Reinforced concrete column subjected to impact

[fa2070]

Hello,

I have to design the corner column (reinforced concrete) for a two-story house at the crossing of two streets, C1 in the picture specifically.

If loads controlled the design, it would be a 20cm x 20cm column with #4 bars, like C2, etc. However, corner columns are given a special consideration due to the potential risk of being impacted by vehicles, which takes precedence over the DL + LL static analysis. It is not uncommon for corner columns of ordinary houses to have a diameter of 50cm or more (or 40cm x 40cm if square) and be heavily reinforced as well. I skimmed ACI 318-08 but didn't find anything in this regard.
Now, my questions are: What is the common practice for designing reinforced-concrete corner columns subjected to the (potential) impact of vehicles? What is the detailing like? Which shape is more convenient, square or circular? Can the longitudinal reinforcement be spliced in these columns? My intuition says it shouldn't... Is it sensitive to use spiral rebar for shear?


Thanks.

 

[JLNJ]

Do they tell you how big the vehicle will be or how fast it will be going?

 

[fa2070]

cars..vans... normal urban traffic...
Beyond municipal codes, which, by the way, are non-existing in this case, I'd like to know what the criteria are for designing such columns.

Thanks.

 

[unclesyd]

Some of the structural guys will probably shoot me down but based on personal experience a circular column, if aesthetically correct, gives the oncoming vehicle a better chance of hitting it with a glancing blow.


Anecdotal:
I have a friend who lives on curve where he looses his mailbox, 8' off the road, about twice a year. The last time he rebuilt it he made the inside a reiforced concrete post 2'x 2' x 4' deep. It was hit about 2 months after he finished and it tore up one car and sent the driver to the hospital. The driver had a blood alcohol twice the legal limit. He is now suing for $500,000 for lost of work, companionship, pain and suffering, etc. The suit says that the mailbox was not of normal construction and the post was deliberately setup to hurt someone.

 

[asixth]

Houses, columns get hit all the time by vehicles (even if the drivers aren't under the influence). I would bulk it up to 500 square with nominal 1% reinforcement.

Can you define what reinforcement ratio constitutes "heavy" reinforcement.

By the way, what CAD program are you using and how would you rate the useability?

 

[MiketheEngineer]

I might consider reinorcing the beams so that they possibly survive if the column was completely taken out - Some call this redundant engineering - big in blast analysis where the loss of one column does not doom a structure.

 

[civilperson]

A pair of bollards to absorb collisions and normal size column is one solution. 6" diameter pipe buried 4 feet in 12" diameter concrete filled holes protects building columns from forklift trucks. The bollards are sacrificial and must be replaced after each major collision.

 

[msquared48]

I agree with civilperson - use bollards to protect the column. If it makes you feel better, put some extra reinforcing in the column too, but rely on the bollards to protect the column.

Mike McCann
MMC Engineering

 

[fa2070]

Thanks everybody.

Can you define what reinforcement ratio constitutes "heavy" reinforcement.

According to a few local contractors, they build these columns 40cm x 40cm with 16 #5 (A_s/A_c=0.02). However, that's not the most widespread pattern I see. I see corner columns as slender as 20cm x 20cm and as thick as 60cm x 60cm and everything in between (for like DL and LL).
My concern is, if something goes wrong and people get hurt, I cannot say to the judge "well, I designed it that way because of word of mouth from local contractors.." I'd better have a more formal and professional answer.

By the way, what CAD program are you using and how would you rate the useability?

I'm using Autocad 2006 with its built-in 3D & rendering functionalities. When needed, I download textures (plain GIF/JPG/BMP files) from the internet. Not the killer user experience, but that's the only package I know. There are many 3D authoring apps that I would like to learn, some commercial (e.g. 3D Studio Max), some free (e.g. Blender). Too many things in the TO-DO list and too little time unfortunately. I do post-processing (like adding dimensions and text) over the rendered model with Photoshop.

I might consider reinorcing the beams so that they possibly survive if the column was completely taken out - Some call this redundant engineering - big in blast analysis where the loss of one column does not doom a structure.

What would you change in the following design ? Too much congestion at the encounter of both beams doesn't leave room to interweave the rebar. Does it make sense? (see pics.)

Thanks.

 

[civilperson]

If you design for every possible contingency, then you will be out of business quicker than the client can see your design compared to the competition. Follow the code, it is considered "good judgement".

 

[fa2070]

Follow the code, it is considered "good judgement".

But I don't even have a starting point on this very subject...

 

[UcfSE]

In the absence of other recommendations, I would take steps to provide as ductile a column as reasonably possible. For instance, provide extra ties for confinement. Doing this for the beams tying into the column may be a good idea also. Keep looking for load information. Using impulse and momentum to arrive at a design force may be possible, at least to a certain degree, though I wouldn't want to go that route.

 

[civilperson]

No requirement for errant vehicles exist in the code, you are imagining things which are beyond the norm. Why not a 747 jet crash or a terrorist bomb? The motorist that hits your column is liable to fix the damage, not you1

 

[gwynn]

You may find some reasonable numbers for impacts in AASHTO or other applicable highway codes, as these are where vehicle impacts are generally considered. For example, I believe CSA S6 provides means for calculating vehicle impact loads.

Impulse and momentum methods can be overly conservative, as it is a fair bit of work to look at anything other than a direct head on impact.

 

[fa2070]

Thanks everyone!