Unbraced Length of a Column

[KOTOR1]

I have situation where a steel lintel (wide flange) supporting reinforced masonry is supported by a steel column in the middle due to the length of the steel lintel (steel lintel is continuous over the column). Will the column be designed with K=1 in both directions or should column be designed with K=1 in one direction and K=2 in the other?

My boss says that since the masonry is reinforced and is braced back (steel angle - 2'-0" above steel lintel) to the joists as well as designed for out of plane bending and braced at the top with the deck, K should be one in both directions.

 

[XR250]

I agree with your boss as long a the beam is braced properly by the masonry.

 

[KootK]

I also anther with your boss.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I also anther with your boss.

OK KootK, since I never heard of the word, "Anther", I googled it.

"the terminal part of a stamen consisting usually of two lobes each containing two sacs in which the pollen matures"


Was that a typo or do I need a better dictionary?

 

[KootK]

I stand by the original statement. The effect of floral genitalia on colun buckling is often not fully appreciated by designers.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IFRs]

"colun" - a beach in southern Chile located south of Corral and west of La Unión. The beach is about 9 km long and runs like many other Chilean beaches from north to south along the Pacific Oceans shores. In a ranking made the newspaper El Mercurio in 2006 Colún was considered the most hidden beach in Chile. The beach has large sand dunes in its southern part and lies west of two freshwater lagoons. Most of the beach is inside the Valdivian Coastal Reserve.

 

[KootK]

Well, yeah. Many a Chilean beach ecosystem has buckled as a result of the pressure that dwindling anther stocks exert on indigenous avian populations. It's not as though flowers have anything to do with post instability.

Speaking of post instability, assuming K=1 for the post implies a lateral bracing demand at the top of the post that would need to be supplied by the supported beam. And that would be additive to any lateral torsional buckling restraint requirements at the post.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KOTOR1]

The steel lintel is designed as completely unbraced. I am not assuming the lintel is braced by the masonry. I will assume K=1 in both directions for the column design.

 

[KootK]

Your beam will require rotational restraint of some form where it passes over top of the post.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KOTOR1]

KootK
Would stiffeners plate at beam web above column provide rotational restraint? That was one of the reasons I wanted to use K=2 in one direction.

 

[KootK]

Often stiffeners will do the trick. It's s difficult thing to asses analytically however. How wide is your beam and column (perpendicular to the beam)? Connection via bolted cap plate? In these situations, I typically have rebar coupled to the top of the beam and lapped with the wall bars above. It is a constructability pain of course.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KOTOR1]

The beam is 8" wide with a 4" column and we are using a bolted cap plate. I have automatic welded studs at 24" o.c. with a grouted bond beam above.

 

[KootK]

Sounds like bracing the beam via connection to the wall may indeed be the way to go.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[albanyYEM]

My opinion is that your K value is probably somewhere between 1.0 and 2.0, but likely much closer to 1.0. I agree a web stiffener at the column location is a good practice, as well as welding dowels/rebar to the top flange of the beam. Seems like a judgment call evaluating how much that K value actually effects the design of your column.

 

[JAE]

I don't think vertical web stiffeners do anything for you here.

What we do is weld a vertical A706 rebar to the top flange of the beam and extend it up into the block in grouted cells.

This creates a rotational resistance for the beam against twisting. KootK is correct above to suggest that any bracing supplied by the beam/wall for the column should be considered in the design of the beam/wall.
The wall will need to resist the beam twisting through bending in the wall...probably not a big deal.



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[KootK]

What we do is weld a vertical A706 rebar to the top flange of the beam and extend it up into the block in grouted cells.

I've been doing this for a long time. It's a detail that I inherited from my mentors. I've never seen one partially constructed, however, and I've always wondered about the constructibility of it. It seems to me that coordinating the bars to hit the masonry cells would be almost hopeless. And if you missed them, you'd end up having to knock out the webs of the blocks for who knows how many courses above the beam. Can anyone comment on this aspect of it?

Some more thoughts on K=2:

1) As described here, it would imply a fixed base connection.
2) Designing the column for K=2 would not, on its own, be sufficient to prevent rollover in the beam at the interior bearing. One would need to ensure that the column was also strong and stiff enough at the top to serve as bracing for the beam.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KOTOR1]

I do have 6" long 3/4" welded studs at 24" o.c. at the top flange of the beam. The masonry course above the top flange is a bond beam. The vertical rebar of CMU does start from this bond beam. In addition, the lintel beam is designed as completely unbraced for LTB.

 

[KootK]

In addition, the lintel beam is designed as completely unbraced for LTB.

This continues to make me nervous. You mean braced for LTB at all three supports, including at the interior post, but nowhere in between, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KOTOR1]

Correct. The span of the beam is considered from exterior support to interior support. I have wall braces going to the joists at 8'-0" o.c. These braces are located 2'-0" above lintel. I dont know if that can be considered as bracing.

 

[KootK]

So the interior support LTB bracing is provided by some combination of:

1) Stiffeners establishing torsional-flexural connection between beam and column and/or;
2) Studs establishing torsional-flexural connection between beam and wall?

Just want to be sure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.