Blade columns? 3

[bart12]

I have been engaged in design of "blade columns", which is the term they used (probably an architect's term) for walls that were used for its thinner dimension compared to normal RC columns. I'd like to ask if anyone knows any specific guidance from codes or texbooks that specifically identifies the behavior of these types of "columns". I know its basically just an RC wall by design but the frequency of its use in our projects invoked me to think that there is more to be looked at on this types of columns.

 

[frv]

So isn't it just a load-bearing wall?

The only issue that I can see that doesn't apply to typical R/C columns is the consideration of slenderness.

 

[hokie66]

The term "blade column" is common where I am. It is used a lot in carparking structures and in residential medium rise buildings. I don't know whether architects invented the term or not.

Blade columns have to be designed using the provisions for walls unless they are thick enough for two layers of vertical bars, restrained by ties, in which case the column provisions can be used.

 

[rapt]

Blade columns were used a lot to get around fire rating rules in older codes (AS1470 in Australia). If the ratio was 4:1 or greater and smaller dimension less than 200mm, they used to be able to be treated as walls for fire rating. This is no longer the case, unless they are fully built into a masonary wall so that fire cannot reach all 4 sides.

In terms of strength design, they are normally columns to get the extra capacity from full design of a column compared to the simplified design method for walls.

 

[bart12]

Based on the way these "blade columns" were used in the project, its dimensions were dictated by wall thickness requirement (limited to 250mm) and span of the flat slab it carries (along its major plane).

From the design we've made, the "blade columns" were checked for the amount of lateral force it carries. Though we've found out (from calculations and analysis software) that they only carry less than 20% of the lateral force, it was quite fair to think or ask, what if they did carry substantial lateral amount of force? If they do, they no longer qualify as bearing type but instead are now shear walls. For zone 3 or 4 as shown in the UBC, special detailing is required for shearwalls though this is quite a task (and expensive) considering the amount of additional shear reinforcement for boundary elements.

Another difficulty is that these walls are offset all the way to the top in different lengths per story. In some cases, the wall below is shorter than the one above.

Thank you all for your replies.

 

[csd72]

Check the fire provisions as this is often the limiting factor. You may not be able to treat them as walls for fire rating purposes as they can have fire exposure on all 4 sides.

 

[asixth]

If the blade columns carry less than 20% of the lateral force, than what element is carrying more than 80%?

I have seen blade columns used to introduce a stiff element into the lateral force resisting system. For example, a suspended slab supported by 450 diameter columns (18") with blade columns around the perimeter to stiffen the structure laterally.