Designing for the Unforeseeable

Design and Construction

By Santiago Pujol, David Nickell, Matt McElmury, Roger Reckers, and Adam Urbanek, M.SAME

In the challenge of designing military buildings to resist progressive collapse, a revised technical approach offers 
a simplified way to arrive at a dependable estimate of material needed, potentially increasing cost controls on government projects.
Using a simplified design approach has been able to provide an earlier and dependable estimate of the structural steel quantity required for a building to resist progressive collapse. Photo by Michael Maddox, USACE Louisville District.

Progressive collapse is a phenomenon where an undetermined, localized extreme event that causes severe local damage cannot be resisted by the rest of the surrounding structure. Critical buildings that are three or more stories in height are required by Unified Facilities Criteria (UFC) 4-023-03 to be designed to resist progressive collapse.

In the UFC approach, as in conventional force-based design, capacity is required to exceed demand.

On these types of military projects, the engineer is tasked with designing a building to survive an unspecified threat, without knowing anything about the event or where it may occur. Additionally, the owner will also want the building to be practical for regular use and within budget restraints. This challenging exercise amounts to asking the engineer to design for the unknown, an expectation that requires nearly unlimited design efforts from the engineering team.

Conversely, a successfully implemented simplified design approach can provide a quick and reliable initial assessment of the UFC-required structural quantities and associated costs. This approach can help control the risks of exceeding government program amounts, design fees, and subsequent cost overruns. This approach uses basic engineering principles without the use of structural analysis software and provides quick and reliable results early on. And this approach produces results comparable to detailed analyses that require more time and can help the initial proportioning of structural quantities that can be confirmed later in the full design process.

Basis For Design

Design and construction in cold regions must consider extreme, low-temperature conditions with freezing and thawing periods. Both infrastructure and facilities such as buildings, roads, utilities, embankments, and other assets in these locations encounter a variety Looking back to engineering principles introduced by John Biggs in 1964 helps understand the basis of the developed design approach. Biggs introduced the main idea that key aspects of the dynamic response of a continuous system can be understood using a single-degree-of-freedom system chosen so that the kinetic and potential energies in both systems are equal. He arrived at this equivalent system by assigning to it a fraction of the mass of the continuous system, strength equal to the total strength of the continuous system for static conditions, and an applied force equal to the total force acting on the continuous system.

The tuned equivalent system produces displacements equal to the maximum displacement in the continuous system.

The response of a floor system to the abrupt removal of a support can be understood in terms of the static strength of the system missing the removed support and the total weight acting on it. The external work done must be equal to the area under an idealized resistance-displacement curve representing the static response of the system.

In this curve, a capacity-to-demand ratio of 1.5 results in a small ductility demand of 1.5. Comparatively, a defensible capacity-to-demand ratio of 1.2 yields a modest ductility demand of 3. The detailing required to achieve the ductility values of 1.5 to 3 is unlikely to be cost prohibitive. This simplified method produces results acceptable with those obtained from detailed and time-consuming nonlinear dynamic analyses.

Current Approach

UFC 4-023-03, Design of Buildings to Resist Progressive Collapse, offers three approaches for design: alternative load paths, enhanced local resistance, and the tie method. Enhanced local resistance and the tie method mostly relate to critical detailing; as a result, the focus becomes alternative load paths and the use of equivalent static analysis to determine initial structural quantities.

In the UFC approach, as in conventional force-based design, capacity is required to exceed demand. The capacity refers to estimates for static conditions modified with factors: m factor, which is related to structure ductility; strength-reduction factor, which is related to uncertainties in material properties; and overstrength factor, which is related to actual material properties larger than nominal properties. Demand is modified in two ways: load factors, where loads may be larger than expected; and load increase factor, which, on average, and according to results from numerical simulations, approaches LIF=1+m.

Lumping all capacity and demand factors into a single ratio of capacity to demand and graphing it, the curve of the current approach in the UFC is remarkably similar to the curve of the simplified method. A well-written code for numerical simulation ought to produce results consistent with basic physics. Both curves suggest again that the problem of the dynamics of a ductile floor responding to abrupt support removal can be reduced to choosing a safety factor and good detailing.

Simplified Approach

This simplified design approach and consideration of detailing can be used in designing a structure that is subject to the UFC progressive collapse requirements and can quickly identify where structures lack redundancy.

The simplified approach requires following a series of a few steps.

  • Design for gravity and lateral loads per the code.
  • Perform analyses to estimate the static capacity of floor systems after support removal (yield-line analysis is convenient for this).
  • Where CDR is less than 1.2, modify the design to reach the desired ductility.
  • Increase member sizes and/or revise detailing.
  • When CDR is greater than 2, perform structural analyses (as a check).

Examining the results of an erected structure can explain the benefits of a simplified approach. In this case study, a three-story, 50,200-ft² instructional building was designed “against” progressive collapse using the simplified design approach and the design was confirmed with detailed calculations.

Based on traditional early design without progressive collapse, the preliminary total steel weight for the building would have been 278-T, or 11.1-lb/ft². Using the design approach, the total steel weight for the building was closer to 433-T, or 17.3-lb/ft². This difference would represent an increase of 56 percent in the detailed design over the initial preliminary design had progressive collapse not been considered from the onset.
Putting this comparison evaluation into dollar values and using a project cost estimate of $11,600/T of moment-connected structural steel, this translated into an additional $1.8 million, or $36/ft² of required steel framing cost.

During the final design for the building and after incorporating all UFC requirements, the total steel weight for the building was 429-T, or 17.1-lbs/ft². This steel take-off was remarkably similar to the quantities obtained from the design approach.

This invaluable procedure provided the design team with reliable structural steel quantities early in the design without requiring over-conservative design assumptions, or worse yet, introducing these costs at a later stage of the design and exceeding the program amount, putting the entire project at risk of overruns.

Dependable Estimates

Based on the requirements of UFC 4-023-03, all buildings three stories or more require a time-consuming design to resist the onset of progressive collapse, which can lead to total building collapse.

The simplified design approach allows the designer to reliably estimate the structural costs of a project early on without the risks of waiting for a full design to be completed. This reliable and proven method can help deliver more dependable estimates of structural material quantities, presenting the government and the contractor with better information earlier in the design process.

A Detailed Effort

The apparent goal of current UFCs that address progressive collapse is not to ensure complete building survival from the unknown event, but rather to provide a level of structural redundancy deemed sufficient.

The specific analyses and procedures required by the UFCs to meet this goal involve extensive computational time and effort, which is often not completed until far into the design process and often after the DD1391 program amounts have been determined by government planners.

The traditional progressive collapse design approach has been to analyze a detailed 3D model with preliminary structural element sizes and then to re-analyze the results by adding additional reinforcement. This approach typically requires a revision to the initial cost estimate in order to then add the additional progressive collapse-driven structural costs onto the preliminary estimates.

Santiago Pujol is Professor of Civil Engineering at University of Canterbury; santiago.pujol@canterbury.ac.nz.

David Nickell, Matt McElmury, and Roger Reckers are Structural Engineers, TGRWA. They can be reached at dnickell@tgrwa.com; mmcelmury@tgrwa.com; and rreckers@tgrwa.com.

Adam Urbanek, M.SAME, is Structural Engineer, exp Federal; adam.urbanek@expfederal.com.

Article published in The Military Engineer, November-December 2024

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