Strategies and solutions for schools and other facilities
For years, school districts have benefited from the efficiency, design freedom, and economy of integrating structural, glued laminated (glulam) wood beams and trusses into the design of their structures.
In addition to offering a natural wood appearance that holds a timeless appeal, glulam beams are specified for their strength and durability. Frequently situated behind finishes or too high to actually see or access, glulam beams commonly fall into the “out of sight, out of mind” scenario and are forgotten in terms of regular inspection and maintenance needs. However, with regular inspections and a preventive maintenance program, glulam beams offer the strength for which they were intended—keeping occupants safe and children in the classroom.
Capitalizing on an earned reputation for striking applications such as vaulted ceilings and other designs with soaring open spaces, the use of glulam beams continues to grow, and they commonly are found as a structural member over gymnasiums, cafeterias, auditoriums, classrooms, and even swimming pools.
Glulam beams offer the design team a means to integrate form and function by adding strength to the structure. Because of their location, however, the beams are often exposed to harsh environments such as extreme heat, cold, or moisture.
Exposure to aggressive chemicals, termite damage, damage from equipment or machinery, poor ventilation and trapped moisture, defects in the original construction, material defects caused by low-grade lumber, and overloading caused by additional roof-mounted mechanical equipment or suspended ceilings are all causes of deterioration and decay of a glulam beam.
It pays to plan
Proper care for glulam beams begins with a planned maintenance program.
In order to evaluate the extent of deterioration and corrosion, periodic inspections of the structure are recommended.
These inspections and resulting preventive maintenance programs have proven not only to decrease the instances of failure, but also to reduce repair and restoration costs drastically. After hiring a specialty contractor familiar with American Institute of Timber Construction (AITC) standards, the first step in the maintenance plan should include an in-depth inspection of the facility, including a written report indicating the condition of the structure and its integrity. The next step is to perform a preliminary analysis to evaluate the effects of deterioration or overstress on the strength and safety of the structure. Remedial work to restore the original strength of the affected members or needed structural strengthening must also be addressed. To avoid damage to other members or parts of the structure, a detailed inspection and maintenance plan must be developed and implemented.
For example, in a recent school project, deterioration was identified when a routine check of mechanical equipment uncovered a severe overloading of the beams. The mechanical equipment that was hanging from the beams was not original construction, rather, it was added during a subsequent renovation.
This scenario resulted in large deflections and cracking, and could have been dangerous if it went undiscovered during the inspection.
Although the two terms sound similar, structural strengthening and structural repair refer to slightly different concepts. Structural repair describes the process of reconstruction and renewal of an existing school or its structural elements. This involves determining the origin of the distress and removing damaged materials and the cause of distress, as well as selecting and applying appropriate repair materials that extend the structure’s useful life.Observation of deterioration and identification of the cause is critical before a repair strategy can be implemented.
Structural strengthening, on the other hand, describes the process of upgrading the structural system of an existing school building to improve performance under existing loads or to increase the strength of the existing structural components to carry additional loads.
For upgrade projects, design engineers must deal with structures in which every element carries a share of the existing load. The effects of strengthening or removing part or all of a structural element—such as penetrations or deteriorated materials—must be carefully analyzed to determine its influence on the global behavior of the structure. Failure to do so may overstress the structural element surrounding the affected area, which can lead to a bigger problem and even localized failure.With upgrade projects, contractors also must deal with critical issues related to access to the work area, constructability of the repair, noise and dust control, and type of construction materials that may not be as critical for new construction projects.
The optimal repair
If damage and its root cause are identified, a typical repair or reinforcement process for glulam beams involves determining the residual strength of the members and then developing an appropriate repair to restore or stabilize the member. The next step is selecting the optimal strengthening strategy to increase the member strength. Options include steel plates, external posttensioning, sectional wood replacement, and composite systems, such as fiber reinforced polymer (FRP) and steel reinforced polymer (SRP).
FRP systems are paper-thin fabric sheets bonded to structural members with an epoxy adhesive that increase the members’ load-carrying capacity significantly.
Usually carbon-based, these systems have been used extensively in the aerospace, automotive, and sportequipment industries, and are now becoming a mainstream technology for the structural upgrade of buildings.
Important characteristics of FRPs for structural repair and strengthening Circle #179 or use InfoDirect at www.gostructural.com Attend the seminar in your area on steel joist construction, and earn .75 CEUs or 7.5 PDHs. Get details at www.steeljoist.org/ceu CEUs ASAP.
PDQ.steeljoist.org/ceu — applications include non-corrosive properties, speed and ease of installation, lower cost, and aesthetic appeal. As with any other externally bonded system, the bond between the FRP system and the existing material is critical, and surface preparation is very important. Typically, installation is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP fabric into the epoxy, and then applying a second layer of the epoxy adhesive. After curing, the FRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate. This is because the carbon FRP has tensile strength approximately 10 times that of steel.
The next generation of composite strengthening systems is SRPs. One example is Hardwire—a family of reinforcements made from ultra-high strength, twisted steel wire cords proven to be 11 times stronger than a typical steel plate. Hardwire reinforces various materials and provides high strength (up to 8 kips/inch) and high modulus (up to 30-million pounds per square inch) in a very thin, ductile envelope.
The product can be molded into or on virtually any structure or part.
Also, strengthening members by using bonded steel plates is a popular option. Steel elements are glued to the structural surface by a two-component epoxy adhesive to create a composite system. The steel elements can be steel plates, channels, angles, or built-up members. Steel elements bonded to the sides or bottom of a structural member can improve its shear or flexural strength. In addition to epoxy adhesive, mechanical anchors typically are used to ensure the steel element will share external loads in case of adhesive failure.
