Multi-Tiered Evaluation of Existing Structures

If you are familiar with FEMA publications related to seismic evaluation of existing buildings, you may have heard of the terms "Rapid Visual Assessment," "Tier 1 Screening Procedure," "Tier 2 Evaluation Procedure," and "Tier 3 Detailed Evaluation Procedure," and even "Seismic Rehabilitation."

Looking at these different types of "assessment" procedures together, I would consider them to be a 6-step overall process of seismic evaluation and rehabilitation design for existing buildings:

1. Step 1 (or I would call Level 0) is a Rapid Visual Assessment procedure, which could take no more than an hour to do. The main output is a "PASS," "FAIL," or sometimes even a "CONDITIONAL PASS" mark for the structure, though not necessarily using those terms. (It is more like "SAFE FOR OCCUPANCY," "DO NOT ENTER," or "FURTHER EVALUATION REQUIRED.")
2. Step 2 (Level 1) is a Tier 1, "Screening" type of evaluation where, using the available information and performing "Quick Check" calculations, checklists are completed to identify potential deficiencies. For some cases particularly those involving newer buildings, this could be simply a code-compliance review. After a Tier 1 evaluation, the owner may choose to perform a more detailed evaluation (e.g. Step 3 or Step 4), or address the identified deficiencies (e.g. Step 5 and/or 6).
3. Step 3 (Level 2) is the Tier 2 "Evaluation" which could either follow Option A (or Step 3A / Level 2A), Deficiency-Only Tier 2 Evaluation which only concerns evaluation of the Tier-1-identified deficiencies, or Option B (or Step 3B / Level 3B), which is a Full Tier 2 Evaluation. A deficiency-only or full Tier 2 Evaluation usually involves 2D or (preferably) 3D linear structural analysis and evaluation. Particularly for a deficiency-only Tier 2 evaluation, a Level 1 or Tier 1 evaluation is a pre-requisite. It is optional but preferred to go through a Tier 1 evaluation prior to performing a full Tier 2 evaluation, more so because it is established more quickly in a Tier 1 evaluation if there is indeed a need for a Tier 2 evaluation, and if so, whether a deficiency-only or full evaluation is more appropriate. For many older buildings, the likelihood of failing a Tier 1 evaluation is high, hence it is justified to proceed directly to a Tier 2 evaluation. If a Tier 1 evaluation is not conducted, a full Tier 2 evaluation must be performed. The output of a Tier 2 evaluation is a list, graphical or linear, of specific elements that are identified to be potentially deficient. The client/owner may choose to perform a more detailed evaluation (e.g. Step 4), or address the identified deficiencies from the Tier 2 evaluation (e.g. Step 5 and/or 6).
4. Step 4 (Level 3) is the Tier 3 "Detailed Evaluation," where there are stricter information gathering requirements which usually require material testing, among others. A linear or nonlinear structural analysis is performed, and actual/in-situ material properties are used. It is optional but preferred to go through a Tier 1 and/or Tier 2 evaluation prior to performing a Tier 3 evaluation, because a Tier 1 and/or Tier 2 could establish more quickly if there are indeed any deficiencies before having to exert the extra effort and expend the extra resources required in a Tier 3 evaluation. But if there is available funding and the extra time and effort is afforded, and there is strong evidence that necessitates a Tier 3 evaluation, the owner may choose to go directly to a Tier 3 evaluation. Furthermore, Tier 3 evaluations are usually ready for use in detailed rehabilitation design (Step 6) which could further justify going straight to a Tier 3 evaluation without having to go through a Tier 1 or Tier 2 evaluation.
5. Step 5 (Level 4) is the Retrofit Scheme Evaluation, which aims to find a suitable, executable, and architecturally acceptable retrofit scheme to address the identified deficiencies. Where availability of materials and expertise permits, more than one retrofit scheme is identified to provide options for the owner. Once a scheme is selected, the owner may choose to go to Step 6. At least one evaluation (Tier 1, 2, or 3) should have been carried out prior to performing Step 5.
6. Step 6 (Level 5) is the Detailed Rehabilitation Design, which aims to provide complete engineering services to provide all necessary details for implementation of the retrofit or rehabilitation. A Tier 3 evaluation is preferred to have been carried out prior to performing the detailed design, over a Tier 2 evaluation, because it is expected that the Tier 3 evaluation could result in retrofit cost savings compared to when results from a Tier 2 evaluation is used. Likewise, such is the case for a Tier 2 compared to a Tier 1 evaluation. Also, obviously a Tier 3 provides a better, more realistic prediction of the behaviour of the system or component being retrofitted, than a Tier 2. And again likewise, a Tier 2 better than a Tier 1.

If you will notice from the above descriptions of the 6 step process, there are no instances of words such as "seismic," "structural," "building," or "existing." Indeed, a similar multi-tiered process of evaluation can be done also for evaluating against wind, flood, and other hazards. Indeed, a similar multi-tiered process of evaluation can be done also for evaluating non-structural components. Indeed, the same process can be done in evaluating not just existing buildings, but even new designs for yet-to-be or newly constructed buildings, or even existing, or new non-building structures.

For the above 6 step process, detailed technical reports have already been published by FEMA and now turned into standards for the (structural and non-structural) seismic evaluation of existing buildings, which are good examples of seismic evaluation tools that are already widely used around the world. We already have experience using the said FEMA procedures for existing buildings for a newly designed building, and they worked well.

The question now is say we need to perform a wind-safety evaluation of an existing lattice tower, what standard do we base our evaluation on? There is now a proposed Level 0 wind evaluation for existing lattice structures used for outdoor advertisements (i.e. "billboard structures" such as those in Metro Manila), but it is still due for further development.

This all is the current challenge at hand for engineers.