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Steel to Wood Connection Design Help Needed...

Steel to Wood Connection Design Help Needed...

Steel to Wood Connection Design Help Needed...

(OP)
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.

RE: Steel to Wood Connection Design Help Needed...

Nat'l Design Spec for Wood Construction from Amer. Wood Council  This will give numbers.  There are several failure modes to be checked.

RE: Steel to Wood Connection Design Help Needed...

Do you have a copy of the NDS?

RE: Steel to Wood Connection Design Help Needed...

I would also note that your loads (which I'm assuming is only one load) are perpendicular to each other.

RE: Steel to Wood Connection Design Help Needed...

A couple of questions first...

1.  Why is the steel section in there to begin with?  Is the wood column existing and too short?  Are you modifying something or is this new construction?

2.  From a design standpoint, this creates a potential lateral load deficiency, with as you suspect, high bolt loads depending on the magnitude of your lateral load.

You can find all the allowable loads for the wood and the fasteners in the National Design Standard for Wood (NDS).  If you do not have a copy, you can purchase electronically from the American Forest and Paper Association.

RE: Steel to Wood Connection Design Help Needed...

(OP)
I do not have a copy of NDS.

The reason for the steel section is for aesthetic purposes only.

One side note is that I am not the engineer responsible for this design, it is something that has been built already and it looked a little suspect to me so I just wanted to run some numbers to convince myself that it is structurally sound.  I have been assured that this has been looked at by an engineer, but again, the design looks a little suspect to me and I just wanted to run some quick numbers to see what loads it is capable of handling.

RE: Steel to Wood Connection Design Help Needed...

(OP)
StructuralEIT

The loads shown on the sketches are depicting two different load scenarios...wind from the East and Wind from the South.

RE: Steel to Wood Connection Design Help Needed...

The NDS provides minimum edge distances from center of bolt to edge of wood.  You don't provide those dimensions on your sketches and these might be critical checks to do.

Is there a cap plate on the 4x4 tube?  I would think so as the welds you show between the upper "U" bracket and the 4x4 would be little welds on the tube wall edges otherwise.

 

RE: Steel to Wood Connection Design Help Needed...

(OP)
Can anyone see the first two pictures, I cannot see them when I click on the link.  They are in .bmp format.

RE: Steel to Wood Connection Design Help Needed...

I can't see them - the file that comes up on Engineering.com just has the word "pic" instead of a usual filename.jpg.

Are you uploading jpgs?

 

RE: Steel to Wood Connection Design Help Needed...

You must be uploading it wrong - every file that Engineering.com refers to in your posts says "Pic".

Why don't we see .jpg or .pdf?  Are you browsing to the correct file?

 

RE: Steel to Wood Connection Design Help Needed...

Still nothing.

RE: Steel to Wood Connection Design Help Needed...

(OP)
ok..maybe no pictures until I can figure out how to post them correctly.

Before I post the attachment has the full name with the extension (.jpg/.bmp/.pdf) but once it gets posted it just says pic??  I may have to post pictures through a third party picture hosting site which I cannot do right now but may be able to do in a few hours.

RE: Steel to Wood Connection Design Help Needed...

Try converting pics to pdf and upload again.

RE: Steel to Wood Connection Design Help Needed...

If you are attempting to create a moment connection between the wood beams and the 4x4x1/4 tube, you need to be concerned about the distance from the bolt to the edge of the wood member which, as you have shown it in "Looking West" appears to be much too small.

BA

RE: Steel to Wood Connection Design Help Needed...

I cannot understand why a moment connection would be required here to resist the lateral load with this connection.  That should be resisted with some other element regardless of the direction of the load.

That being said, I do not like the connection at all as it is a hinge connection in a column, inherently unstable, regardless of the length of the tube column.  The wood column should have been extended to the underside of the beam and framed with a beam to column hanger.  

Sorry to say, but this appears to me to be an example of an archectural detail gone awry and unchecked by a structural engineer.

Mike McCann
MMC Engineering

RE: Steel to Wood Connection Design Help Needed...

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.

 

RE: Steel to Wood Connection Design Help Needed...

Is this a column for a pole building?   

Mike McCann
MMC Engineering

RE: Steel to Wood Connection Design Help Needed...

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.  sadeyes

Mike McCann
MMC Engineering

RE: Steel to Wood Connection Design Help Needed...

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.   

RE: Steel to Wood Connection Design Help Needed...

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!

RE: Steel to Wood Connection Design Help Needed...

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.

RE: Steel to Wood Connection Design Help Needed...

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.  

  

 

RE: Steel to Wood Connection Design Help Needed...

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.

RE: Steel to Wood Connection Design Help Needed...

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

RE: Steel to Wood Connection Design Help Needed...

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).

RE: Steel to Wood Connection Design Help Needed...

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

RE: Steel to Wood Connection Design Help Needed...

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

RE: Steel to Wood Connection Design Help Needed...

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".

RE: Steel to Wood Connection Design Help Needed...

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.

  

RE: Steel to Wood Connection Design Help Needed...

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

BA

RE: Steel to Wood Connection Design Help Needed...

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

RE: Steel to Wood Connection Design Help Needed...

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

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