Stacked HSS Columns at Steel Stair Tower

[courtnvm]

I have a 6 story stair tower which has a stacked HSS column the whole height of tower. The columns are intersected by the landing members. In essence what I have is a HSS6x2x3/8 column and MC12x10.6 landing member. The HSS6x2 columns have end plates at each end and are welded off to the MC12x10.6 landing member. I was thinking that there was some sort of guidelines for stacked columns. I have never used stacked columns like this before. My bottom column has an axial load of 37 kips.

Do I assume that the axial load is transferred through direct bearing on the channel flanges with stiffeners? Do the welds have to transfer any of the axial load or is there a percentage of the axial load that the welds have to transfer? Any help would be appreciated.

Val

 

[flyby911]

in my code (the egyptian code of practice for steel structures) it assumes that 60% of the axial load will be transfered through the weld and 40% through direct bearing and the weld is designed to resist the 60% of the load

p.s :the whole weld area (flanges & web) resist the axial load not just the flanges weld

 

[271828]

Assuming that I'm understanding your problem, you need to design your channel web / stiffener combo for the cruciform (12tw or 25tw) section from AISC 2005 J10.8. You should probably put stiffeners under each face of the HSS column.

As for the weld, are you asking about designing the stiffener-to-channel flange weld? If so, then you need to design it for the load also from J10.8. If your web crippling, web yielding, and web buckling strengths are high enough, you might not have to design the weld for much, if any, load.

 

[jike]

I have several questions. Why aren't the columns continuous and the landing members framing into the sides of the column? This is more typical of what I have done and have seen others doing. Why are the columns such an odd size (6x2)? Why not square?

 

[courtnvm]

Well originally the columns were designed as continuous, but the contractor did not locate the stair tower walls correctly, therefore geometry concerns come into play. In order to get the correct stair widths to meet code egress requirements the columns had to be stacked. My original question was regarding the weld between the column and the channel flange. Does the weld have to transfer 100% of the axial load or does the column transfer all of the axial load thru bearing on the channel flange?

 

[271828]

The column to channel flange weld doesn't have to be designed for the weld if it's the end is finished like any other column. Think about a huge W-shape column welded to a base plate with minimum fillet welds--same idea. Somewhere in the AISC Manual is guidance on this, but I forgot where.

 

[dik]

I'd typically design the column sections so that they were stressed to about 70% just to avoid complete penetration welds and grind these smooth to accept primer touch up and top coat.

The channels would previously be welded to the face of the column, wherever they occur. Stair stringer and landing loads are generally small compared to the column or channel capacity and the attachment forces are generally small, even for thin wall HSS sections.

Dik

 

[jike]

Just like gravity column splices, load is transfered thru bearing if the members are finished properly so that uniform bearing is achieved. In column splices, there is an AISC requirement to shim any gaps as the columns are pulled into plumbness. How do intend on shimming with this stacked system?

I would be concerned about the overall stability of the whole system when having columns stacked. You are creating knuckles (pinned condition) at the top and bottom of the landing stringers.

 

[courtnvm]

jike, the landings are supported at three corners using stiffened angle seats welded to steel embed plates and the fourth corner has the stacked column. As of right now, the fabricator want to only weld the 2" side of the tube to the channel flange, which would only give me 3 inches of weld. I am tempted to make them weld the accessible 6" side with a partial penetration weld for stability reasons.I know this is not the first time columns have been stacked at steel stair towers for geometry reasons.

 

[courtnvm]

Okay, I found the AISC specification regarding bearing joints for compression members. Spec section J1.4 states the following:

"When columns bear on bearing plates or are finished to bear at splices, there shall be sufficient connectors to hold all parts securely in place.

When compression members other than columns are finished to bear, the splice material and its connectors shall be arranged to hold all parts in line and shall be proportioned for 50% of the required strength of the member.

All compression joints shall be proportioned to resist any tension developed by the factored load combinations stipulated in Section A4"

This statement was taken from AISC 3rd Edition. Other editions are similar to this wording.

 

[csd72]

courtnvm,

The stiffeners will be about 1/8" shorter than the distance between the flanges , the welds therefore need to be designed to take the whole load.

You also need to check the length of 'stiff bearing'at the unsupported end of the channel flange between the stiffeners. Use this to check the stress at the base of the column.

Regards

csd