HEAVY INDUSTRIAL FACILITY MODIFICATION

by Patrick R. McCormick

Brander Construction Technology, Inc.

Presented in New York at the

September 1992 ASCE International Convention & Exposition

In 1991, Brander Construction Technology, Inc. was asked by a Minnesota paper company to design modifications to a roof structural system over a paper mill building at their facility. The challenge was that the modifications would have to be done without disturbing the normal papermaking operations below. The modifications were proposed to address deterioration of the roof framing system and to upgrade the air handling system inside the building which houses two paper machines.

Brander was first asked to investigate the condition of the structural framing system and to help the owner determine what could be done to address the deterioration and to increase the volume of the building, which was required for the upgrade of the air handling system.

Description

The original building was built in 1907. At that time the roof was a continuous, 249′ long, single elevation system with steel fink-type trusses spaced 16′ on center. The trusses spanned 80′ across the width of the building and were supported by brick masonry pilasters.

The top chords of the trusses were fabricated with a slope of 6 in 12 and consisted of two 12″ channels back to back; the bottom chords consisted of two 10″ channels back to back. The web sections were either single or double angle members. The trusses supported redwood beams, a redwood roof deck, and a roofing system.

Sometime after original construction, a 112′ long section of the roof was raised approximately 10′ to accommodate a new dryer and hood system for the machines below. The hood system, which was also made entirely of redwood, was installed in and supported by the raised roof trusses. Numerous air handling units were also installed on the raised portion of the roof.

Planning

In our initial planning for the project, there were several things that had to be taken into consideration, such as:

1. The Owner’s objectives for the project. Besides repairing the deteriorated structure, what was the Owner hoping to accomplish with the rehabilitation?

2. Requirements of the field investigation. What information did we need and what was the most efficient way to acquire that information?

3. Interior environmental conditions. What conditions had to be accounted for in the roof system design?

4. Rehabilitation alternatives. Instead of presenting only one design, we wanted to give the Owner a choice between two or more feasible options so the Owner could determine what would best fit the budget and schedules of the mill.

5. Construction scheduling. We had to recognize that mill operations below were not going to cease, so we had to be able to adapt our schedule to the mill schedule.

6. Special safety considerations. There were some unique settings that required special consideration to minimize the potential hazard to mill personnel in the area.

Owner’s Objectives

In discussion with the Ownerâs representatives, it was clear that they viewed safety as a major concern. In the recent past some wood beams had failed and had been temporarily repaired, but there was still concern about the possibility of failure of additional beams under the traffic of maintenance personnel on the steep roof. The Owner wanted not only to address the deterioration of structural members, but also to reduce the slope of the roof to allow easier access for maintenance of the roof-mounted equipment. It seemed appropriate to consider modifications that would not only provide the additional building volume needed for the new air handling system, but also would address the Ownerâs concern about the slope and maintenance access.

Investigation

Before any repairs or modifications were recommended, an intensive field investigation was conducted to determine the condition of the existing structural system.

The investigation began with a review of the existing drawings. Despite the age of the building, we were fortunate to have a good set of original drawings of the trusses and general building layout. With those drawings we were able to determine what modifications had been made since original construction of the building.

Next the temperature and humidity conditions in and around the existing structural system were determined. The environments in paper machine buildings are typically hot and humid and this was no exception. It was found that the temperature inside the building near the roof structure was near 160 degrees F, and relative humidity was approximately 70%. In such an environment, airborne moisture will condense on a personâs 98.6 degree skin. It was clear that temperature and moisture conditions would be major factors in the design of the new roof and wall system and, because of the access required, would have to be taken into account in the scheduling of inspection and rehabilitation work.

During a scheduled machine maintenance shutdown, detailed observations of the structural system and the roof system were made to find out the condition of the individual structural members. It was found that numerous wood beams had failed or were near failure, primarily around penetrations. With all the deterioration noted in the wood members, it was determined that they would all have to be removed or replaced, depending on the type of rehabilitation chosen.

