Log Structure Engineering Jennifer Anthony, PE 1 Please note: the author of this design guide has made every effort to eliminate error and omission from data supplied herein, and accepts no responsibility or liability for any problems resulting from use of this information. Copyright Jennifer Anthony How do Log structures differ from conventional framed structures? Structural members are round. Full section vs sawn round properties Shear stress for round members Shear at coped beam TPI and other log grading agencies. Sample TPI log grading values and comparison to NDS TPI contact information Other log grading agencies Typical details and design considerations hancrafted and milled logs log taper and average diameter notched log walls and offset coursing log wall types- chinked, scribed, piece en piece Shrinkage and settlement design of screwjacks and ways to hide them non-bearing post and wall detailing other ways to accommodate settlement Resisting lateral loads (wind and seismic) in horizontal log walls Interpolated IBC seismic design R value Load resisting methods (wood or steel dowels or pipes or rods, through bolts, lag screws, log screws) If you are a rookie at log engineering, it is best to make sure you are working with highly qualified log contractors. They can help teach you the ropes. There are lots of things that can go wrong with log construction. Full Logs Sawn Round Logs
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Log Structure Engineering Jennifer Anthony, PE
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Please note: the author of this design guide has made every effort to eliminate error and omission from data supplied herein, and accepts no responsibility or liability for any problems resulting from use of this information. Copyright Jennifer Anthony
How do Log structures differ from conventional framed structures?
Structural members are round. Full section vs sawn round properties Shear stress for round members Shear at coped beam
TPI and other log grading agencies. Sample TPI log grading values and comparison to NDS TPI contact information Other log grading agencies
Typical details and design considerations hancrafted and milled logs log taper and average diameter notched log walls and offset coursing log wall types- chinked, scribed, piece en piece
Shrinkage and settlement design of screwjacks and ways to hide them non-bearing post and wall detailing other ways to accommodate settlement
Resisting lateral loads (wind and seismic) in horizontal log walls Interpolated IBC seismic design R value Load resisting methods (wood or steel dowels or pipes or rods, through bolts, lag screws, log screws)
If you are a rookie at log engineering, it is best to make sure you are working with highly qualified log contractors. They can help teach you the ropes. There are lots of things that can go wrong with log construction.
Shear*: fv = (4/3) X V / A (round members only) Bending: fb = 12 M / S
Deflection: delta = standard formula for deflection (depending on load type) with I as calculated for round section.
* fv = VQ/ Ib Q= Area half section x center of gravity of half circle Q= (A /2)x(4xr)/(3xphi) I= Ax(r^2)/4 b= 2xr
shear at coped beam: Technically the full depth of the member is bearing on a crossed beam at a half coped notch. Because there may be some additional taper in the supported member, it is prudent to check for shear of ½ the standard area (a concentrated shear due to a square notch is not valid, as it is not a square notch). Adjustments may need to be made for removing part of support beam to accommodate shear- both members need to be checked then for reduced sections.
Fv = (approximately) (4/3) V/ A (A = phi * r^2/2 or half of full area)
LOG GRADING:
How logs are graded will need to be verified with the log supplier. In the west, it is most often TPI. The only job in the east I have worked on used Log Homes Council standards. It is best to verify with the log contractor if possible.
