HSS Column extension 5

[Oussama Maham]

Howdy,
I am raising a roof from 28' to 35', and i am checking the possibility of using the existing column.
Hallow 8x8x1/4 taking 110 kips and sides moments of 15k' and 8k'.
It seems that only option here beside using an entire new column is putting a splice and between the existing 28' and new 7' column. How the existing 28' column be reinforced ?
Thank you

 

[r13]

Do an analysis on the 35' column and strengthen per resulting demand.

 

[canwesteng]

Easiest way to reinforce a column is often by bracing it at discrete points instead of adding plate to it.

 

[JLNJ]

You can CJP weld on the column extension (very carefully) but the resulting column will likely be too slender. It seems like you will need some welded reinforcements - at least partial length.

 

[Oussama Maham]

Thank you all for your responses. If i treat the entire thing as one column, the HSS8x8x1/4's axial capacity drops 75% (28'to 35').
when you say do an analysis, you mean to check for the limiting states per chapters E and F? and how do i determine where the reinforcement go and for how long?
Thank you

 

[r13]

That's almost like a new design for the existing column to stand 35' tall, from analysis, you know it might not work, instead of go get another larger section as in a new design, now you have to look at the entire structure, and to find where in the structure can provide help with modifications to absorb the additional strength requirement, and find ways to do it. After analysis, you have the reactions on hand, it becomes just a normal design task. Show us a cross section, maybe somebody can provide a hint or two.

 

[Oussama Maham]

 

[r13]

How much is the increase in gravity load? If not much, you might be use a stepped column approach - 7' new larger column with the original on top. The problem is how to manage structural demand in direction of the roof joist, which seems have no better way to brace it.

 

[spieng89]

If the roof is being raised and therefore removed (?) why not just put in a new column. The cost of steel would prob end up being cheaper than welding up an extension

 

[CANPRO]

I agree with canwesteng that adding bracing is the best option, if it is an option. Removing and replacing the column sounds easy enough, but now you're into foundation work (assuming foundations are ok for reno) - might be cheaper/easier to reinforce in place once you consider the foundation work.

See below for an option to extend. Looks like you have joists on top of the column, I would cut the column square just below the cap plate. You can place a new 7' piece on top of the 28', bearing plate between the two to transfer your vertical load, and then lap the sides with channels to transfer your bending moment and just as importantly - maintain bending stiffness across the joint so you don't reduce your buckling capacity. You might be able to get this done with channels (2) sides, your might need (4) sides. Worth talking to a local fabricator on this, you might be able to shop attach the bearing plate and channels to save some field work.

 

[r13]

The main concern is the direction along the roof truss (transverse dir.), as there is no room for bracing. Only after an analysis identified the magnitude of demands, it is very difficult to come up good ideas.

 

[Hokie93]

Is encasing some length of the existing HSS8x8 steel column in concrete (a cylindrical cross-section) an option? The length and diameter of the encasement would be determined from a stepped-column analysis. How many columns are we talking about? Have you investigated filling the extended HSS columns with concrete and analyzing them as composite? Adding lateral bracing is probably the most efficient option if geometrically feasible. You could weld on lengths of steel members (say, steel channels) to each face over some length (say, middle one-third) to increase the cross-sectional area and moment of inertia.

 

[mfrad]

Contact liftroof.com specialty contractor that uses hydraulic jacks to raise the entire roof system from the slab level below and placing new column sections. According to their web page they can increase warehouse heights to 100% of original height.[pre][/pre]

 

[r13]

Is there space available to sandwich the existing column with two shapes on each side?