Structural Considerations in the Design of a Log Home

Note: prescriptive design for log construction is not provided by the local building codes. (But see Code Issue Update at end.) As such, the designer of a log home may need to hire a structural engineer. A list of issues the Structural Engineer must address follows. And some general discussion items follow the list.

Building Code and Site Considerations

Governing Code

Snow Load - Ground or Roof (and Exposure Condition)

Occupancy Live Loads (Residential, or ...)

Basic Wind Speed (Exposure Condition and Topography)

Seismic Design Category (or Ground Accelerations)

Soil Conditions (Allowable Soil Bearing Capacity)

Prevailing Frost Depth

Other (Energy Code, etc.)

LOGS

Species and Size

Green or Seasoned

Equilibrium Moisture Content Conditions (Anticipated)

Applicable Technical Information and Standards

Details by Log Provider (if Applicable)

Gravity Load Considerations

Roof

Determination of Applicable Snow Load(s) per Roof Slope, Surface, etc.

Drifting / Sliding of Snow onto Lower Roof Surfaces and Decks

Roof Sheathing / Decking

Roof Joists or Rafters / Purlins

BeamsAND/OR Log Trusses

Overbuilds

Eaves, Rakes (and Outriggers)

Columns

Settling Devices

Connection Hardware

Porches

Roof Sheathing / Decking

Roof Joists / Rafters

Roof Beams

Posts / Columns

Settling Devices

Connection Hardware

Walls

Headers (Log and Other)

Upper Floor / Loft

Sheathing / Decking

Joists / Purlins

Beams

Connections

Upper Decks

Decking

Joists

Beams

Ledgers

Posts / Columns

Connection Hardware

Main Floor

Ditto with Upper Floor / Loft

Decks

Roofs (See Porch Roof)

Framing (See Upper Deck)

Footings

Basement / Foundation

Walls (Concrete All Around or Daylight w/ Framing ... framing `Stick' or Log)

Footings (Perimeter)

Footings (For Columns and Interior Walls)

Lateral Load Considerations

WIND

Main Roof

Roof Fastenings (per Manufacturer)

Roof Sheathing and Nailing (Roof Diaphragm)

Load Path / Fastenings (into Log and Other Walls)

Porch Roofs

Bracing (Straps, Knee Bracing, or whatever)

Walls

Wood Dowel / Steel Screw Schedule for Log Walls as Shear Walls

Other Brace / Shear Walls (as needed)

Load Path (Fastenings)

Floor(s)

Floor Sheathing / Nailing

Fastenings (Load Path)

Foundation

Anchor Bolts

Other Foundation Connections

Wood Protection (Concrete/Earth Contact)

Site Grading

Crawl Space (or Slab on Grade)

Foundation Drain / Drainage

SEISMIC (Assuming Wind `Governs' ... in this case)

Anchorage of Beams, Girders

EARTH PRESSURES

Lateral Earth Pressure (Basement Retaining Walls)

Surcharges (Snow Drifts, Parking, etc.)

Other

Construction Drawings / Details - Provided by Contractor / Manufacturers as Needed

NOT CONSIDERED

Energy Code Requirements

Electrical Plan

Plumbing Plan

Mechanical Plan

Coordination of Plans

Construction Bracing

Non-Structural Framing

Site Stability

Site Plan

Discussion

Several main issues come to mind with regard to designing a log home residence. I list them and then expand on them a bit. And then after those I discuss some more issues.

1. Use of logs as spanning members (purlins, beams, rafters).

2. Use of log walls as brace / shear walls.

3. Added weight of log construction (no big deal as long as it's considered)

4. Shrink/swell of log walls.

5. Connections.

1. Logs as Spanning Members

Using logs as spanning members is not uncommon but takes a bit of out-of-the-ordinary number crunching. It will be necessary to obtain species and size information, from which we will get some design values. Calculations will probably be by hand unless we can do some `equivalent' size member number crunching with our typical engineering design software. Logs are not `magic' and cannot be expected to span great distances, despite their size.

Some Owners want to have log trusses. Fine. Log trusses will certainly require some `custom' engineering.

2. Log Walls as Brace / Shear Walls

Generally the course-to-course steel screws or wood (generally hardwood) dowels are counted upon to provide in-plane lateral load resistance for a log structure. Log-to-log friction is generally not counted on, nor are the interlocking corners (at least by structural engineers), though obviously both provide at least some resistance.

So, for bracing, we'll determine screw/dowel schedules.

3. Added Weight of Log Construction

This is no big deal, really; we just do it. Log construction would `kick us out' of trying to use prescriptive footing sizes. But prescriptive footing sizes would only be available for conventional light frame wood construction, anyway. If we are in an area of high seismic risk, we'll obviously need to consider the (added) weight of logs (over lighter construction materials).

4. Shrinkage / Swell of Logs

This is a big item. Not so much with regard to the logs, but logs with regard to every thing else. For example, a log wall would be made up horizontal logs. Wood tends to shrink/swell much more across the grain than along the grain. Hence, these logs would shrink/swell more `up and down' than they would horizontally. Fine, until we construct them alongside other things that don't. For example, stick framed walls have their wood members oriented primarily with grain directed vertically. So, if you believe what I said about the logs, and the same is true of the studs, then the log walls will be shrinking/swelling much more than the stud walls. Thus, we must let the log walls move up and down ... slip adjacent to the stud walls. This is no big deal with non-bearing walls. We just leave gaps, and then hide the gaps (with trim, or whatever).

If the stick frame walls are bearing walls, then we will need to provide some settling devices (that accommodate loads).

Likewise, if we have columns, we will need settling devices (that accommodate significant loads).

If our non-log walls also comprise some of the lateral load resisting systems of the structure, we need to come up with some way of carrying the load across these `gaps'. (Yikes!) (And, so, we'll probably use the log walls for brace / shear walls, and not any of the stick frame walls.)

5. Connections

Unlike for dimension lumber and engineered lumber and standardized timber products, we will not generally be able to go to the `hardware' (building supply) store and grab connections for our logs. As such, we'll need to design custom hardware or provide custom designed connections (probably with lag screws, thru bolts, etc.). In many cases the log package provider can give us a start on connections, but it will ultimately be the responsibility of the structural engineer for the connections. And these connections might be detailed connection-by-connection, or perhaps more generally ( ... `log beam end bearing at least 3 in. onto wall with two 5/8 in. lag screws each end' ... or something of the like).

Also

6. Snow

If our log home is to be in the `mountains' we may be facing gargantuan amounts of snow. A `primer' on snow loads is provided here.

7. Updated Code Issue

Apparently there is a log construction standard put out by the International Code Council (ICC), Standard on the Design and Construction of Log Structures, 2007. I do not yet have this document. And to my knowledge it has not been adopted into the local codes (in my immediate region of practice).

8. Log Building (Construction) Standard

The International Log Builders Association (ILBA) has a standard for Log Construction (here).

References

Snow Loads on Roofs and Decks , Associated Content

Standard on the Design and Construction of Log Structures, ICC 400 - 2007, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478, www.iccsafe.org.

International Log Builders Association, P.O. Box 775, Lumby, BC Canada V0E 2G0, www.logassociation.org.