Glued laminated timber

The Keystone Wye Interchange Bridge, built in 1968, is a three-hinged arch bridge constructed of glued laminated timber.

Glued laminated timber, or glulam, is made of wood laminates that are glued together that the grains of all lamination are longitudinally parallel. Laminations are usually 1 or 2 inches thick but can be made in any length or width. The glued laminates can be made into straight timbers to serve as columns and beams or curved for arches or trusses. Glued laminate timbers are popular because they permit unique architectural effects and can span longer than solid timber.

History

The creation of structural glued laminated timber was first seen in Germany in the 1890s, a development of Otto Hetzer. He received patents for glued laminated timber in varoious European countries in the early 1900s. The Hetzer construction method was soon born and would refer to the glued laminated timber arches in Europe for years to come. The Hetzer firm continued to build structures for the next 20 years. It constructed arches and beams for factories, workshops and gymnasiums. The shortage of casein glue during World War I caused the use of glued laminated timber to slow in Europe. In the United States, research for improving the glue in plywood and other laminates helped develop advancements in glued laminated timber. The McKeown Brothers Company of Chicago used adhesives in combination with nails for bowstring truss chords it made in the early 1920s. A German, named Max C. Hanisch, Sr., introduced glued laminated timber to the United States. Hanisch had learned glued laminated timber techniques from Hetzer; upon immigrating into the United States, in 1934 he founded Unit Structures, Inc. in Wisconsin with the Thompson brothers. The first glued laminated timber building in the United States was the gymnasium/auditorium at the Peshtigo High School in Peshtigo, Wisconsin.

Manufacturing Process

Manufacturing glued laminated timbers usually requires strips of lumber that are between 1 and 2 inches thick and gluing them together. They can be glued to take almost any desired shape and is often selected on its strength and appearance. Before gluing can begin, it is important that the lumber is planed, cleaned, and seasoned so that the moisture content is less than 16% at the time of bonding. Glued laminated timber is custom manufactured and the finished members can vary widely by shape, size, and finish. If one lamination is not the desired width, placing multiple pieces edge to edge may be required. Joining laminations from end to end may also be necessary. Butt joint were the original way of joining these laminations together but was later replaced by stronger joints like hooked, scarf, or finger joints. After the timber is prepared, equipment places adhesives on the faces of the lumber, the laminations are placed together, and clamps are applied to create uniformed pressure until the adhesive can cure. As advances in adhesives have been developed, the improvement of glued laminated timbers also improved. Before World War II, the primary type of adhesive was water-resistant casein glue which was acceptable for the interior of buildings but not the exterior. Synthetic-resin glue was introduced in the 1930s and improved the glued laminated timber and made it possible for use in bridges and other weather exposed structures. Resin adhesives, such as urea formaldehyde and phenol-formaldehyde, were used late until the 1950s.

Uses and Installation

The use of steel reinforcement in the Peshtigo gymnasium was an attempt to appease skeptical state authorities, who later realized that the reinforcement was highly unnecessary. Research soon proved that the adhesives in the glued laminated timber provided bonds as strong as the wood itself. The Forest Products Laboratory at Madison, Wisconsin used glued laminated timbers manufactured at Peshtigo to construct a storage building to prove its strength and durability. It had 7 arches, five of which had three-hinge glued laminated arches. The arches created a span of 46 feet and a ceiling height of 19 feet. Forest Laboratory loaded sandbags onto the arches, weighing roughly 31,500 pounds, for a year to test the durability of the timbers. The experiment caused the roof peak to deflect 1 ¼ inches and spread the shoulder about ¼ inch each. With this building and the beginning of World War II, glue laminated timber saw a dramatic increase in use. This material was used for barns, churches, storage facilities, warehouses, and gyms. St. Leonard's Catholic Church at Laona, Wisconsin was the first church built using glued laminated timbers. Before World War II the use of glued laminated timber had not reached many points outside of Wisconsin. The war required large amounts of steel and wood, leaving the door open for glued laminated timber industry to expand. Unit Structures was capable of persuading the military to use glued laminated timber to make storage facilities, aircraft hangers, and drill halls at the U.S. Naval Training Station. The arches reached up to 115 feet and game the buildings nearly 42 feet at the peak of the roof. Due to the success of these buildings, glued laminated timber saw a rise in popularity and became an acceptable building material for many commercial buildings. Churches, supermarkets, factories and warehouses used extensive amounts of the glued laminated timber. The American Institute of Timber Construction was formed in 1952 with the first president being Ward Mayer. Between 1954 and 1963 the glued laminated timber industry more than doubled its production, growing from 31,420,000 board feet to nearly 86,000,000 board feet.

Conservation

It order to restore glued laminated timber structures, the expertise of a structural engineer is highly suggested. These engineers will observe and measure the building to learn how is has performed in its life. They need to develop an appropriate stabilization plan, as well as create a repair or replacement program for historic glued laminated timbers. Analyzing the structural capacity of the building should also be done before any attempts to rehabilitate are initiated.