Log Grading agencies: Timber Products Inspection, Inc. (770) 922-8000 http://www.tpinspection.com/ Patrick Edwards Vice President of Engineering 770-922-8000 ext 153
Log and Timber Homes Council Log Grading Program 1201 15th Street, NW Washington, DC 20005 (202) 266-8357
“The purpose of visual stress grading of logs is to provide designers and engineers of log and timber homes and commercial buildings the allowable stress values they require to build a safe and economical structure that meets the requirements of the nation’s building codes. All Log Home Council members must participate in a monitored Log Grading Program before joining the council.” Comparison of Wall Log, Sawn Round Log and NDS Design Properties Sawn round timbers can be completely round or sawn or shaved along one surface with max shaving of 0.3X Radius. Wall logs can be used for all material 4.5” or greater in its least dimension. Timbers can be run to any profile or be left round. Wall logs may be used for log walls, beams, joist, rafters etc. Wall Log Design Values (PSI) Doug Fir (TPI) Grade Fb Ft Fv FcT Fc// MOE X 10^6 Premium 1500 1000 165 630 1050 1.6 Select 1300 875 165 630 900 1.6 Rustic 1150 775 165 630 800 1.4 Wall Log 40 850 575 165 630 600 1.3 Wall Log 30 650 425 165 630 450 1.3 Sawn Round Timber Design Values (PSI) (sawn round) Doug Fir Larch (TPI) Grade Fb Ft Fv FcT Fc// MOE X 10^6 Unsawn 2050 1150 240 630 1100 1.6 #1 1700 925 240 630 900 1.6 #2 1400 775 240 630 750 1.6 #3 800 450 240 630 425 1.3 Design Values for Visually Graded Timbers (5” X 5” & larger) Doug Fir Larch (WWPA)- Posts and timbers Grade Fb Ft Fv FcT Fc// MOE X 10^6 Dense SS 1750 1150 170 730 1350 1.7 SS 1500 1000 170 625 1150 1.6 Dense #1 1400 950 170 730 1200 1.7 #1 1200 825 170 625 1000 1.6 Dense #2 850 550 170 730 825 1.4 #2 750 475 170 625 700 1.3
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Typical Details and Design Considerations
•Handcrafted Logs •Log Taper and Average Diameter •Notched Log Walls and Offset Coursing •Milled logs •Chinked, Scribed and Piece en Piece Log Wall construction
Handcrafted Logs and design considerations Hand peeled logs are tapered, with a tip, or thin end and a butt, or thicker end. The amount of taper depends on the type of wood and growth conditions- this can be verified with the log supplier. The required diameter is generally considered as the average (midpoint) unless special design considerations require a certain size at either end. For example if you have a high shear at one end, check thick tapered sections for maximum load and require butt end to be placed at high shear end.
Generally structural log diameters (beams and joists, rafters etc.) should be within 2”-3” of wall log diameters. This should be verified with the log builder.
Logs are notched at corners, with a full course sill log starting on all walls in one direction, and a half course sill log starting in the perpendicular direction. This means that perpendicular walls are one half-course offset. This affects the layout of floor joists and rafters or purlins.
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Whidbey Island Log Guest Cabin, Jean Steinbrecher Architects
Structural beams supporting other log members (log floor joists, purlins or rafters) are not to be notched at the member they are supporting, rather the member should cope over the support beam.
Log floor joist must be one half course above supporting wall- any intermediate tie beams would also be at the same elevation as the top of log wall below the floor joist. Photo from Log Building Construction Manual by Robert Chambers
It is possible to have log beams or joist in the same plane as the support and dovetail into support, in that case the support section must be checked for capacity with greatly reduced section properties. Since the logs are round, a great deal of housing is required to have substantial section below the bearing point. One of the most common errors of inexperienced architects designing log structures is to draw all members in one plane. It is best to work with them early on in the process and even encourage that they consult with an experienced log architect. It is also ideal to identify the log contractor as early on as possible and to coordinate detailing with how they like to build. Just like with timber framing, there are many ways to get to the same result.
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Milled Logs A milled log is one that has been put through a saw mill, lathe, or planer and has been cut into a specific shape or profile. When you look at a milled log home all the logs will be uniform size. Logs may have a flat top and bottom surface, may be coped, or may be tongue-and-grooved. The sides of the log can be sanded smooth, left with a rougher or more rustic look, or hand peeled with a draw knife. Milled logs are usually air and or kiln-dried to a specific moisture content before they are used.
Types of Log Wall Shapes
Chinked Log Walls
Moravian logs and chinking (clay & hair) Modern chinking is flexible and longer lasting
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Over scribed or coped log walls
Handcrafted Milled
Piece en Piece Log Walls
Port Angeles Log Home, Washington
Murray Arnott Design and Nicola Logworks
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Shrinkage and Settlement in log walls:
Whether dry or green the designer must account for log shrinkage and accommodate settlement- screwjacks are required at any non-shrinking supports such as posts or beams
supported by posts, and fireplaces if they support structural members. Green logs may shrink from 4-10% depending on species. It is best to check with log supplier for specific species and moisture content to approximate the amount of settlement to expect.
