Impact Loads for Warehouse Columns? 5

[BTStructural]

Working a case where a forklift collided with an HSS column in a commercial warehouse for general merchandise (not government warehouse or super sensitive materials inside) and the roof came down. Is there an impact load that the columns need to be designed for in the IBC/ASCE 7? I don't know of one. What about a standard procedure or something for wrapping the columns with something that deforms to take the impact load? A reflective safety color at least?

Thank you!

 

[KootK]

I know of no standard load case for this. Some options that I've seen for managing this situation:

1) Concrete encasements around the bottoms of the columns.

2) Little guard fence things offset a bit from the column to, hopefully, alert folks that they're close to the column.

This issue comes up with some regularity but, until now, I've never heard of it taking a roof down. That's unfortunate.

 

[BTStructural]

Cool, thanks KootK!

Thankfully no one was injured in this one.

 

[phamENG]

ASCE 7-10 2.5 "Load Combinations for Extraordinary Events"

(0.9 or 1.2)D+A[sub]k[/sub]+0.5L+0.2S

where A[sub]k[/sub] = the load or load effect resulting from extraordinary event A

It's an optional load case, though - "where required by the owner or applicable code" - and I don't know where it is in the IBC. So unless the owner required it when the building was constructed, I would not necessarily expect it to be considered.

With that said, I always look at this for warehouses. Maybe it's because one of my early projects was a government warehouse where the same thing happened (not a collapse, fortunately, but you could see some daylight where it tried), but I either convince them to put in bollards around the columns, design for an impact load, or design the structure above to remain standing without the column (see 2.5.2.2, Residual Capacity - also known as progressive collapse prevention in some circles).

 

[bones206]

There are some HDPE column protectors on the market that are designed to absorb impact.

 

[le99]

Since the column is to be replaced obviously, I think it is imperative to incorporate an impact load in the redesign. It requires help from the plant to determine the impact load - the type of vehicle, and the expected operating speed (industry plant usually set/has speed limit for inhouse traffics). Limiting approaching distance by visible painting, or physical barriers, along the traffic path, will also help. But nothing is foolproof if the plant does not enforce the rules.

 

[hokie66]

Protection for column bases in industrial building is always good practice, but often ignored. Had a call out once to an aluminum smelter. Plant manager showed me a W14 column twisted 90 degrees, wanted a 'quick fix'.

 

[RangeRock]

For efficiencies sake... I would think bollards/crash protection barriers would be far more practical than potentially increased str. steel tonnage by say 20% as a result of impact loads at the column base. Guess it would depend on spacing/impact capacity of the columns without bollards. Thinking the owners space requirements and aisle maneuverability could also play a role in final decision.

 

[dik]

...a 30" dia reinf conc base x 3' high; we used to call them 'elephant feet'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[KootK]

Another thing to consider, depending on the setup, is whether or not an impact load low on your column will fail/tax your base anchor bolts in shear breakout. If you've got the typical, compressible material around your column at the SOG, it may not be prudent to assume that your SOG is taking the shear load. Granted, as long as the column isn't taking appreciable uplift or lateral load, this would likely not be a catastrophic failure.

 

[centondollar]

One way to solve this is using dynamic amplification factor method - a quasi-dynamical method suitable for low-speed impacts. First, estimate the location of the impact (distance from pinned, stiff or elastic support); then calculate the bending (and axial and shear) strain energy of the column and beams (e.g., a single frame out of several frames in a row) and use conservation of energy to equate it with the kinetic energy (1/2*m*v^2) of the vehicle. Finally, solve the impact force (previously used "M = P*e" in the strain energy integrals, where e = distance to location of impact mentioned earlier) and calculate maximum normal stress (stress = P/A +- M/W) in the column.

Size the column to stay well below yield for a predefined (mass, velocity) impact. Increasing the size of the beam (or truss) and opposite column of the frame will also reduce the load, since it will cause them to absorb more strain energy.

 

[centondollar]

Do note that the required member sizes to resist appreciable loads (a few tonne truck moving at, say, 5 m/s (18km/h)) will be very large, and a protective barrier (stiffened steel cylinder or concrete cylinder with pilasters) may be much cheaper and faster to implement in practice - as others have already alluded in this thread.

 

[bones206]

I prefer the plastic guards because they are designed to crumple and absorb impact energy. Concrete or steel column protection is a relic of the old school before engineered plastic guards existed. Even bollards have evolved with technology.

 

[dik]

thanks, bones...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[KootK]

To some extent, it's always seemed to me that the elephant's feet probably just transfer the damage up to the column at the top of the concrete and maybe spread out the load delivery to the column a bit. That, especially when folks add the concrete after the fact and the concrete sits on SOG rather than the foundation.

 

[dik]

They do that, Kootk... but their purpose is not for major impacts. If you anticipate there will be major impacts then you accommodate them. Most damage to columns that i've encountered is alight and accidental 'nudging'. They work fine for that. The client is advised of their purpose. If he wants more, then he gets more... I should have added, you often find small 'paint scuffs' on the surface so they seem serve their purpose. I don't know how they compare in cost to 'plastic bumpers', but they don't require replacement with a slight nudge. Not often, but I've encountered where forklift trucks have perforateed an HSS column... not with the elephant feet. More added... they are placed around the columns before the SOG is placed and the sonotube 'form' isolates them from the slab.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[bones206]

Dik, just to be clear - I didn't my previous statement to sound as critical as it did towards the elephant feet approach. I've used them myself in the past. Just trying to convey that there have been recent advancements in these types of safety systems and we may have some catching up to do as a profession. Similar to how many engineers and architects are still consistently specifying caged ladders.

 

[KootK]

Similar to how many engineers and architects are still consistently specifying caged ladders.

I'm still specifying caged ladders. Can you share the story on that?

 

[GC_Hopi]

Cage ladders are out.

 

[dik]

I agree bones... I think 'plastic' bumpers have great use and will use them in future for some work. I didn't mean to knock them. You see them as traffic 'barriers' all the time.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik