Factor of Safety

The factor of safety is the ratio of the capacity of a structural element to the demand placed upon it, and the building code requires that this ratio be greater than one — significantly greater, because the code accounts for the uncertainties that engineers cannot eliminate, the variability in material strength, the variability in construction quality, the variability in the actual loads versus the assumed loads, the variability in the analysis methods, the variability in everything — and the code prescribes load factors and resistance factors that, when applied, produce a design that is stronger than it needs to be by a margin that is sufficient to absorb the variability, and this margin is the factor of safety, and the factor of safety is the distance between the actual state of the structure and the failure state, and the distance must be large enough that the structure never crosses it, not under the worst combination of loads, not under the most unfavorable conditions, not in twenty years or fifty years or one hundred years.

The parking structure at Lake and Wabash had no factor of safety on the east wall connections.

Ada submitted the preliminary report to the City of Chicago Department of Buildings on a Thursday, fourteen days after the collapse. The report was forty-seven pages. It included the field observations, the document review findings, the structural analysis results, and the preliminary conclusions, and the conclusions stated, in the language that Ada had constructed with the precision of a structural detail:

"The collapse of the Lake-Wabash parking structure was initiated by the failure of the gravity load connections at the east wall, grid line F, at the third-floor level. The connections were designed for lateral loads only, based on the classification of the east wall as a shear wall in the original structural calculations. The east wall was, in fact, functioning as a bearing wall, supporting the gravity loads of the double-tee floor members at each level. The resulting demand-to-capacity ratio for the connections exceeded 1.0 under dead load and design live load, indicating that the connections had no factor of safety against failure. The root cause of the collapse is a design error in the original structural calculations, specifically the omission of gravity loads on the east wall at grid line F."

She printed the report. She signed it. She stamped it with her own stamp — the inch-and-a-half circle that contained her name, Ada K. Nowak, her license number, 062-047891, and the state, Illinois — and the stamping was the act that made the report official, that made it her opinion, her declaration, her professional assertion that the findings were true and the conclusions were sound and the analysis was correct, and the stamp was the heaviest thing she did that day, heavier than the forty-seven pages, heavier than the box of documents, because the stamp was her bearing wall, the structural element that carried her career and her reputation and her liability, and she pressed it onto the page and the ink transferred and the circle appeared and her name was on the report the way Hardin's name was on the drawings, and the two names — hers and his, student and teacher, investigator and investigated — were now on opposite sides of the same failure.

She delivered the report to the building department in person, because this was the kind of document that you did not entrust to email or courier, this was the kind of document that required the physical act of handing it to someone, the transfer of weight from one hand to another, and she handed it to the deputy commissioner, a woman named Janet Park who had been with the department for twenty years and who received the report with the practiced neutrality of a person who has received many reports, some of which said nothing alarming and some of which said the thing that changes everything.

"I want to flag the key finding," Ada said. "The engineer of record is Martin Hardin, of Hardin & Keane. Professor Hardin is well known in the structural engineering community. The report identifies a design error in his calculations. This will generate attention."

Janet Park looked at Ada with the expression of a woman who had been deputy commissioner of the Chicago building department for long enough to know that "generate attention" was an understatement the way "the building has experienced partial structural failure" is an understatement, and she nodded.

"We'll review it," she said. "The legal team will need to see it. And the families' attorneys."

"I understand."

"Dr. Nowak — is the remaining structure safe?"

"No. The remaining structure should be demolished. The standing portion on the west side is stable under current loading, but the connections at grid line D — the line where the collapse stopped — are damaged and the load redistribution from the collapsed portion has overstressed several elements. I've included shoring recommendations in Section 7, but my recommendation is full demolition."

"How soon?"

"The shoring should be installed within a week. The demolition should proceed as soon as the forensic documentation is complete. I'd recommend no more than sixty days."

Janet Park wrote something on a notepad. She placed Ada's report on the stack of documents on her desk — the desk of the deputy commissioner, which was its own bearing wall, carrying the accumulated weight of a city's building stock, every permit and every inspection and every violation and every complaint, the entire structural history of Chicago compressed into the surface area of a desk — and she said, "Thank you, Dr. Nowak."

Ada left the building department and walked to her car and sat in the car and looked at her hands on the steering wheel and thought about the stamp she had just used, the ink circle that was now on page forty-seven of the report, and she thought about Hardin's stamp on the drawings, the ink circle that was on sheet S-1 of the structural set, and she thought about how the two stamps were the same size and the same shape and served the same function — the assertion of professional responsibility, the bearing of weight, the carrying of consequence — and the difference was that her stamp said the building had failed and his stamp said the building was safe, and one of these assertions was wrong, and the wrong assertion had been there for twenty years, and three people were dead.

She drove to the office. She parked in the garage beneath the building on South Wacker Drive. She took the elevator to the fourteenth floor. She walked to her office and sat at her desk and looked at the model on her screen, the mathematical ghost of the building that was now scheduled for demolition, and she thought about factors of safety.

