Steel to Wood Connection Design Help Needed...

[rulljs]

I work mainly in the structural steel world so the steel analysis of the attached sketches is not a problem, when it comes to the interaction with the wood is where I get a bit confused.

I believe my main concern with this connection would be the bolts ripping/tearing out of the wood, (as long as all the steel strengths are adequate to carry the load), but I'm not sure how to find out how much load can be put on the bolts before the wood fails.

I've attached two .pdf files, (tried to at least) to further detail the situation I have. Any input will be appreciated.

Thank You.

 

[csd72]

I suspect that mike is right in that this is an architects details rather thatn an engineers one.

I do believe that it might be okay if it is only lightly loaded though, but such details are often difficult to justify.

Is there another load path for lateral stability that would negate the need for the lateral load into the column?

A few general rules in timber design:
1. The steel sections are virtually never critical (if they are then you are possibly not analysing it the way it actually behaves).
2. bolt capacities in douglas fir are only about one kip for 3/4" (dont quote me on that but this gives you a rough ballpark and also explains rule 1.)
3. Edge distances usually dictate bolt layout, size e.t.c. that can be used for any given connection. These are much higher than for steel.
4. There is no substitute for a fully copy of the timber code.

 

[msquared48]

Is this a column for a pole building?

Mike McCann
MMC Engineering

 

[msquared48]

If this detail MUST resist a lateral force, then a much better detail would have been to use a tube section one to two feet long in place of the side tabs to the wood column. The wood column could be trimmed for a tight fit into the tube member and enough thru-bolts added to resist uplift. The walls of the tube section would resist the lateral load in bearing with the length of the tube section sixed to limit the bearing stress on the wood via leverage. The same smaller tube section could be welded to the top plate too.

So much for water over the dam though.

Mike McCann
MMC Engineering

 

[bdSE]

The good news, even if the detail had been done "properly" as suggested above, the column would still not have taken lateral load of any consequence unless the base of the 6x6 is fixed against rotation. It is rare to get a true moment connection in wood design. Likely the base and top of the column were designed as pin connections carrying mainly vertical loads.

The bad news, you've got an inturrupted column supporting your structure, yuk.

Be sure to keep trying to load the pictures, this type of situation can be acceptable or quite bad depending on the application and surrounding framing.

 

[hston284028]

.pdf

 

[hston284028]

Sorry for the long delay, for some reason I was unable to get logged on for about a week, I sent quite a few e-mails to eng-tips support with no response, finally just joined under a different name. I just posted a .pdf of a few pictures to give a better idea of what's going on.

Hope the pictures give a better explination of what's going on.

Thanks for all your replies so far!

 

[TehMightyEngineer]

Well, I can see why that caught your eye. I second what damronb said, these columns are unlikely to see any lateral bending unless they're fixed at the base. I'm guessing they're only going to see axial loads.

P.S. That also happened to me, I had to create account after account and each one kept having problems. I just rejoined again last week after a few months away, hopefully this account stays put.

EIT with BS in Civil/Structural engineering.

 

[bdSE]

These are good pictures to show the connection, but the rest of the story is important as well. This looks to be a roof structure of some kind. Is it free standing (rephrasing MMC's question about it being a pole structure)? Is it connected back to a more ridged structure like a home or retaining wall or something like that?

If there is no other structure restraining the lateral forces this design could fail in a strong gust. This is not due mainly to the segmented column detail, but due to the fact that nothing else is resisting the lateral loads. The pictures only seem to show gravity loads resisted by the columns, but I can't see everything so my assumption is reserved for the rest of the information.

 

[Ron]

As Mike noted before, this is a hinge in the column. Depending on the wind load, I think you have a problem. If the wood column is fixed at the bottom (unlikely but possible), then you'll have a significant couple at the two through bolts in the top of the wood column.

Since this is essentially an open structure, if the wind blows from the left in your picture orientation, you'll produce a significant load in the lateral direction to the right.

Consider knee braces at least at the outside columns.

 

[BAretired]

You have two details at the top of the HSS...four bolts where the beams abut and two bolts where they are continuous. In the four bolt connection, there does not appear to be enough distance from the bolt to the end of the member.

At the top of wood post, even a slight moment would tend to split the wood at the bolts...a very bad detail.

Remedial measures would seem to be required in this structure.

BA

 

[hston284028]

The structure is free standing and is not connected back to a rigid structure, that is what had me concerned.

I'm not sure exactly what is meant by a pole structure but this particular roof is supported by a grid of 16 columns, 4 columns by 4 columns, the roof is sloped in one direction. Supported by drilled shaft type foundations (don't know the details of the concrete).

 

[BAretired]

The columns in a pole structure extend down into the soil, i.e. the column and pile is one piece. If you have concrete foundations, you don't have a pole structure.

Is there any possibility of introducing some diagonal bracing in the exterior frames?

BA

 

[msquared48]

BaRetired:

Depending what he means by this "Supported by drilled shaft type foundations", it may in fact be a pole structure if the columns extend unbroken into the drilled piers.

Is there any way to post a detail or picture of the column base here?

Mike McCann
MMC Engineering

 

[hston284028]

I say "supported by drilled shaft type foundations" because I wasn't able to see the foundations (they are under the a deck) but I was told they were drilled shafts, I really don't have any further details on them.

If I had to guess I would think that the wood columns extend into the concrete shafts, therefore being a "pole structure".

 

[bdSE]

This project will need remedial work.

There is insufficient lateral capacity in the columns, at the column to roof connection. If indeed this is a pole framing configuration or the post sits on top of the foundation I believe the remedial work would be similar if not exactly the same.

Without looking at the whole project I think the suggestion earlier to provide knee (diagonal) braces at the exterior columns may be your best approach.

Don't waste your time trying to make the current connections work, even if they "calc out" you are still left with a sub-par design. Although, if they "calc out" you won't have to spend any more time or money on the project.

See sketch below for one idea on how to brace the existing structure. Of course there are many ways to do this connection well, this is just a starting place for you if you want to do any remedial work on this project.

 

[BAretired]

Knee braces are not sufficient if the bottom of the wood columns are hinged.

BA

 

[msquared48]

You need to identify if the structure is a pole building or not, performing whatever non-destructive inspection techniques are necessary, but to include a local dismanteling dismanteling the deck if required.

This information is critical to the ultimate solution.

Mike McCann
MMC Engineering

 

[bdSE]

Good point, if hinged at the base the knee bracing is not sufficient.