log homes shearwalls 2

[gtcdesign]

How do one calculate the shear strength (plf)of a 6" log wall. Each course of logs are lagged together with 3/8" dia x8" screws @ 24" o.c.. What about the openings (doors and windows).An adjustment factor can be applied for perforated shearwall (framed).

What about holddowns at the end of it shearwall segment. There's not a corner post to attach a simpson connector.

 

[DaveAtkins]

You can only use the perforated shear wall method for a wall with plywood or OSB sheathing, so you cannot use it here (or for GWB shear walls, for that matter).

I would check the lags at the lowest log for shear and pullout. Use all the lags along the entire length of log, taking into account butt joints between logs.

You are correct that you cannot use Simpson holddowns, so 60% of the dead load on the shear wall must be able to prevent overturning of the shear wall due to lateral loads.

DaveAtkins

 

[nrguades]

I would like to add additional clarifications from this tread.

I've also seen details where logs are connected together with pipe 7/8" x 19 1/2" spaced at 96" o.c. and the shear is stated to be equal to 270(max.). Does it means that the pipes, determine the value of the shear for the log wall?

A 3/4" through-bolts c/w nuts and timber washer is install at each end of the logwall. Will this bolt take care the overturning moment acting at the wall?

thanks

 

[RockEngineer]

Most log walls are relatively low and long and have a lot of weight. They don't have as much problem with overturning as they do in racking shear. That is why you tend to look at a load per length along the walls and then size some type of connection (thru bolt, spike, pipe, dowel pin) that will transfer this load and keep the logs from sliding relative to each other.

Even in framed housing there is usually a lot more problem with racking of the wall than there is with overturning unless you have a high aspect ratio where the wall is much taller than it is long.

 

[nrguades]

thanks RockEngineer for the information, but im concern also of the topmost logs to overturn if it is not properly connected to the log below. I guess it is alright to consider the resultant weight of the log to resist the overturning effect of the lateral loads and uplift forces if the logs will be acting as one solid wall. But in the case of the topmost log, there is a possibility that the deadweight of the log multiplied by at least 60% will not be sufficient to resist the overturning effect and the uplift. thanks...

 

[RockEngineer]

Uplife is one of the reasons I haven't liked the drift pin method because the logs are not mechanically tied together. If you use full height rods attached to the foundation or bottom log, lag screws, pounded in rebar or spikes there is some mechanical resistance to uplift because the connectors between logs. If you try to stack a bunch of short logs on top of each other to make a wall section you will tend to get buckling out of plane. That's another reason for having top logs that run the full length of the structure and limiting window sizes in log homes. You see a lot of log home designs with huge windows but in those areas you need to use a lot of caution in the design

 

[nrguades]

tnx RockEngineer.. But I would like to know where do you usually place your through-bolts, are you placing it at every end of each panel same as placing simpson's holddowns? Or are your placing it at certain spacing. Or are you considering it as a whole panel with openings.
thanks

 

[RockEngineer]

Thru bolts are placed at spaced lengths usually about 4 ft o.c. Where there are openings which block that equal spacing you place the bolts on both sides of the opening. There is usually a thru bolt in the overhang at the corners just because it is an easy place to be able to adjust it.

Some designs have the thru bolts on springs or other automatic tensioners to allow for wall shrinkage.

Having the bolts near the corners and on both sides of openings takes care of the uplift similar to the Simpson ties on framed construction.

If you look at a lot of old log structures they have weathered many wind storms and earthquakes and many have never even been tied to the foundation. As a engineer you need to follow your numbers though. It can give you a little piece of mind to know that there are forces at work which hold your structure together that you are not taking into account in your calculations. It is difficult to account for friction between logs, notching and all the other things that go into a well built log home.

 

[nrguades]

thanks again RockEngineer for sharing your expertize and experience with regards to log homes construction and design.

I would just like to clarify again about the 4 ft spacing of the through-bolts, it seems to me that the bolts are so closely spaced. Is this because your also designing the bolts to carry also the shear forces? Or are you placing this together with your lag bolts,or maybe dowel or pipes?

 

[RockEngineer]

The maximum spacing of the thru bolts is dependant upon your shear stress. The actual spacing is more often based on where your openings are as long as you don't exceed the maximum.

