The need for existing structural drawings as a part of the evaluation of an existing structure is obvious — it should be the first question asked of clients or owners at the onset of an investigation.
However, the ease at which an existing structure can be analyzed in the absence of structural drawings varies depending on the nature of the system and the extent to which the structure is concealed or exposed, and accessible.
For instance, the analysis of an existing exposed structural steel building is a much easier task than that of a reinforced concrete structure simply because the steel members can be measured and their capacity quickly determined in the context of any historical material strength information that is readily available. With a reinforced concrete structure, although it is fairly easy to determine material strengths based on historical information and nondestructive testing, establishing what the internal reinforcement is in order to facilitate the calculation of the member capacity can be a very difficult and challenging task.
Some of the options that can be used to determine the capacity of reinforced concrete structures (and other similar internally reinforced systems, such as one- and two-way clay tile and unit masonry joist) where no drawings are available include:
- Determine the building usage at the time of the initial construction, research the building code from the same time period, and establish the minimum live load required for the original intended use. Examples of older building code minimum loading requirements are tabulated on the table in the next column.
- Create small exploratory, demolished openings to expose the internal reinforcement at areas of the framing not susceptible to removal of small amounts of material. This approach, used in conjunction with a Profometer (Pachometer), can sometimes enable the determination of existing reinforcing in large areas surrounding the exploratory opening.
Exploratory demolition can also be utilized in many different situations to reveal hidden or concealed structural systems. For example, the photos to the right and in the next page (top) illustrate how a portion of a 2nd floor plaster partition wall was removed to reveal an existing story high structural steel transfer truss in order to measure the components of the framing as required for the analysis of the load carrying capacity of the truss over an existing assembly room. The photos were taken at the Scarborough Hall building, which was constructed in 1912, at Doane Academy in Burlington, N.J.
- X-ray the members in question to locate the size and spacing of the internal reinforcement. X-ray use can be very economical, with the daily cost (even during weekend hours) ranging from $2,500 to $3,000. The use of X-rays to locate and determine internal reinforcement also requires a clear understanding of the projected shadow effect associated with the process, as opposed to the reflective nature of photography. An example of this process is illustrated in the following two X-ray images and related diagram. The X-rays were taken at the Northampton County Courthouse parking garage, which was constructed in 1975, in Easton, Penn.
X-rays of hollow flat clay-tile arches (in which the tie rods are not visible) are an invaluable tool for determining the load carrying capacity of this type of system since once the size and spacing of the rods are known, the strength of the framing can be more accurately established. Because hollow flat clay-tile arches are very susceptible to damage as a result of exploratory demolition, it is preferable to locate the tie rod spacing using non-destructive tools. However, since the tie rods are typically located more than three inches above the soffit or bottom of the tiles, it is difficult to locate the steel rods using a Profometer. In addition, due to the significant amount of internal cavities associated with hollow clay tiles, it is also difficult to locate the rods using ground penetrating radar (GPR). X-rays are hence the best method of locating the tie rods. X-rays can determine both the spacing and diameter of the rods without damaging any of the voussoir action of the tiles.
|X-ray 1||X-ray 2|
|Interpretation of x-ray charts.|
Another approach often suggested by some structural engineers to determine the load carrying capacity of a reinforced concrete, or other similarly internally reinforced structure in which no drawings exist, is to load test the framing. This approach can be problematic, primarily because the absence of any information on the actual capacity, strength or stiffness of any system makes it impossible to establish what the safety factor is or the appropriateness of the serviceability (i.e., deflection) results of any given load test. Therefore, the use of load testing is not recommended, unless you already know the structure's calculated design capacity.
Structural engineers often also seek to track down historical load tables for precast or other similar concrete, clay tile or unit masonry proprietary systems in hopes of determining the capacity of an existing framing system. In this situation, even if copies of the load tables for the desired system exist, unless there are specific mark numbers on the product in the field that clearly identify the member in relationship to the load table, it is not advisable to establish load capacities of an existing structure using historical load table data.
In the case of open web steel bar joists, it is more practical to rely on load tables because of the wealth of historical information available through the Steel Joist Institute (SJI) and the ability of an engineer to field measure the actual joist components and, in turn, analyze the member as a simple truss to confirm that load table capacity. However, it is still difficult to determine the type of joist that one is dealing with in any given building in the absence of drawings, so the author of this article developed the flow chart provided below as a guide when dealing with existing open web steel joists.
Wood buildings are similar to structural steel buildings in that the members, if exposed and readily accessible, can easily be measured in the field to facilitate the analysis of the individual members to determine load carrying capacities. Unfortunately, it can be difficult to establish the appropriate allowable stress of existing wood that should be assumed for the structural analysis. To solve this dilemma, structural engineers can take small pieces (no larger than a toothpick) of the timber framing and send the samples to a lab to determine the species of the wood. Once the species is determined, along with the age of the building, it is possible to easily determine reasonable allowable stresses from historical resources.
In older structures it is common to encounter a situation in which the superstructure is constructed with steel but the members have been encased in concrete for fire protection. An example of this type of condition is illustrated in the photo below. The photo was taken at the National State Bank Building in Newark, N.J., constructed in 1912. The photo reveals an exposed steel column only, which was protected by hollow clay tile blocks that have been removed, while the steel beam framing above is still encased in concrete.
The existing concrete encasement made it impossible to measure the steel framing. However, the need to perform exploratory demolition at this structure was avoided because existing drawings were found during the due diligence effort associated with locating documents for the building. Historical records of the building revealed that the architect was Cass Gilbert, who also designed the famous Woolworth Building in New York City. As a result, it was also possible to determine that the New York Historical Society had archived drawings for a number of different buildings designed by Gilbert, including the National State Bank Building.
The existing drawings for this building consisted of linen sheets that had been folded for storage in several boxes. Because the Historical Society would not allow the drawings to be removed and scanned to create electronic copies, it became necessary to photograph each sheet after it was spread out on a table to be able to document the information for future use. This was accomplished by first photographing the entire drawing and then taking additional close up photos of portions of each sheet, as illustrated above.
This project serves as a good example of how engineers should take the time to track down any lead they may have when attempting to locate existing drawings. A thorough search may involve rummaging through any documents that may have been left in the building itself and a search of the local building department archives. In addition, if the building is of historical significance, a search of the Historic American Building Survey and Engineering Record online archives at the Library of Congress (http://memory.loc.gov/ammem/collections/habs_haer/index.html) is also recommended.
If you do have access to the existing drawings, it is also recommended that a thorough site visit be conducted in order to confirm that the as-built conditions agree with the drawings. This approach is recommended because, more often than not, it is common to encounter as-built conditions that are different from those shown on any existing structural drawings.
Structural engineers involved with renovation and rehabilitation projects need to be aware of and educate themselves concerning the specifics of the existing framing system so that the most unobtrusive and non-destructive solutions can be developed as a part of an adaptive reuse project. This is particularly true if an antiquated structural system is encountered. This approach enables the project to be more economically viable because of the resulting limitation of the extent of structural costs associated with a typical renovation project. In other words, without any knowledge of an existing structural system it is still possible to develop a structural solution. However, this approach will always be much more intrusive, and therefore more costly, than if the engineer has a sound understanding of the system involved.
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB is the structural division manager at the corporate headquarters of Pennoni Associates Inc. in Philadelphia, Penn. Stuart has 35 years of experience as a practicing structural engineer and is actively licensed in 21 states. He can be reached at MStuart@Pennoni.com.