lateral load design for log houses

[IslandEngineer]

Engineers,

I need to develop the structural requirements for the repair of a 1982 (2) story log house damaged by fire. The house is not a "kit" type log house; it was hand built. The 2nd story is a total loss and will be removed and subsequently rebuilt using standard 2x6 light frame construction. The 1st story, 26' x 32' x 9 ft (to the 2nd floor), has not been damaged except for the local (6 linear ft of charring) sections of the top 2 logs. The charring is approximately 3/8" deep (max)on these logs which is important as these are the continuous top intersecting linkage logs above doors and windows.

The log walls consist of 10-11 12" diam to 6" diam doug fir logs held together with interconnecting coved joinery at the corners and a series of 1/2" diam x 3 ft long staggered rebar driven vertically through the logs at 3-4 places along each wall. There is a minimum of 1 log vertical overlap at each stagger. These rebar are the core of my inquiry. I intend to use them to assist with the transfer of lateral load thru the walls. The foundation is in good condition - concrete footing and stemwall with the sill log attached with 1/2" AB at about 6 ft c/c.

I obtained the ICC 400-2007 Standard on Design & Construction of Log Structures but I can find no guidance except for Sections 403.1 & 404.2.2 & 406 which note the need for load tranfer but not the how.

Please give me your comment, advice and/or recommendation on transfer of lateral (and vertical) load through the above described log system.

Thanks for Your Input.
Island Engineer

 

[msquared48]

I know that there is a lot of dead load to these structures - designed them for years. I relied on thru-rods, top log to the foundation with allowance for settlement, for the lateral capacity. Normally had to use 3 to 5, 3/4" diameter rods per side, to get the required shear, normally working with 10-12" diameter logs.

I have heard of driving rebar as you describe, but never used it due to the problem with settlement allowance. Are the driven rods in oversized holes for the first log or two, with the lowest log driven into? This would have worked for settlement when constructed although it is not an issue now.

For the lateral capacity of the rods, just go through the lateral bearing equations in the IBC or AITC to determine the load values, I think there are 7 or 8 failure mechanisms to consider if I remember correctly.

I have not done a log home for about four years now. Many of the designers and suppliers are out of business. Hopefully things will change.

You might want to contact the International Log Builders Association in Canada too as they have a lot of useful design information.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RockEngineer]

It sounds like the home is a mix of several styles of log building. The Log Home Builders Association of North America (LHBA) teaches using 1/2" rebar for connecting the logs. I have engineered homes for them for almost 20 years along with other companies and log building methods. I use the NDS method for dowel bearing in single shear to determine the lateral load capability of the log walls as the failure method. When you use the rebar you often don't get much contact between the logs so you don't count on friction between the logs. I usually just check the narrowest section through the building (wall length minus all doors & windows in that wall line) and the base shear against the calculated allowable from NDS.

The LHBA doesn't usually use the notched corners so that is why I say it is a mixed method. There aren't any tests I am aware of that give values for any lateral resistance by the notched corners so I usually ingnore them.

When log builders use the thru bolting Mike was talking about the initial failure is usually slip between the logs until the rod in an oversized hole finally bears on the logs and the ultimate failure method is then wood crushing on the rod in single shear as I talked about above. With the rebar the hole is not oversized so all you have is the single shear of the logs and rebar.

 

[IslandEngineer]

Msquared48 & RockEngineer,

Thanks very much for your response.

Mike asked "Are the driven rods in oversized holes for the first log or two, with the lowest log driven into?" Answer is...per the builder, the holes where not oversized, the wood was "green" & the holes were drilled slightly undersized to the No. 4 (1/2" diam) rebar (7/16" diam drilled hole was the estimate). The No. 4 rebar was "pounded" into place "at a slight angle". Each rebar was about 3 ft long (max length of the "ship makers" drill bit)with the first rebar driven thru the first (3) logs above the sill log; second rebar is staggered about 1 to 2 ft away and driven through the next 2 - 3 logs and into the top log of the first 3 logs set in place. That staggered driven rebar process is continued upward through all logs.

The undersized hole and the green wood appear to have worked well as the wood appears to have tightened about the rebar as it dried. There are a few places where I have been able to verify the angle of the rebar (about 5 deg from vertical).

For lateral capacity of the rebar "dowels", both of you reference the Yield Limit equations. Rock Engineer advises using " the NDS method for dowel bearing in single shear" is just the path I intended to follow, checking the 6 yield modes of single shear in Section 11.3 of NDS 2005.

For lateral load development, I am in a Design Category D2 and am using Section 12.14 of ASCE 7-05 with the 2.5 overstrength factor applied (due to what I consider as discontinuous walls).

Thanks once more for your time and recommendation. I note that both Msquared & Rock are licensed in WA State. I too am a WA State engr and this project is on Whidbey Island.

Regards,
IslandEngineer

 

[msquared48]

For my edification here, being green logs with no apparent method for allowing for normal vertical settlement due to cross grain shrinkage of the logs:

1. Are the logs checked horizontally?

2. Has the chinking been replaced over the years, or in need of now?

You know, if the wood has shrunk to the rebar as you said, then the rebar are supporting the logs vertically from log to log, possibly with some space in between the logs. If this is so, that space will affect the lateral capacity of the rebarf and the structure, effectively lowering it. Just a thought.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IslandEngineer]

Mike,

You ask..."being green logs with no apparent method for allowing for normal vertical settlement due to cross grain shrinkage of the logs: 1) Are the logs checked horizontally? 2) Has the chinking been replaced over the years, or in need of now?"...and you note..."the rebar are supporting the logs vertically from log to log, possibly with some space in between the logs."

The construction sequence included using a chain and "come-along" to pull the "green" logs tightly together and down to the foundation prior to driving in the rebar. This appears to have worked well as the logs remain tight even as shrinkage (including cross grain) has taken place. The logs are not smooth round; they retain their rough surface including knots & undulations and have coved, interlocking corner notches (with rebar!), so Tom Haney's model in his 2000 article readily applies.

Chinking has been replaced but it is non-structural in my opinion.

There are a few log-log locations were knots etc create a space between logs that the rebar must span (max about 1.5 inches) but most appear tight. That is a great observation and I believe Yield Mode IV will develop the critical shear resistance figure. Looks like I may use some interior plywood sheathing panels (behind cabinetry) as necessary to supplement the lateral resistance.

Thanks for your thoughtful questions.
Jack H.