The exposed steel elements must be protected with a suitable system immediately following installation.
Regardless of the specified corrosion protection system, its long-term durability properties and maintenance requirements must be fully considered.
External post-tensioning is yet another technique that has been used effectively to repair glulam beams.
Active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads.
Because of the minimal additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive cracking and deflections. The posttensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structural element. Prior to external post-tensioning, all existing cracks are epoxy-injected and other needed repairs are performed to ensure post-tensioning forces are distributed uniformly across the member.
Although FRP, steel plates, and post-tensioning are all proven methods for the repair of glulam beams, the best strengthening solution balances aesthetics of the repaired member, fire resistance considerations, and constructability. A variety of factors, including technical (engineering), constructability (construction methods), aesthetics (architectural), and economics (return on investment), each play a role. Many opportunities exist for engineers, contractors, and material suppliers to work together to supply their perspectives to an upgrade project.
This explains the trend of design/build-type teams for delivering cost-effective solutions to school districts. See “Success stories” on page 33 for examples.
Contrary to industry perception, strengthening assessment and design is far more complex than new construction, and should not be treated lightly. Challenges usually arise because of unknown actual structural states such as load path and material properties, as well as the size and location of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements share the loads must be evaluated and properly addressed in the upgrade design, detailing, and implementation methods.
In addition, school facility professionals should consider the procurement process for specialty repair and strengthening projects to be different from new construction services.
Engaging specialty engineering and contracting firms that are familiar on a day-to-day basis with all of the critical aspects highlighted here will ensure the most cost-effective and long-lasting results. Although it may appear there is an up-front financial benefit to obtaining these specialty services from firms with experience in new construction, the real risk is that the repairs will cause an endless “repair of repairs” cycle, resulting in additional disruption and expenditure to owners. When it comes to structural repair and strengthening, the mantra “do it right the first time” pays dividends.
Last, but certainly not least, safety is paramount when it comes to glulam beam repair. While all property owners have the responsibility of ensuring the safety of tenants or residents, school districts are challenged with implementing repair solutions that minimize disruption of the many inhabitants of their facilities.
When faced with the possibility of repairs, it is optimal to select a contractor that specializes in, and is familiar with, all elements of the repairs, including the safety of workers, the structure, and the public.
Mark Sitar is a branch manager and industrial division operations manager for Structural Preservation Systems, Inc., a provider of structural repair, protection, and strengthening services, and a subsidiary of Structural Group, based in Baltimore, Md. He can be reached at 847-551-1012 or firstname.lastname@example.org.
Two examples of glulam strengthening Structural Preservation Systems (SPS) was retained by a Midwestern school district to provide a design-build solution for the repair of some glued laminated wood beams located in the school gymnasium. When school personnel noticed cracks as large as an inch in size during a routine mechanical inspection, they contacted SPS and a structural engineer, and emergency shoring was installed immediately. Once the beams were stabilized, a repair and strengthening plan was initiated.
SPS and the structural engineer worked hand-in-hand on the proper repair strategy, and the repairs were implemented during school breaks and at night to minimize the impact on the teachers and students. The successful repair of the cracked and deflected roof wood beams required a high level of coordination between the school district and the SPS design-build team. Further, coordination between the roofing, electrical, wood, steel, and masonry portions of the work and the school district was imperative to ensure that the repair strategies were cost-effective and constructible.
Another example of a successful repair of glulam beams was for the one-story Gaywood Elementary School in Seabrook, Md. Part of the Prince George’s County Schools, the school houses kindergarten through sixth grade and was originally constructed in 1958 with a brick façade and wood beams supporting the roof. After an extremely harsh winter with heavy snowfalls, the beams became cracked because the flat roof could not withstand the severe weight load of snow.
Visible to the naked eye, the cracks were clearly evident to all who entered the school. Although officials had taken proper steps and shored up the beams, the parents and the Parent Teacher Association were concerned about the safety of students. School officials then contacted SPS to develop a repair strategy for the beams.
Understanding the importance of this repair project, SPS developed a two-prong repair approach. Before either strategy was implemented, the team performed surface preparation that required removing the paint and injecting the crack with an epoxy. The team opted to use the Hardwire system on beams that had moderate cracking. This approach was non-intrusive and it was not visible that repairs had been made to the structure, which helped to ease parents’ concerns. The second approach involved using external posttensioning on beams that were severely cracked and would need extra load-bearing capacity. Both of these options saved the school from having to tear out the beams and replace them. Further, SPS’s repair team did not remove any existing lighting; rather, they worked around existing light and classroom fixtures.To avoid any damage to the walls, there was a great deal of shoring and scaffold work. This approach provided a tremendous cost-savings to the school and ensured a non-invasive procedure.
Safety was paramount since this project occurred on a school campus. Both approaches did not use volatile organic compounds (VOCs) or hazardous air pollutants (HAPs) and clean-up occurred everyday. Dust from the surface preparation process was minimized because a dustless girder system was used. Further, steps were taken by SPS to minimize noise disruption since summer school was in session while the project was taking place.
Officials from the Prince George’s County School District were extremely pleased with the result of the projects. The repairs were so seamless that it is difficult to distinguish which beams had been repaired. The overwhelming positive results of this project have prompted officials to consider strengthening beams as a preventive measure for other schools that have a similar construction. The detailed construction phasing schedule developed by SPS and school officials ensured that the project was completed during the summer before all students returned in the fall.