Cantilevering Structural Grating 2

[jtayl71]

I have a prefabricated building that i am designing an elevated structure for it to sit in. The client wants a deck around the full perimiter. This building has lifting lugs that stick off the side so my support beams have to be outside of these lifting lugs for it to fit in place. This means for my platform to be close to the building, i need to look at cantilevering the grating about 8 inches past the support beam. Has anyone seen this done before or have any literature on a possible standard for this situation? I've attached a model view showing where the grating needs to extend over the lifting lug and also a sketch of what i am trying to accomplish. Any insight is appreciated. [/url]

 

[jayrod12]

I'm fairly certain most grating is designed on a first principles type method, not testing. Therefore I don't see why you couldn't have the grating cantilever provided you've checked it for the possible failure modes.

That being said, I probably wouldn't think twice about it, I'd be overly conservative on my bearing bar spec and be done with it.

 

[atrizzy]

What is the simply supported span length capable of being supported by the grating? As long as you cantilever less than half of that distance you should be fine. I don't see an issue with your 10". Just make sure it's spanning the right way and that you can handle unbalanced load on the cantilever.

 

[WARose]

Most grating catalogs will give you enough info to hand check it. If it comes up short, one trick a guy I once worked with use to do is to have a angle (instead of a toe plate) going around the perimeter to pick up the end. If the cantilever was short enough, the angle leg could just attach to the top flange of the beam.

 

[dik]

Some jurisdictions require a toe plate to be 5" above the floor...

Dik

 

[Once20036]

Why not call the manufacturer?
The ability of decking to handle negative flexure will vary based on the specifics of the deck.
The deck we typically specify has bearing bars in one direct, and small rounds in the other direction. These rounds will brace the compression side of the bearing bars for positive flexure, but not negative flexure. Like the cantilever beam over a column/inflection point bracing question that comes up routinely, if you're using this type of deck you`ll have a huge unbraced length for the compression (bottom) flange, and hence very little capacity.

 

[TehMightyEngineer]

I've cantilevered grating in similar situations; it's not hard to find the approximate section properties of the grating. I'd be worried more about deflection than strength, though.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[weab]

I cantilever grating all the time. If using 1 1/4" grating and 50 psf (or even 100 psf), 8" should be fine. Of course, you know that the orientation must be just as you have it shown. If you find the loading table for this grating, you could back into the stress in this case.

If grating goes, say, 1/4" from the building, you could probably eliminate the toe plate. But your detail is fine if you need it.

Looking at your detail, would it be possible to run a small channel to support the grating end rather than having it cantilever? I think 8" should provide enough space for connection clearance.

 

[wannabeSE]

The grating manufacture is a good resource. For general information, NAAMM has several publications on grating:

https://www.naamm.org/division/publications/8

 

[JedClampett]

I'd either weld it down or add mechanical hold downs. Someone putting their full weight on the cantilever might lift of the other end.
As far as the mechanical properties, I had to check some grating just today, and from what I can tell it's just a simple section modulus calculation. They must assume 24,000 psi allowable due to lateral bracing, because that what their spans indicate.
I'm not sure how many engineers or technical staff are left working for the grating companies anymore. I've tried calling and all I've gotten is someone who can go to the same page of the online catalogue that I'm already on.

 

[jtayl71]

Everyone,

Thanks for the replies, I'm not too concerned with the strength of the cantilevered section as I agree i believe it is just a simple span situation, my main concern is anchoring as JedClampett has described. We usually call out grating clips to secure grating but I was wondering if in this situation if those would be sufficient or not if someone were to stop on that edge. I will probably do as Once suggested and just call the manufacturer and see what they would recommend. It is good to know that this is not uncommon though. Thanks again.

 

[thaidavid40]

I would weld the grating, especially on the backspan. If the backspan is very long (say 4 or 5 feet), then you might want to weld a square (or round) bar to the underside perpendicular to the midspan to brace the bearing bars.

Thaidavid

 

[dik]

I would think that the normal saddle clips would secure the backspan and not disturb any coating system.

The cantilever distance is quite small.

Dik

 

[Nor Cal SE]

Good input from all. I 2nd the thought that the cantilever is not large and your stress is almost certainly adequate if your simple-span condition is at least twice that span. Checking deflection is also a good idea.

And I agree with others that you should check the uplift reaction on the end of the grating piece opposite the cantilever edge, and see whether your typical saddle clips would have the appropriate capacity. This reaction might not be very large if your back-span is significantly larger than your cantilever length.

 

[canwesteng]

Just as an aside, but the moment from a cantilever is 4 times not 2 times greater than the moment from a simple span. For a cantilever of equal length to the simplet span, moment is either PL or wL^2/2 as compared to PL/4 or wl^2/8. The cantiliever should not exceed 1/4 the simple span but this appears to be satisfied in any case. I've cantilevered grating like this often with no issue.

 

[Nor Cal SE]

Great point, canwesteng, you are correct.

 

[thaidavid40]

@dik,
I have also used those grating clips myself, at times. However, I don't really like them much because in the environments in which I work they tend to loosen, and then they can be dislodged, or even fall off. I sometimes use them for securing small electrical support posts and such on grating, or securing small removable grating sections, but I've mostly quit using them for regular fastening.
Dave

Thaidavid

 

[UcfSE]

I've used the section properties and listed allowables in the past when I needed to do something outside the manufacturer's load tables. As others have mentioned, you can derive the load table values with calculations, so you get some confidence you're doing it correctly.