Only one truss end was found to have any significant deterioration, and that was caused by a roof leak directly over the truss. At that truss end, ultrasonic thickness tests were done to determine how much of the cross sections of truss components had been lost to corrosion. The data was later used in the analyses of the trusses and the subsequent design of repairs. The trusses in general were in excellent condition, which suggested to us that it would be feasible to use the existing trusses in the rehabilitation.

The investigation also included detailed inspections of the supporting masonry walls and pilasters. Those were found to be in generally good condition, except for two locations at which weathering of the exterior wythe of brick had reduced the effective thickness of the pilasters. The weathering was caused by years of exposure to rain water and melted snow flowing down the outside faces of the pilasters. Because of the deterioration observed, it was determined that either the pilasters would have to be repaired or some of the truss loads would have to be removed from the pilasters.

The investigation revealed that some changes had been made to the original structural framing. Numerous cross bracing members had been removed to allow for the installation of mechanical or process equipment and numerous hoist beams had been installed on the bottom chords of the trusses for use in maintenance of the machines below.

The condition of the structural framing of all the adjacent buildings was also investigated and documented, since the rehabilitation of the new roof could cause snowdrift loads to accumulate on those structures. One of the more interesting items noted during the investigation was the condition of the ends of the redwood beams in the adjacent lower sections of roof. Over the years, both condensation and roof leaks at the wall-roof junction had saturated the ends of the beams and the beams were severely deteriorated.

Rehabilitation Alternatives

Based on the field investigation, two rehabilitation alternatives were prepared and submitted to the Owner for consideration.

The first alternative was to install columns on the existing masonry pilasters to support new trusses that would span the full width of the building. With this alternative, the existing trusses could either be removed, creating an open area that would provide more usable space, or could be left in place to support auxiliary loads, such as the existing hoods and hoist beams.

The second alternative included a column, beam, and steel joist system to be installed over and to bear on the peaks of the existing trusses. This alternative was suggested to take advantage of the excess capacity of the existing trusses that would result once the existing roofing system and air handling equipment were removed.

While both systems satisfied the Owner’s objectives for creating a low slope roof and increasing the building volume, the second alternative was chosen by the Owner. Using the existing trusses as part of the new roof structure proved to be the more economical alternative.

New Roof System

Since the new roof system was to be designed to incorporate the existing roof trusses, analyses were performed of those trusses, using all existing loads and the loads that would result from the new roof system that was designed to bear on the peaks of the trusses.

The analyses indicated that the maximum stresses in the existing interior trusses that would result with the new roof system were slightly higher than the stresses from the original roof loads, but those stresses were still within the allowable range, primarily because much of the dead and live loads of the new system were transferred directly to the masonry pilasters through the new columns, rather than being transferred to the pilasters through the existing trusses. This allowed for the installation of the new framing system without reinforcement of the interior trusses.

The analyses of the two end trusses had to take into account not only the new roof system loads, but also the design loads of snowdrifts that would develop along the ends of the building because of the elevation differences between the new roof and adjacent lower roofs. The end trusses carry the majority of the snowdrift load on the lower roof.

The analyses revealed that, with the new roof and snowdrift loads, the top and bottom chords of those end trusses would be overstressed and reinforcement would be required. The reinforcement consisted of installing T-sections between the existing channels.

Analyses of the beams of the lower roofs adjacent to the raised roof area revealed that they, too, were inadequate to support the new design snowdrift loads and would require reinforcing. The reinforcing method chosen was to install channels alongside the existing wood beams.

Another challenge that we faced with the new roof and wall system design was the extreme environmental conditions that existed inside the building at the roof level. The high temperatures and high humidity resulted in dew point temperatures of approximately 132 degrees F. An extremely tight and properly insulated interior envelope, which had to accommodate numerous penetrations, would have to be designed to minimize the potential for condensation in the building.

Construction Scheduling Challenges

Construction scheduling for rehabilitation projects in manufacturing facilities poses a unique problem in that most work must be done while mill operations continue business as usual. Rarely are machines shut down to accommodate construction- only if absolutely necessary. In the case of our roof rehabilitation project, not only were mill operations to continue, but all rehabilitation work was being done directly above mill operations.