Screwjack at base of post
jack may be hidden by sleeve or in subfloor
Screwjack hidden in floor
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Screwjack in top of stud wall
Screwjack at Fireplace
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Screwjack hidden in post top
This is a unique solution to hiding a screwjack in the top of an exterior column in a damp climate. The log plate beam hides the connection and it is weatherproof. This requires the use of a more expensive worm drive screwjack- the plus side is that the jack may be raised or lowered with a screwdriver (there is a small access port on the side of the column) and the weight on the jack does not need to be temporarily lifted.
Whidbey Island Log Home- Unique
Timber Corp.
Another unique approach tested on the same project and deemed too costly in terms of labor - a log wedge at the top of the column- wedge can be pounded in as log walls settle and gap forms at top of post. A neat idea, and still functioning on this project.
Screwjack Design: Single screwjacks should be designed assuming a minimum eccentricity of 5% of the height, so that bending and axial load will influence design of the jack. None of the simple off the shelf jacks I have seen take eccentricity into account, further, they often have plates that are too small and have no positive connection to the log or beam they are supporting, or to the foundation or support.
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Single Screwjack Rod Design
(P/A)/Fa + (M/S)/Fb is less than or equal to 1 Fa depends on KL/r K= 0.5 (fixed fixed) (see AISC steel manual) Fb = 0.75 Fy – for 36 ksi steel Fb = 27 ksi
Example Screw Jack Threaded Rod Design:
For an expected travel of 6” and a 5% eccentricity:
M= 0.05*6”*P = 0.3P r = (I/A)^1/2
Then P (capacity) is less than 1/(1/(Fa*A) + 1/(90*S)
Diam. A S I r kL/r Fa (from chart) Pallow
1” .7854 .0982 .0491 .25 12 21.05 5.76K
1.25 1.2272 .1917 .1198 .313 9.6 21.16 10.37K
1.5 1.7671 .3313 .2485 .375 8.0 21.25 16.62K
For Multiple rod assemblies, the eccentricity may be lowered. I assume a 3% eccentricity for a 3 jack assembly.
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Worm Drive Screwjacks Worm drive screw jacks are more costly but very easy to use. They do not require removal of heavy loads to let down, (with a heavily loaded conventional screw jack, you may have to temporarily jack some of the load off of a post or beam to be able to let the bearing jack down). Worm drive screw jacks may be lowered (and raised) with a cordless drill.
The manufacturer can engineer a jack for specific applications, including some lateral load capacity, which is impossible with a simple threaded rod and bolt jack. Whidbey Island Log Home
Other Settling Details to be considered
Door and window bucks, non bearing walls, plumbing and electrical, fireplace walls to log wall connections, all must allow logs to come down, and still provide a connection. (a separate hours lecture at least)
Non Bearing CMU Shearwall
Plumbing and non-bearing wall slip joint
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Log wall to CMU Wall
Moment Frame Slip Connection
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Separation of log structures and non settling structures
Whidbey Island Log Home
Orcas Overlook Log Home (2001) Orcas Island Washington Jean Steinbrecher Arch., Unique Timber Corp.
Entry wing separated from main wing to allow log settlement
Chinked logs must accommodate shrinkage with green logs, as per above. Some log suppliers using dead standing chinked logs do not allow or accommodate settlement. Verify with the log supplier the method that they use to dissuade settlement, if there is any possibility of settlement, it should be accommodated with screw jacks. Note that gable ends constructed with horizontal log walls and upper level floor walls with varying eave height will allow differential settlement. Piece en Piece or log veneer can be used to accomplish the look of full log structure while eliminating differential settlement. Some log builders using chinked log systems use rebar at 45 degree angles to eliminate settlement. I can not attest to the long term effectiveness of this solution.
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Seismic Loads on Log Structures: R value: this is where it gets tricky. In the old Uniform Building Code, R= 5.5 for shear panel walls and 4.5 for other light framed shear walls. In ASCE 7, shear panel walls with wood structural panels rated for shear resistance have a value of 6.5, other non-rated shear panel walls 2 and walls with flat strapped bracing 4. Log walls are not mentioned at all. I feel comfortable using R = 4, as it is similar to the UBC equivalent, and was used for many years with great success in high seismic areas. Also log walls have at least as much strength as flat metal bracing, which is given the same R value. You may have problems with the code official, however, who may require testing or other methods of proving this R value. In that case good luck- I recommend contacting engineers in high seismic areas who have had projects that performed well in major earthquakes, and referring the building official to a list of time proven structures.