The factor of safety for a typical structural connection, designed according to the building code, is approximately 2.0. This means the connection can carry twice the load it is expected to carry before it fails. The factor of safety for the east wall connections at Lake and Wabash was 0.68 — the inverse of the demand-to-capacity ratio of 1.47 — which meant the connections could carry only sixty-eight percent of the load they were actually carrying, which meant they had no safety margin, which meant they were in a state of chronic overstress from the day they were installed, which meant the building was never safe, which meant the certificate of occupancy was issued for a building that did not meet the building code, which meant every person who had ever parked a car in that garage had been inside a structure that was closer to failure than anyone knew.

Not just Marcus Webb and Diane Ostrowski and James Chen. Everyone. For twenty years.

Ada thought about the thousands of people who had driven into that garage and parked and walked to the elevator and taken the elevator down and walked out onto the street, never knowing that the wall beside them was overstressed, never knowing that the weld above them was undersized, never knowing that the factor of safety was less than one, and these people were alive because the structure had reserves — the concrete was slightly stronger than specified, the welds had some capacity beyond the calculated value, the loads were usually less than the design loads — and the reserves had held, had covered the deficiency, had provided the factor of safety that the design had not, and the reserves were a gift, an accident, an unintended grace.

Until the reserves ran out.

This is what Ada thought about when she thought about factors of safety — not the abstract ratio, not the number in the code, but the human reality of the margin, the distance between standing and falling, the space that the engineer provides so that the building can absorb the things the engineer did not foresee. The factor of safety is not conservatism. It is not over-engineering. It is not waste. It is the acknowledgment that the engineer does not know everything, that the materials are variable, that the construction is imperfect, that the loads are uncertain, that the future is unknowable, and in the face of all this uncertainty the engineer provides margin, provides room, provides the structural equivalent of grace, and the grace keeps people alive.

Hardin had not provided the grace.

Not intentionally. The error was in the calculation, not in the intent. Hardin had not decided to under-design the connections. Hardin — or Keane, or whoever had written the zero on page 87 — had made a mistake, a misclassification, a failure of attention, and the failure of attention had propagated through the design the way a crack propagates through concrete, following the path of least resistance, finding the weak point, spreading.

Ada sat at her desk and thought about attention. She thought about the specific quality of attention that structural engineering requires — the ability to hold an entire building in your mind, every element, every connection, every load path, simultaneously, the way a conductor holds an entire orchestra in mind, every instrument, every entrance, every dynamic, and the failure of attention is the dropped note, the missed entrance, the moment when the conductor looks at the brass and the strings fall apart, and the building is the music and the engineer is the conductor and the attention is the only instrument the engineer has.

Had Hardin's attention failed? Had he looked at the lateral analysis and missed the gravity loads? Had he trusted Keane's calculations without checking the fundamental assumptions? Had he been busy, distracted, overworked, compromised by the ordinary pressures of running a practice — the deadlines, the budgets, the client demands, the need to produce drawings on schedule and move to the next project — and had the pressure caused him to review less carefully than he should have, to trust more than he should have, to stamp without verifying?

Ada did not know. She could not know from the documents alone. The documents showed the what but not the why, the error but not the cause of the error, the zero on page 87 but not the hand that wrote the zero or the mind that directed the hand or the circumstances that shaped the mind at the moment of writing. The documents were structural elements — rigid, measurable, fixed — but the human story behind the documents was the live load, the variable weight that changed depending on the day and the hour and the person, and the live load was harder to measure than the dead load, harder to predict, harder to design for.

Her phone rang. The screen showed a 312 number. She knew the number now.

"Professor Hardin."

"Ada. I've been told you submitted a preliminary report to the city."

"I did. This afternoon."

"I'd like to see it."

"The report will be made available through the building department. I can't provide it directly — you'll need to request it through your attorney."

"My attorney." A pause. The particular quality of the pause told Ada that Hardin had not yet engaged an attorney, that the word "attorney" was new in this context, that the need for an attorney was a load he had not yet accepted. "Ada, I don't have an attorney. I haven't done anything that requires an attorney."

"Professor Hardin, I'm not able to discuss the contents of the report with you. The report speaks for itself. I would strongly recommend that you retain legal counsel."

"So it's that bad."

Ada did not answer immediately. She let the silence carry the weight, the way a bearing pad carries the weight — passively, without moving, without doing anything except being there, between the beam and the column, between the question and the answer.

"The report identifies a design deficiency in the original structural calculations for the east wall," she said. "The deficiency contributed to the collapse. The report recommends further investigation and attributes the deficiency to the original design documents, which bear your stamp."

Another silence. Longer. Ada counted the seconds the way she counted her mother's respirations — involuntarily, automatically, the engineer's habit of measuring time.

"The east wall," Hardin said. "Grid line F."

