein she spoke of Mr. John Osteraas, Principal Engineer, Failure Analysis Associates, Menlo Park. Mr. Osteraas' firm is capable of analyzing the effects of the failure of the Tower—the impact it would have on our built environment. Sutro Tower is the only such tower in the country built in the heart of a highly populated urban area. In addition, it is close to three of the City's main reservoirs, a large elementary school, a fire station, and a hospital. Its collapse on or flying debris impacting the homes or other facilities near the Tower will cause casualties, serious injuries, and property damage. Even its partial collapse could demolish the next-door water reservoir, creating a water shortage in the aftermath of an earthquake. Loss of communication lines will cause a serious problem for emergency response and recovery throughout the Bay Area. Flying debris could ignite fires in the Sutro forest that, given a high wind, would quickly spread. And it is not a farfetched idea that we could have a strong earthquake during a strong wind. Inasmuch as the Sutro Tower is located within 5 miles of the San Andreas fault and would suffer near-source effects, the City might be interested in such a failure analysis. Mr. Osteraas has volunteered to provide advice in this regard.
In his letter of September 10, 1997, Lloyd Cluff indicated that the Sutro Tower should be considered an essential facility. On December 3, Jim Hutchinson of the Building Department declared it so. Kline Towers, immediately responded that the importance factors appropriate to an essential facility, as specified in the 1995 San Francisco Building Code, had been used, and that following their seismic upgrade, the Tower will meet essential-facility requirements. The importance factor only requires that they add 25 percent to the normal seismic demand, which we know is inadequate to begin with.
We were astonished that the Building Inspection Commission, did not think it was necessary for the Tower to undergo a full dynamic analysis. Sutro Tower surely is one of the few structures in California's building stock that demands and deserves refinements above and beyond prescriptive building-code requirements. The purpose of the Building Code is to provide minimum standards for life safety by preventing total collapse; the new codes are aimed toward continued performance following an earthquake. California has been at the forefront in developing new and more stringent seismic codes, and this information is not foreign to the Planning Department. San Francisco's City Hall has just been upgraded using state-of-the-art technology. It has been upgraded to a standard much higher than the 1995 Building Code so that it will be functional following the big one. Although the Building Commission and the Building Department acted within the strict limitations of their jurisdiction, the time has come to add new tools to current procedures and to introduce responsibility and accountability into the process. The City should have a vested interest in protecting its citizens from death and injury, protecting its economy, and ensuring that essential services continue to function after earthquakes.
We have raised serious questions regarding the stability of Sutro Tower during a large earthquake, and we have put the City on notice that, in our opinion, the addition of DTV antennae to Sutro Tower would be foolhardy without a full dynamic analysis conducted by structural engineers fully knowledgeable of the new seismic engineering data from the Loma Prieta, Northridge, and Kobe earthquakes. We are asking a lot—we are asking that you challenge people who lack the will, knowledge, or support to deal with a hazard that has the public-safety and economic implications of earthquakes. Does the City really believe FEMA will keep bailing it out? Given this knowledge, how long will Sutro Tower be able to keep its insurer?
We have the talent in the Bay Area and we have the data and the technology. The seismic safety of Sutro Tower must overcome benign neglect, denial, procrastination, politics, and ignorance. Thank you for your consideration.
Abolhassan Astaneh-Asl, Ph.D., P.E.
cc: US Senator Barbara Boxer
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