1997 UBC V base=(2.5*Ca*I*W)/R (eq. 30-5) V design = V base/1.4 for allowable stress design (UBC 12-9) Per code use soil type Sd when soil is unknown (UBC 1629.3) Z= Seismic Zone Factor (table 16-I) Ca = Seismic Coefficient related to soil (table 16-Q) Na = near source factor (table 16-s) I = importance factor (table 16-K) EXAMPLE- Riverside, CA, Seismic zone 4, soil unknown, one story residence with log shearwalls. R= 4.5 Z= 0.40 seismic zone 4 Ca = 0.44*Na Na = 1 (no fault within 10 km) I = 1 V design = (2.5*0.44*1*1*W)/(1.4*4.5) = 0.1746*W
ASCE 7-10 and 2012 IBC Risk Category (ASCE table 1.5-1) Seismic Importance Factor (ASCE table 1.5-2) Seismic Design Category (ASCE table 11.6.1) Permitted Analytic Procedures (ASCE table 12.6-1) Simplified Analysis Procedure for Seismic Design- ASCE 12.14.8: V base = F*Sds*W /R F=1 one story, 1.1 two story, 1.2 three story (eq. 12.14-11) R = response modification coefficient (table 12.14-1)
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Sds :use USGS program with longitude and latitude go to https://earthquake.usgs.gov/designmaps/us/application.php for this program (use site class D if unknown, unless building official requires otherwise) V design = 0.7 V base (ASCE 2.4) (use full V base to determine deflection) EXAMPLE- one story log home Seismic location Riverside, CA soil type D Occupancy Category I Seismic Use Group I Seismic Design Category D Sds from website based on site specific info Sds=1.0 Permitted Analytic Procedures – Simplified (12.14.8) allowed I=1.0 (residential)
R= 4.0 for Log Walls V design = (0.7*Sds*W)/R= (0.7*1*W)/4.0=0.175*W Note that the IBC load is almost identical to UBC load, the load would increase with more stories. IBC is site specific, while UBC is more general in terms of earthquake zones. The fact that the IBC load is usually the same or higher than UBC justifies the use of an R value of 4.0.
Lateral Load Resisting Methods: (Use load duration factor of 1.6 for wood design) Wood Dowels- do not rust or sweat and shrink and swell with walls. Capacity based on wood dowel strength in species of wood. (see NDS Appendix I- and attached spreadsheet for example of wood dowel calculation).
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Steel pins or pipes- high capacity allows wide spacing. Capacity based on NDS tables for steel bolts in single shear, with reduction for drift pin as per code. Use Table llA with 7 ½” main and 3 ½” side (pin is driven through top course into lower course, but not bolted) and your wood species. Through Bolts- not my favorite. They overdrill so much to get these in that I question the effectiveness in shear. If holes are drilled ¼” larger than threaded rod, a 10’ wall with 12” logs could deflect 10X.25” = 2.5”. If you must use through bolts require self tightening washer at top to ensure friction of logs, and dissuade use wherever possible. Lag Screws – lower capacity than steel pins or pipes, will hang up settling walls. Use on dry chinked homes only, but encourage pins. Load capacity per NDS. Oly Log Screws – low capacity- in high seismic or lateral load areas impractical due to very tight spacing required. Ok in lower lateral load zones. Load capacity- see wood screws NDS Table 11L.
Log Shearwalls
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Something to check on with log builder is length of continuous logs. Generally quality handcrafted log homes have splices only at wall intersections, but milled logs are often short pieces spiced randomly, surprisingly, you have to tell these builders not splice over windows and other openings. The length of log is especially important when considering overturning forces- if continuous logs are used, you would have to uplift the entire wall, and intersecting corners, to have a resulting uplift force on a log wall. It is rare. In addition, I use the height to width ratios with design for force transfer around openings for log shearwalls with continuous logs, in a pinch I will check overturning force on the actual header beams. (Refer to NDS Wind and Seismic Design 4.5.3.)
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When all else fails and there are not enough wall to use for shear… moment frame hidden in logs
Moment Frame hidden in truss or plate log
Whidbey Island Pool House- 2 moment frames hidden in walls