If you are using lag bolts or spikes or pounded in rebar you usually will not include thru bolts. You will just attach the bottom log to the foundation and let your lag bolts, etc, provide your shear resistance between logs and the uplift resistance.

There is another thread in eng-tips.com about log home shear wall design which may help. Thread337-25324 I believe there are some others too. In this thread I think I was a little confusing about dowel bearing. If you use the bolt bearing strengths and all the additional factors for wind and seismic you will get a reasonable spacing for you bolts, screws, spikes or rebar. Look at the NDS for bolting and the formulas gives you some extra shear for having relatively long bolts and thick members connected together.

 

[nrguades]

Actually, what I am thinking now is the combination of the two anchors to make the design more simpler and maybe perhaps more economical. I will just let the pipes or dript pins carry the the shear load and let full height rods carry the overturning moments and uplift at the wall ends and corners. thanks again

 

[nrguades]

In using pipes, I observed that what governs is the bearing strees of pipes on the log not the shear stress developed in the steel.

 

[RockEngineer]

I haven't used the pipe method but unless you have the pipes tight in the hole full height of the wall, the pipes will not bear unless there is some movement in the logs. This means you are relying on the friction between the logs most of the time and the pipes are only there to resist ultimate failure when you have overcome the friction between the logs and get enough movement that the pipes will actually bear on the wood. This is not bad. It is the general idea behind both the long thru bolts or rods and drift pins and even lag bolts when you oversize the hole in the upper log to allow for settling. No one ever talks about it though. It makes a lot of engineers nervous. This is one of the things that makes log home engineering a challenge. If anyone could do it and was willing to stand out there and engineer log homes without a lot of full size home shear wall tests it wouldn't be such a challenge.

I laugh when I talk to engineers for big companies when they tell me they can't make any changes to their design or provide engineering without full scale tests to see how it would perform no matter what their numbers tell them. I have very seldom had the luxury of full scale tests. Log home builders don't have that type of money and aren't that organized. They also use too many methods which react differently. Use your brain and a little common sense. Look at how things really work. Take advantage of all the additional factors the codes and NDS give you. If you get too conservative you won't have any clients and it will be too difficult to build.

Good Luck and enjoy.

 

[boo1]

Consider reviewing the Rough Draft — ICC Standard on Log Construction at:

http://www.iccsafe.org/cs/standards/is-log/draft060105.html

 

[bylar]

I live in the mountains of NC and we have a lot of log homes constructed and most of them have no engineering. I have not heard of any failures. That is not to say that they don’t need engineering. We also have areas that require designs for winds up to 130 MPH.

Remember with all log homes there is a great deal of shrinkage (up to 1-1/2” in 8’ high wall) and the design has to allow for this. So if you have a thru bolt it has to be tightened as the wall settles. There is a builder in Pigeon Forge TN. That has a great idea. The threaded rod has a high strength spring between the top log and the Nut. This maintains tension on the rod even after shrinkage. I believe they space the rods at 48” o/c.

 

[RockEngineer]

Bylar,
The Canadian log building standards published by the International Log Builders Association recommend allowing for up to 1/2" per foot settlement when building with green logs. That would be 4" of settlement in an 8' wall. I have had some builders who recommend designing for up to 3/4" per foot settlement.

There is one method of log home building I know of that doesn't have the wall settlement. The Log Home Builders Association of North America teaches a butt and pass method where 1/2" rebar is pounded from one log into the log below every 24". The logs shrink toward their centers so the gap between the logs gets bigger but the wall height does not change. It is very labor intensive and must be used with chinking. It has lots of shear resistance due to the rebar. This method is popular with people building their own log homes because it doesn't require a lot of skill just lots of hard work.

For those engineering log homes the link Boo1 gave to the draft ICC log home standards is one of the few places you can get the allowable stresses for TPI graded logs without going directly to TPI. TPI is one of the few timber grading societies that grades full logs for log homes or other uses so engineers have an allowable stress to work with. Using sawn timber values from WWPA or NDS is not always practical for logs because WWPA and most grading societies don't grade the raw logs. They grade the sawn timber.