Timber Frame Pergola: Against the Wind, Under Snow and Seismic Loads

Aluminum Pergola Wind Blown DownSnow Collapsed Vinyl Pergola

Over the last 50 years engineering research has been conducted and wind force data collected to improve designs of structures for prevailing high winds and determined seismic flexibility; which has resulted in varying state and local building codes and national load standards for buildings and other structures. The building codes continue to change and efforts are made to improve and ensure structures in high wind and earthquake prevalent areas.

With the natural occurrences of storms, strong winds, heavy snows and seismic shifting it is essential to have a thorough understanding of load paths and design properties.

Timber Frame Railroad Bridge

Timber frame bridges have been among the most visible and noble testimonies of the integrity of timber. Some still in use date back as early as the 16th century; proven masterpieces of engineering, technology and long lasting endurance spanning great distances against the violence of water and storms.

In the United States, the USDA Forest Service has more than 1,000 timber bridges spanning 20 feet and beyond, 90 years old and older which are still in service today with more being built every year. There are also more than one thousand five hundred miles of trestles and timber bridges to service the railroads. Timber bridges still remain a choice of engineers today for its ability to bear the dead-load and the live-load of pedestrian, vehicular, railroad while remaining steadfast under continual exposure of powerful winds, seasonal freezing, thawing and resisting the side effects from adverse de-icing agents.

No matter what the environment is like, properly seasoned and treated timber does not deteriorate and corrode, rust like steel or crumble like concrete. The weight/strength ratio of timber is another consideration as concrete weighs 4 times as much as timber of a similar thickness. Timber meets the strength, lightweight and structural requirements of energy absorbent properties desired in construction of a safe bridge for conformance highway safety of HL-93 and AASHTO HS20-44 standards in bridge building of vehicular bridges fording continuous traffic.

Pressure-treated/well-seasoned timber is inherently more durable. Government required testing for well over 20 years of pressure treated timber stakes demonstrated that timber will withstand burying without beginning to disintegrate over time. Although a pergola does not have buried portions of abutments like a timber bridge does, it does give added assurance to the durability of timber.

Timber Has A Natural Greater Ability To Withstand Wind, Snow and Seismic Shifting

The natural elasticity of timber and is more conducive to responsible architectural design than inelastic materials being able to compensate for structural movement. The earth invariably has seismic vibrations due to natural and sometimes artificial causes. The earth moves up and down as well as side-to-side. An architectural structure will carry its own weight vertically (or it would fall down) by resisting the up and down loads but the greatest weakness in a building is in the lateral movement from side to side. Structures with added cladding will experience stronger forces.

Timber is stronger than conventional framing and can be constructed in any kind of weather without being detrimental to the material; which is another safety benefit.

Inelastic Materials Must Compensate With Artificial Mechanisms

In inelastic load bearing materials such as concrete, steel or aluminum beams, beams are required by architectural engineers to compensate for the rigidness against inevitable seismic movement by installing kinematic mechanisms and plates. These artificial mechanisms are also to help guard against spalling, vapor bubble and plate impacts subjected to localized low-pressure. When a beam yields a little at its highest point without losing its resistance; the joints and weight maintain its bearing resistance; the resonant frequency will dampen out and they will hold full strength.

The marvel of timber is that it does not require or need any artificial mechanisms to dampen out resonate frequencies to maintain its integrity. This integrity is shown in timber framing dating back hundreds of years that have allowed for wide-open spaces, cathedral ceilings, commercial construction, great rooms and more. The oldest recorded man made structure in the world is a primitive timber water/bridge structures in Thames, England.

The Dovetail Notch System For Added Safety

The oldest timber frame architectural buildings and bridges implemented the ancient architectural joinery of dovetails, mortise and tenon or what we call our “Western Timber Frame Dovetail Notch System”. The dovetail joint has a trapezoidal angled shape that interlocks acting like a wedge with another angled timber for a puzzle-like fit that resists pullout. This gives the timbers 100% more strength against positive and negative wind load actions with added power to perform its inherit quality of expanding and contracting dampening out resonate frequencies. The natural air pockets within wood also gives the structure stability and is less affected by temperature extremes or prone to condensation as aluminum, metal or vinyl is. The dovetail joint does not depend on pinions to stay against tension. The dovetail works as an extension of timber allowing for shrinkage and weather variations while remaining secure and snug.

With superior construction techniques, properly engineered designs and expert craftsmanship a pressure treated timber pergola can be expected to last even longer than timber structures of ages past. Timber framing is an investment in children and grandchildren proving to be safer, durable structures more resistant to powerful coastal storms, high winds, heavy snows of high mountainous regions and seismic shifting, maintaining structural integrity.