What Independent Review Buys

Independent review, which the rare and difficult buildings Michael Stern writes about make unavoidable, is made of two physical things: a criteria document and a comment log. A structural engineer writing for STRUCTURE magazine describes both. The criteria document becomes the de facto code for the project, describing the performance objectives, the design loads, the proposed exceptions to prescriptive rules, the assumptions behind key elements and which parts of the lateral system will be allowed to yield; the comment log tracks every review comment to its resolution.¹ A third material belongs beside them for the envelope, a full scale specimen of the wall itself, pressurized and soaked in a laboratory. This page is about what those three things cost and what they have, on record, prevented.

What a second engineer costs and when

The engineer’s account calls the practice Independent Design Review, the term the loading standard ASCE 7 uses, and explains where it is required: for base isolation and viscous damping in seismic design, for designs using nonlinear response history analysis, and for any design that claims equivalent performance in order to depart from prescriptive rules, which on the West Coast often means a residential or office tower exceeding a structural system’s height limit.¹ The panel is a practitioner who has designed similar structures, an expert in the hazard, and often an academic in the system proposed, all without conflicts on the project.¹ Timing is the whole economics. The review should start during conceptual design, when the structural system and configuration are chosen, because reviewer input there changes the design cheaply, while a review that starts late can force redesign and delay.¹ The author’s summary is that, properly run, the process costs the project a small additional amount and, he hopes, no schedule, in exchange for being allowed to build what the prescriptive code would not otherwise permit.¹

Figure

Who sits on a review panel and what they read

1. 01
   A practitionerAn independent engineer who has designed similar structures and has no conflict on the project.
2. 02
   A hazard expertA specialist in the earthquake or extreme wind the design must resist.
3. 03
   An academicOften, a researcher with expertise in the structural system proposed.
4. 04
   The criteria documentThe project's own rulebook, setting performance objectives, loads, proposed exceptions and which elements may yield.
5. 05
   The comment logA running record of every review comment and its resolution, so disagreement is settled before drawings are final.

Cities have turned the same idea into a permit path. A code consultancy summarizes Chicago’s program, an optional route, in place for more than a decade, that speeds a building permit along under rules that took effect at the start of 2018. The applicant engages a Registered Structural Peer Reviewer from the department’s register before filing. That reviewer must hold an Illinois structural license, have practised for at least six years, with three of those years in Illinois, have handled five comparable jobs within the past five years, finish the department’s course, carry insurance, and stay clear of the project for 18 months after the review closes.² The report has to confirm the codes and criteria used, the site investigation, the fit between structural and architectural drawings and whether the documents are ready for permitting, and the department may still choose to review the drawings itself, at random or for cause.²

Figure

Chicago's reviewer, by the rules

1. 01
   LicenseA current Illinois structural engineering license, at least six years licensed anywhere and three in Illinois.
2. 02
   ExperienceFive comparable jobs within the past 60 months as professional of record, project lead or reviewer.
3. 03
   Training and insuranceThe department's course, renewed every five years, plus liability insurance.
4. 04
   IndependenceNo financial or other interest in the structures reviewed, kept for 18 months after the review ends.
5. 05
   The reportVerified codes and criteria, the site investigation, drawing coordination and a certification in fixed wording with no conditions attached.

New York writes the requirement into its building code. The structural peer review provisions specify where review is required and by whom, require verification that the design is in general conformance with the code, and provide that the reviewer is a qualified independent structural engineer retained by or on behalf of the owner.³ The connection to special inspections, the construction stage counterpart, is drawn on that page. In each jurisdiction the economics are the same shape: a defined fee for a second engineer, paid before the drawings are final, against an undefined cost if the drawings are wrong.

What being wrong once costs

The case that structural engineers teach is a tower on Lexington Avenue. ArchDaily’s history of the Citigroup Center describes a 1977 office building raised on four columns centered on each face instead of at the corners, so that a church on the site could be preserved, braced by chevrons designed by the engineer William Le Messurier and kept from swaying by a heavy tuned damper under its sloping roof, the frame having been kept deliberately light.⁴ In 1978 an undergraduate, Diane Hartley, recalculated the wind loads and included the case the design had not, wind arriving at a corner and loading two faces together. Above a certain speed the bracing joints would fail. She phoned the engineer’s office, was reassured, and the matter rested there for close to two decades.⁴

Le Messurier checked and found the student right, and the reason was a substitution made during construction to save money: connections in the braces that had been designed as welds were built as bolts, and the corner wind case exposed the difference.⁴ A local news site notes that the damper, which needed electricity to run, was the only thing keeping the building safe in a strong storm, that an emergency repair welded steel plates across the bolted connections, and that Hurricane Ella’s failure to make landfall that year may have mattered.⁴⁵ The review that caught the error was unpaid, unsolicited and twenty years late in being acknowledged. The one that should have caught it, a second set of eyes on the quartering case before the joints were changed, would have cost a fee. The value engineering entry reads the same story from the side of the substitution.

Figure

What a laboratory mockup is for

1. 01
   VerificationWind, rain, earthquake movement and heat and cold are simulated against the specified performance.
2. 02
   Cost and timeProblems found in the chamber are corrected before fabrication instead of as rework on site.
3. 03
   ComplianceThe record shows the wall met the standards the jurisdiction expects.
4. 04
   DurabilityMaterials, methods and assembly are shown to hold up before decades of weather arrive.

The envelope has its own version of the second opinion, and it is physical. A curtain wall consultant recounts a laboratory test of a custom unitized wall destined for a tower in Asia: a specimen 22.7 meters wide and 12 meters tall, made by the factory that would produce the real units and shipped to Florida. It cleared the air leakage, water and racking stages. When suction equal to 2.5 kilopascals was applied to mimic wind pulling on the cladding, part of the anchorage gave way, dropping four units from the test frame. The anchors were redesigned before a single production panel was made, and the consultant’s conclusion was that without the test the tower would have been clad with connections that could not hold against a modest wind.⁶

The test is routine rather than heroic. A testing laboratory describes pre construction mockup testing as building a full scale model of a facade in a controlled environment to catch problems of design, manufacture and fitting before construction, and reports more than 1,300 laboratory mockups tested since 1974.⁷ A building enclosure institute explains why the laboratory has to be someone else’s: for results to be accepted, since the codes demand it, testing must be done by a laboratory holding accreditation, which is the formal assurance that the testing body is independent and unbiased and has the equipment and competent staff to run the tests. The same paper describes three phases of testing, in the laboratory, during installation and after occupancy, each adding assurance of long term performance.⁸ The mockup entry goes through the procedures.

What independent review buys, then, is a known price for an unknown error. The structural reviewer’s fee is small against the project and fixed in advance; the mockup specimen is one bay of a wall built twice. Against them sits the cost of being wrong once: a redesign after fabrication, a wall that leaks for decades, or a crew welding plates over bolted joints with a hurricane in the Atlantic. Where precedent is thin the second column is longer than anywhere else, and that is the entire case for paying the first.