"I can't discuss the specifics."

"You don't need to. I know the building. I know the east wall. Grid line F was a shear wall."

"The report addresses the classification of the east wall."

"Ada." His voice was different now. The authority was still there, the resonance, the professor's voice that she had listened to for three years in lecture halls and seminars and thesis meetings, but there was something beneath the authority, something that the authority was trying to cover, the way a building's facade covers the structure — the facade is what you see, the structure is what holds, and sometimes the facade is intact while the structure is failing. "Ada, I need to ask you a direct question. Did the east wall fail because it was carrying gravity loads?"

"I'm not going to answer that."

"Because the answer is yes."

Ada closed her eyes. She sat at her desk with her eyes closed and her phone against her ear and she heard the wind outside the window — the fourteenth floor, the wind louder up here, the building swaying slightly in the gust, a movement that was designed for, that was within the allowable drift, that was the building doing what a tall building does, flexing rather than breaking, yielding slightly to absorb the force — and she heard Hardin breathing on the other end of the line, and the breathing was the breathing of a man who was beginning to understand the weight that was coming, the dead load that was descending, the permanent weight of three names and a zero and a stamp.

"I designed that wall as a shear wall," Hardin said. "I remember the project. Keane did the calculations. I reviewed them. I reviewed the drawings. I stamped them. The east wall was a shear wall."

"The east wall was carrying gravity loads. The double-tees bear on the wall ledge. The gravity loads flow through the wall to the foundation."

"That's not — the double-tees were supposed to bear on a beam at that line. An inverted-tee beam. The same system as the interior bays."

Ada opened her eyes. She looked at the framing plan on her desk — sheet S-5, the third-floor plan — and she looked at grid line F and she saw the wall and she saw the double-tees bearing on the wall and she did not see a beam.

"There is no beam at grid line F," she said. "The framing plan shows the double-tees bearing directly on the wall."

"That's not what I designed."

"That's what the drawings show. Your drawings. Your stamp."

"I need to see the drawings. I need to see my own drawings."

"Professor Hardin. Please retain an attorney. The report has been filed. The process will proceed. I am telling you this as a professional courtesy and I am telling you this because you were my teacher and I owe you the courtesy. But I am not able to discuss the technical findings further."

"Ada —"

"Good night, Professor Hardin."

She hung up. She set the phone on the desk. She looked at the framing plan. She looked at grid line F. She looked at the wall where Hardin said a beam should have been and was not.

A new question had entered the investigation. If Hardin had designed a beam at grid line F, and the drawings showed a wall instead, then there was a discrepancy between the design intent and the construction documents, and the discrepancy was not a calculation error, it was a drafting error, or a coordination error, or a decision made somewhere between the design and the drawings that changed the structural system without changing the analysis, and the question was who made the decision and whether Hardin had known.

Or Hardin was lying. Or Hardin was remembering incorrectly. Or Hardin was constructing a narrative that distanced him from the error. Ada could not determine which from a phone call. She could determine it from the documents — the design development documents, the correspondence between Hardin and Keane, the correspondence between the engineer and the architect, the record of the decisions that had shaped the building, the decisions that had determined which walls would bear and which would not.

She needed more documents. She needed Hardin's complete project file, not just the calculations and the shop drawing reviews, but the design sketches, the internal memos, the emails, the meeting minutes. She needed the architect's drawings. She needed the contractor's records.

She needed the full structural history.

She called Brin. "I need you to subpoena the complete project records from Hardin & Keane, from the architect — who was the architect?"

"Marston Cole Associates."

"From Marston Cole, and from the general contractor."

"What are we looking for?"

"A beam. A beam that may have been designed but never drawn. Or a beam that was drawn and then removed. Or a beam that never existed. I need to know what happened at grid line F during the design phase, not just the construction phase."

"This is getting bigger."

"It was always this big. We're just seeing more of it."

She hung up. She sat at her desk and looked at the city through the window and the city was there, the buildings were there, standing, holding, and somewhere in the city Martin Hardin was sitting in his own space looking at his own view and thinking about an east wall and a beam that he said should have been there, and Ada did not know whether the beam was real or invented, remembered or fabricated, and the not-knowing was a load, another load, added to the loads she was already carrying, and the factor of safety — her own personal factor of safety, the margin between her capacity and her demand — was shrinking.

She thought about her mother's words: fair is not a structural term.

She thought about the factor of safety and how the code provides it for buildings but no code provides it for people, no code says that the demand on a person shall not exceed sixty-eight percent of the person's capacity, no code provides load factors for grief and resistance factors for endurance, no code accounts for the variability in human strength or the uncertainty in human circumstance, and people carry what they carry without margin, without redundancy, without the engineered grace that keeps buildings standing.

She drove home. She parked in the garage. She looked at the columns.

She went upstairs. She stood at the window. The city stood. She stood inside it.

The factor of safety held, for one more day.