Deterioration

Glued laminated timber has some distinct qualities from solid timber. At a moisture level greater than 20%, the glue laminated timber becomes prone to fungi and termites. Deterioration will occur after the lamination because of the moisture variations. The glue in the materials can act as a barrier for termites, trapping them in an outer layer while preserving the interior layer; but this can be for a short period of time and the termite and moisture issue should be dealt with immediately. Large pieces of glued laminated timber should be checked often because they are highly susceptible to deterioration which can cause extreme structural problems. Moisture related failure is common on the ends that are more likely to come into contact to wet surfaces. Deficiencies in design or manufacturing can cause failures, as well as a change in service conditions. Radial tension, bending, or horizontal shear may cause the glued laminated timber to split at the end. Horizontal shear usually happens at connection or notched ends, as well as heavily weighted beams. It is common for the wood to shrink around large bolt connectors and cause the wood to split. Also, inadequate face bonds can cause horizontal shear failure at glue joints.

When assessing glued laminated timber, it is important to identify the type of wood, the preservative finishes, and the type of adhesive applied, this information can commonly be found on the shop drawings. These drawings are important because they show what was constructed and most of the dimensions for the building. Finding the dates of construction will also assist in assessing the glued laminated timber. It helps to establish design values with a certain type of lumber combination. Design standards are important aspects of assessing the glued laminated timber and should be discovered for each particular structure. Water-proof and water-resistant adhesives were used heavily before the 1960s. Later, preservative techniques, such as pressure treating, became popular. The most popular treatment for glued laminated timber is a petroleum gas called pentachlorophenol , which prevents warping that is caused by a wetting and drying process. If the finish is dark, it is likely waterproof; if it is gray or white, it is likely water resistant. These samples may require laboratory testing because pieces can become soiled. When the glued laminated timber’s adhesive begins to fail, hollow cavities and weak areas will be formed. These areas can be found by tapping a carpenter’s hammer or by finding extreme discoloration. Other signs may include: swelling, brittleness, or a raised grain surface. Small pinholes can be caused by insects boring into the surface of the wood and should be visible on the surface. The taking of core samples can be taken to evaluate the level of decay; ultrasonic devices can also prove helpful in identifying problem areas. Moisture meters can assist in determining areas that are prone to deterioration.

Conservation Techniques

Due to decay, reinforcing glued laminated timber with steel or an epoxy system may be required, especially at the ends of arches or A-frames. Welding steel plates on the original steel shoe along with bolting sheer plates to the faces of the laminated timber will transfer all bending forces to the steel and off of the timber structure. Temporary jacking may be required to alleviate the load so that repair may be done in place. Shear reinforcing to prevent failure from other members is done by inserting lag screws to join the beam together. For best results, these should be applied through the roof or floor deck where they are perpendicular to the axis of the beam. Holes are predrilled (sometimes the full depth of the beam) and then lag screws are put in place. Exposed joint hangers may also be used to reinforce the glued laminated timber. This technique includes placing a notched beam, steeled sided plates, and a steel reinforced plastic dowel into vertical holes and then epoxying the dowels in place. Radial tension, which may occur in the curved portion of tapered beams, is remedied again with the use of lag screws. If only a few tension laminations are damaged, it can be cut away and be replaced with new, high-quality boards of the same thickness. This work may be difficult because it requires a skilled artisan and a manufacturer who can supply finger-jointed replacements. Also, the cause of the failure should be identified before the replacement can take place. Adding more layers of laminations may be a solution to the problem. Segmental infill, the adding of new laminations for greater depth, is commonly done in the field. The laminate is applied to the existing piece, cured, sanded, and finished to match the original piece.

If the glued laminated structure is bending, reinforcement with steel side plates, bottom plates or flitch plates may be required. A flitch plate is a vertical steel plate that is commonly placed between at least two timbers to act as a beam. If a steel plate is used, it should only be used to support the same load amount as the glued laminated timber. Engineers commonly design the steel plate to carry more of the weight than the glued laminated timber. These plates can be painted to match the original timbers. Flitch plates are installed into a rout that is cut into the center of the beam. These cuts are usually made with chain saws that are attached to a guide for improved accuracy. The plate is inserted and an epoxy is added. It is imperative to determine plate thickness, height, length, and method of connection, and may require extensive investigation. Exposed steel tie-rods or bottom plates can also be applied to the beams for reinforcement but they are aesthetically unpleasing.

If the columns or arches are exposed to moisture, deterioration may occur. The location where two laminated timbers connect to a supporting steel shoe is critical because the shoe will transfer vertical forces and horizontal thrust through anchor bolts into the foundation. If this type of connection is required, it may be necessary to weld steel plates to the existing shoe and connect them to the glued laminated timber member so that all forces are resisted. Steel side plates to glued laminated timber might require the field installation of shear plates, which should be inserted flush to the face of the timber in a notch cut specifically for the piece.

Replacement

Replacing these pieces may be easier than in-place structural reinforcement or repair because laminated timber structures are known for having multiple components. Glued Laminated timber can be easily matched in size, detail, and finish. The American Institute for Timber Construction and the American Society for Testing and Materials has established standards that should be followed when replacing or repairing glue laminated timbers.

References

• McNall, Andrew, and David C. Fischetti. "Glued Laminated Timber." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 136-40. Print.

External links

• [http://www.fpl.fs.fed.us/documnts/pdf1997/moody97a.pdf Russell C. Moody and Roland Hernandez, "Glued-Laminated Timber", in Smulski, Stephen, eds., Engineered wood products-A guide for specifiers, designers and users (Madison, WI: PFS Research Foundation, 1997), pp. 1.1–1.39.