Grating Design 1

[Mb87]

NOTE: This is in Australia so will be referencing AS

I have a walkway on one site that has had a confined space davit attached (been there for probably 10 years), and we want to have records of design check comps so I'm producing a few reports.

I'm familiar with AS1657/AS1170 and will use that as the basis for "floor loading" with the addition of the requirement per the AS1891 series.

However, refer to the image below for the existing mounting. I need about 10kNm moment and 22kN normal/down thru this baseplate.

For the complexity of the mesh do I ignore the fixing thru the mesh and comp it with just the 3 fixings into the frame? Im only at a .13 utilization for tension and .42 for baseplate stiffness.
I am just concerned separating the "floor loading" and this live load through the mesh will be overlooking a potential critical area
How would you approach this?

 

[human909]

We are going to need plenty more context than one photograph.

Eg;
-ignoring the grating is probably reasonable when the moment is puts the gating in direct compression and no bending. But likely not reasonable if you are putting it in shear and compression.
-following from that what direction is the moment acting in? All possible directions?
-Is the davit arm only 0.45m long that seems quite short.

I'm surprised at your 0.13 utilisation in tension. I'd presume the bolts might be working harder than that given the small separation and high moment. (Potentially higher than 10kNm?)

 

[Mb87]

Thanks for the prompt reply.

The photo was just to show the fixing thru the mesh

To answer your questions,
It can be used in all directions, so need to assume it might get used in all directions.
yes, the arm is very short. it's for confined space recovery directly below its position, In normal use it will never be used its only if there is a need for emergency recovery of an operator who is harnessed, but normally they enter and exit the area via ladder.

Sorry for the bum steer on the fastener utilization
The .13 utilization comp is actually this ones sister which has a larger baseplate and the bolts are in the corner not right under the base

 

[human909]

In that case I'd be checking the suitability of the grating to carry the compression load and moment.

I suspect you'll have a bending failure if there is not joist running parallel with the grating span. Or if there is a joist you will need to check the buckling of the grating between the plate and the joist.

In reality it will never see 10kNm or 22kN in normal use. But fall arrest or in this case confined space recovery isn't something to skimp on.

 

[JoshPlumSE]

Yeah, I wouldn't have relied on grating for anything structural.... other than a basic uniform live load or dead load. If I had a moment that needed to be resisted like this we would have used small angles or such to resist that moment directly. Bolts passing through the grating would be okay though

 

[GC_Hopi]

Agree with JoshPlumSE. Often the grating is not bolted or screwed down and if it was the attachments are "lost" over time. Life safety is not something to be played with just put in some real structural framing for the loads.

 

[Mb87]

Thanks for the replies,
This is the approach I was working on in the first post. for the Davit just assume the grate does nothing (for the Davit) and the Base plate and its bolts into the frame are the only thing carrying the forces for the Davit

 

[hokie66]

What frame? I see a member running one way which supports the grating and in turn one edge of the davit base. Based on the position of the bolts, that member may be a channel. But what about the other direction?

 

[human909]

Thanks for the replies,
This is the approach I was working on in the first post. for the Davit just assume the grate does nothing (for the Davit) and the Base plate and its bolts into the frame are the only thing carrying the forces for the Davit

But how does the baseplate and the bolts carry compression into the frame when it is resting on the grating?

You cannot simply ignore that it is there.

 

[ifitsmoving]

I did checks on about 15 of these items on a site in Central QLD approx. 8 years ago now. None of them were on the mesh yet we still struggled to justify the supporting structures for about half of them. They were often putting small PFCs into heavy torsion and other nasty things.

I'd be very uneasy about relying on the mesh in compression as anything other than a "packer" and even then it's probably a bad idea. Your bolts will be unrestrained over a significant length (U/S of baseplate to T.O.S. of trimmer). In shear loads, how do you stop your bolts from bending, or just forming a mechanism that allows everything to shift sideways? It wouldn't shift far but it could put the heads / nuts into higher prying loads or do other weird things. I wouldn't rely on it at all in tension (I've seen far too many missing / loose grating clips). Remember that the manufacturers of this mesh design it for nominal UDL type loads, not whacking great point loads.

How hard is it to get the operators of the site to just cut the mesh and bolt the davit down to the structure below?

Assuming this structure is similar to most other Australian industrial sites I'd also check more than just the immediate connection details - make sure you trace the load path back to something hefty.

 

[JLNJ]

This is for a fall protection anchor point?

 

[human909]

Pink star for you ifitsmoving. You said what I have been trying to say and impress upon Mb87 for a while now. While I haven't dealt with this particular type of post mounted anchor system, I've designed the connection others previously. It is not abnormal to end up with sizable members and connections as the moments are not insignificant.

This is for a fall protection anchor point?

Essentially yes. Though it seems more for retrieval than fall prevention, either way falls under roughly the same requirements.