Earthquake and wind forces act on a building in the lateral direction or parallel to the ground. Engineers call this lateral loading.
Earthquake and wind are natural phenomena laterally acting on a building.
However, they are fundamentally very different forces that call for unique and specific design solutions.
Earthquakes:
Earthquakes produce the most natural destruction to our homes on a terrifying grand scale.
The Haiti Earthquake of last month is the latest deadly reminder that we must design our homes to withstand natural destruction. The knowledge and building technology is available. Log construction is one of many solutions…
Earthquakes alone do not kill. The buildings we design kill its occupants as they collapse over our heads because they are not adequately designed and engineered to withstand local seismic forces.
In January 1995 a devastating earth quake struck Kobe in Japan.
Since 1993, I had been designing log homes mainly for the Japanese market. Log homes are popular in that country because Japanese consumers believe log homes are able to withstand seismic tremors better than other type of construction. Watching the news, I was shocked to view the almost total devastation of the city… except a few homes were still standing untouched among all the rubble. Those were log homes…
Log homes are uncommonly strong to lateral movement created by earthquake or wind forces because the interlocking log cross-corners, the friction between logs in the long laterals and the weight of the logs can provide great flexibility and resistance against lateral forces. Earthquakes displace the ground laterally thus shaking our buildings from the foundation all the way to the roof.
The key to designing earthquake proof buildings is to find ways to absorb as much seismic energy within the flexibility of the building material and structural deformation but keep enough structural stiffness to hold the building together so it will not collapse.
Engineering adequate anchorage of the bottom logs to the foundation is essential to prevent uplift of the log shell from seismic forces. Using 5/8” diameter anchor bolts anchored a minimum 8” deep in concrete foundation and spaced at 32” on center to tie in the sill log to the foundation is a common construction specification to keep friction between the sill or bottom log and the concrete foundation. However, anchor bolts will lose tightness as the sill logs lose moisture content and shrink.
Using thru bolts tied to the anchor bolt with couplers all the way to the top log is a good solution favored by engineers.Thru bolts are only necessary in seismic areas and are not used in low seismic zones like Texas or Minnesota for example. This works as long as the thru bolts are regularly tightened during the settling of the log walls which can take up to five years when using green logs.
Unfortunately, experience tells us that home owners almost always forget to maintain thru bolt tightness during the settling of the log walls.
Some log builders go as far as visiting the log homes they built to tighten thru bolts and adjust screwjacks at bottom or top of vertical log posts to ensure the log shell will settle evenly and keep friction to a maximum. Using automatic pressure springs at top of log walls in this instance is a solution to keep adequate friction between log to log and log to foundation.
For detailed information on log homes versus seismic and wind lateral loads, please refer to the most excellent study written by Structural Engineer Tom Hahney @2000 “How log buildings resist lateral loads” published in Log Building News, October 2000.
Tom Hahney concludes: “Log buildings survive quite well in an earthquake as long as they stay on their foundation. In fact log joinery helps dissipate the seismic energy”.
Wind:
Lateral forces created by wind depends on the wind velocity at the building site, the building shape and roof slopes and as well how much the site is protected by topography and trees.In hurricane areas, roof slopes are usually low to lessen lateral wind load. Hip roofs are also best as they do not offer much grip to the wind compared to roofs with many valleys.
During a wind storm the roof is likely the weakest part of the house and it will be the first part of the building to be blown off, so special attention must be taken to ensure the roof is securely attached to the log walls.
As log home roof overhangs should be designed to protrude at least 4’ to 5’ beyond exterior log walls to protect the log work from weathering, roof systems must be engineered to be fastened with lag screws to top plate logs, and plate logs in turn may need to be lagged or thru bolted to logs below to prevent wind uplift forces.
If the logs are large diameter, the weight of the top logs is often sufficient to anchor the roof.
If the local wind load is expected to be high, then the top logs need to be lagged or thru bolted to lower logs and possibly foundation to maximize friction between logs and the overall dead load the roof is attached to (Figure 3).
Large glass towards the local wind direction needs special attention during the design and engineering of the building.
Avoid large garage doors and opt for multiple single car garage doors instead of the double type.
A quick internet search will guide you to many historic examples of log homes surviving hurricane force winds in Florida, North & South Carolina…
Screwjacks (adjustable threaded rods) at vertical posts supporting deck roofs, (used to be adjusted to handle log wall settling) should be installed at the bottom of the post instead of the top where it creates a weak hinge point vulnerable to wind loads. Too many deck roofs have collapsed due to that simple design mistake.
As a general rule screwjacks should be installed under posts to avoid structural failure to lateral forces, inside and outside the log shell. 90 degree upright screwjack installation is crucial to maximize its load bearing capacity. A five degree off plumb position can easily half the bearing strength.
Screwjacks must be engineered for all point loads in the log structure.
Last word…
Log home design has evolved from the simple almost windowless log cabin with four cross corners built by the American pioneers two hundred years ago, to large complex log homes with huge glass walls and high vaulted ceilings maximizing view and grandiose interior space, thus weakening the log home structure ability to withstand lateral loads.
Log walls with cross corners at both ends have unique lateral load resistance capabilities superior to conventional wood frame building systems and have great seismic dissipation characteristics allowing them to survive powerful earthquakes.
However, it is crucial that the design takes in account local seismic and wind loads. Further structural engineering analysis is a must to ensure your log home will stand against nature’s extremes.
