Steel unbraced lengths

[penpal97]

2 different questions on same topic

Lets say I have a typical 2 story building, 4 columns (1 at each corner). There are floor stringers from column to column, floor beams from floor stringer to floor stringer.

1) Is a typical shear connection w/ clip angles of the beam to the stringer enough to consider the stringer compression flange to be laterally braced? Assume beam is coped so top of steel is same for beam and stringer. What if the top of beam is lowered 3" below top of stringer? Does this require that the stringer be studded to connect to the concrete deck or welded to the SIP forms?

2) The corner column supports 2 stringers: 1 framing into the web, 1 framing into the flange. Is the unbraced length of the compression flange (that does not support a stringer) 1 story or 2 stories?

 

[Lion06]

1) a - yes; b - depends on depth of stringer, but most likely stil yes

2) as long as the beams are connected to the diaphragm (which, in turn, has a lateral system to work with), then the column is braced in both directions (i.e. the unbraced length is 1 story)

 

[penpal97]

Thanks StructuralEIT.

Could you expand further on 1). When and why is it considered braced? Does this all assume connection of some sort to the concrete?

2) I understand the column is globally braced in both directions (say for axial compression). However, what about checking bending capacity? Is the unbraced length of the compression flange 1 story or 2 story?

 

[Lion06]

1) You can read about bracing in appendix 6 in the 13th edition steel manual, but basically the clip angle provide some rotation restraint (against lateral-torsional buckling) about the axis of the member (because even though we assume it to be a shear only connection it does have some nominal restraint. It doesn't need to be connected to concrete, but you can get a brace point if you have studs into a concrete deck and provide full depth stiffeners right under the studs.

2) Follow the same logic as for 1) above with your specific details to determine whether it is braced or not.

 

[penpal97]

Appendix 6 is very informative. Thank you.

 

[GalileoG]

I'm slightly confused, would appreciate it if someone would clarify this matter to me:

For the beam that spans into the web of the column, say that it is connected using a single shear clip angle to the web, wouldn't the leg of the angle brace one flange (assuming it is within 3") when it acts in compression? Are double clip angles often used in order to brace both flanges of the column in case the other flange acts in compression?

 

[penpal97]

I would guess the 2 clip angles brace the 2 column flanges. 1 clip angle would only brace the 1 adjacent flange.