Platform Grating Behaviour & Tributary Areas 1

[Dinga17]

Hi all,

I am design a platform which will have grating supported on two I-beams which are then connected to columns etc. I will be using either steel grating or FRP from Webforge (this stuff is pretty standard in Australia). Selecting the grating is easy - the manufacturer has load tables and you select from there.

I am sizing the I-beams to support the grating. If I apply a live loading of 2.5kPa onto the grating, what are the loadings applied to the I beams?

Do I simply determine the area of the grating (load bar area) and then determine the total force from the 2.5kPa loading? This would then be equally distributed between the two I-beams. I can then apply a point load on each I-beam to size it etc.

I haven't used AS1170 standards a lot but they mention tributary area and I am wondering whether I need to know more about this or whether it's more specific to buildings with floors and multiple columns.

What's the correct way of determining the reactions from platform Grating onto supports?

PS. My structural engineer is certifying this design so calculations will be checked. I'm just trying to learn and get it in the ballpark.

Cheers all

 

[human909]

Nothing complicated here. You seemed to have figured it out mostly and are on the right track. Except I'd expect to be applying a line load (UDL) on the beam rather than a point load.

Don't forget appropriate load factors. Also deflection rather than strength would normally govern most designs. All this is in 1170.

I'm hesitant to help you along much further as I have no desire to encourage non locals and non engineers performing structural design for Australian purposes. But I will say that you are mostly on the right track.

 

[Lomarandil]

You're pretty close. Assuming that the grating only sits between the two beams, yes, you can assume that the total load on each beam is half. This can be applied as a point load at the middle of the beam (a little conservative, but that's usually good in industrial settings) or as a distributed load across the length of each beam. (Edit: Human909 beat me to it)

I wouldn't worry about live load reductions (which might be what you're referring to from AS1170) in this sort of case. They typically wouldn't apply.

Factors that might complicate things -- if the grating extends beyond the beams, if there is a handrail also supported by the beams, if the beams themselves have multiple spans across supports.

----
just call me Lo.

 

[Settingsun]

area of the grating (load bar area)

Just to be abundantly clear, you use the full area of the platform, not just the solid area of the grating.

 

[Dinga17]

Human, you're right, a UDL would be more accurate. I can work out pressure per load bar and work out force per metre. Forgive my ignorance, but what factors are you talking about? Sounds like I need to read more of AS1170 I am an engineer just never designed platforms/structures, hence why the structural engineer I work under will be checking it all. Just trying to learn these codes in my spare time

Lo, yes I have read about the reduction factors and have ignored that section. Probably provides a more conservative answer ignoring them anyways. The platform does have handrails - however my understanding is that loadings on the handrails themselves are a seperate load case to platform loading. I do need to consider dead loads as well though together with the live loading of the platform.

Thanks guys.

 

[Dinga17]

Steveh49, I was initially confused about your comment.I suppose in real world practice a person standing on grating can't just stand on the load bars and not the gaps, so the load would be applied over an area. So using the total area to find the total force does make sense. Or I can find the total force per metre of grating and apply a UDL.

Thanks

 

[human909]

Steve49. Nice catch.

I am an engineer just never designed platforms/structures, hence why the structural engineer I work under will be checking it all. Just trying to learn these codes in my spare time.

Fair enough. But are you an engineer who resides in Australia or somebody who is designing from overseas?

It matters. As soon as you are dealing with a non locals and/or non structural engineer responses may change and be more tailored to suit the audience. As Steve49 caught but most of us missed, you seems to have been heading down the path of completely mis-applying the loads by a factor of ~7x.

Steveh49, I was initially confused about your comment.I suppose in real world practice a person standing on grating can't just stand on the load bars and not the gaps, so the load would be applied over an area. So using the total area to find the total force does make sense. Or I can find the total force per metre of grating and apply a UDL.

Thanks

Real world or concept world the answer is the same. There is no engineering world where a floor live load only applies to the contact edges of the floor and not the area of the floor. If this isn't obvious then take several steps back and revise the basic.


Sorry if this response is blunt. Your initial question is quite polite and descriptive which is an excellent approach in getting answers. I and others here are happy to help, but nobody want to enable people who are well out of their depth. Hence the cautious response.

 

[Dinga17]

Human909, yeah I am an engineer in Australia working under a Registered Professional engineer in Australia. I respect your blunt response and I get that it matters. But I am here trying to learn and better myself. I haven't had to do statics in years hence why I am revising and giving this a crack.

I wasn't applying the live load to the contact edges of the grating where it meets the beam but the actual surface area of the grating. Anyways, I have understand this mistake.

Anyways, thanks for your advice. I will revise my basic statics further.
Thanks.

 

[human909]

Cool mate. Good luck with it.

For what it is worth. I work for a firm that is mostly in the business of mechanical design. Most of my colleagues would have a working knowledge less than yours, only one could name AS1170. So I totally get where you are coming from as I am mostly a lone structural engineer in a mechanical firm. (We design and build for manufacturing firms. Plenty of industrial steel like you describe.)

But I am here trying to learn and better myself.

Great! I and most of us are here for the same reason.

 

[human909]

Oh and one last thing. For this type of design, sticking with a span to depth ratio between 20 and 30 will pretty much cover you for most things you can throw at it. For walkway and light equipment loads you'll be coved. Start getting into 'heavy' equipment or more complicated things then just guess on the below 20 side of things and let the engineer handle the rest.

EG if the beam spans 5000mm then a 180PFC/200PFC/200UB is suitable (5000mm/200mm=25).

Yes it is almost that simple for basic spanning beams. If somebody is checking your work anyway then you almost don't need to dig much deeper.

 

[Dinga17]

Human, I've never actually heard of the span to depth ratio. That sounds too good to be true! It does make sense though.

Yeah very true, but is also worthwhile going through the checks myself to familiarise myself with it all too.

 

[eng-erik]

Yes, as someone mentioned above, the tributary area and the load applied on the beams should be half if I understand your description correctly. If the structural configuration or the area loads were more irregular I would use something like Tribby3d to calculate the tributary areas and the load.

Good luck!