Since there were three two-day machine maintenance shutdowns that were already on the schedule, the interior rehabilitation work was scheduled during those shutdowns. End truss reinforcing required that construction personnel work inside the building in the areas in which, during normal operations, the environmental conditions were too extreme for the work required. Such work was scheduled to be done during shutdowns, which allowed for some cooling of the roof areas and also allowed workers easier access to the trusses. Existing exterior walls adjacent to the end trusses were temporarily removed, then replaced when the work was completed.

Work done outside the shutdown periods, which was above the existing roof and away from the high interior temperatures, proceeded smoothly and on schedule.

Scheduling around the mill shutdowns required that the contractor plan appropriately to ensure that the proper materials were on site and available for the shutdown periods. The contractor scheduled workers around the clock to fully utilize the time during which access to the building was available. At no time throughout the project were the shutdowns scheduled by the mill extended or addition shutdowns required for the construction work, primarily because of good planning by the contractor.

As soon as we developed our preliminary drawings and details, we contacted and met with a representative of the local Building Inspection Department to review the project. There was a limited amount of time available to obtain the required building permit, a process that often involves a lengthy review by the building officials, but because there had already been numerous conversations and meetings between the building officials and Brander personnel prior to the review, the officials were familiar with the project and the review time was reduced considerably.

There was also very good and open communication between the general contractor and our office. According to Minnesota state code, which generally follows UBC, the Engineer of Record or his/her representative must, in most cases, inspect all welding of structural steel and bolt installations, which meant that personnel from our office were on site one or two days a week during certain phases of the rehabilitation work. That visibility made contractor personnel at ease to raise questions and discuss the construction as it progressed, eliminating much of the guesswork that might otherwise have taken place in the field.

The communication between the building officials, contractor, Owner, and our office was very brisk throughout the project and we found it to be invaluable to the successful completion of the project.

Another factor that proved to be invaluable was the decision to erect a tower crane at the closest available location. The crane was used by all trades (building, mechanical, and roofing) involved in the project. Moving the materials overhead, rather than over adjacent roofs, minimized damage to those roofs and reduced the costs for repairs once the project was completed.

Special Safety Considerations

Because mill operations proceeded normally below the roof structure during the majority of the project, safety of the mill personnel and construction workers, as well as protection of the operating machines was critical. The existing roof was temporarily reinforced in the deteriorated areas and was then used as both a working platform and a debris net during a majority of the construction sequence. This minimized the amount of additional safety netting and the number of debris shields needed inside the building. The existing roof was left in place until all but one section of wall panel were installed, after which the roof was demolished.

The threat of fire inside the building was very high, considering all the wood used in the construction of the hoods over the paper machines and in the ends walls of the building. For that reason, two fire watches were posted continuously at each welding or cutting location inside the building during those operations. The fire watches were equipped with either extinguishers or hoses and with walkie-talkies.

Because of the steps taken by both the Owner and the contractor to see that all personnel and machines below were adequately protected, no major construction related injuries or damage occurred.

Final Notes

The roof reconstruction project at the paper mill offered some unique challenges, not only in terms of structural design, which involved integrating the new structural system with the existing structural system, but also in terms of field data acquisition and construction scheduling. Some of the main things that we learned, or maybe relearned, from this project were:

1. Accurate and comprehensive field documentation of the existing structure or structures affected by the proposed work, before the final design and detailing were begun, minimized the number of field changes required during construction.

2. Maintaining very open communication lines between the contractor, Owner, and engineer all but eliminated any guesswork on the part of all involved, right from the bidding phase to completion of the project.

3. Because we were directly involved with the project from design through construction, potential misinterpretations and conflicts were avoided.

Since the roof rehabilitation project has been completed, output from the machines in the building has increased and production records have been broken. Though we cannot claim that the increased output of the machines is a direct result of the structural rehabilitation of the roof system, it was a nice ending to a successful project.