Why is roof live load unreducible? 1

[hetgen]

Consider a large steel roof with a truss girder that supports 600m² [ 6500ft² ]; the roof is inaccessible and located in a no-snow area. Why is the roof live-load unreducible in this case?

It seems conservative to design the girder with a full live load, according to the eurocode the total live load will be ~ 15.3 Ton [ 33,720lb]?

 

[Rabbit12]

To account for roofers stacking a whole crap load of materials in that one particular spot?

 

[dik]

Because it can really happen, in spite of what happens below...

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

-Dik

 

[JLNJ]

ASCE 7 allows for roof live load reduction. For most roofs this means going from 20 psf to 12 psf for members with large tributary areas. The Eurocode has no similar provisions?

Your numbers (33720#/6500sf) yield a really small live load (~5 psf). How much smaller do you want it to be?

 

[JoshPlumSE]

I'm with JLNJ and Rabbit12 for here in the US.

Roof live load is often reducible, but not for small areas. Especially since (to a large degree) this account for construction load situations where you lots of roofing materials stacked up in one relatively small area.

 

[hetgen]

@JLNJ Eurocode does not take into account the dependency on the tributary area for inaccessible roofs. Oddly, the need for an increased live load on a smaller roof area similar to ASCE and Australian standards, as shown in the figure below, is not present in Eurocode. The code recommends ~8 psf live load but allows the member countries to use a lower value.


Anyway, my question is specifically for a very large roof area. Considering ASCE's 12 psf x 6500 ft² = 78000lb [35.4 T] .. I just don't see this much load happening on a light steel roof.

 

[Settingsun]

8 psf is 0.4 kPa, but your graph shows 0.75 kPa for Eurocode?

Hail caused five warehouses to collapse in Australia in 2015 in a no-snow region. Depending on your point of view, the AU load is either too low, or just right since this problem has been very infrequent.

 

[hetgen]

@steveh49 You are right the graph is showing a higher value for Eurocode incorrectly. The Eurocode read as below. 0.4kPa is the recommended figure

 

[dik]

In Canada, snow load is distinct from live load and no reduction is permitted for the reason noted above.

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

-Dik

 

[MIStructE_IRE]

Over here we design roofs with access for 1,5kN/m sq. Its a hell of a lot but we do it to meet the code.

We had a very sensible client before who wanted the roof to be designed for 1.5kN/m sq on any one square meter at any given time. Ie, two men and a toolbox, not a full blown party!

Personally I think a lot of the code designated load requirements are incredibly conservative. And then we go and safety factor them up again!

 

[dik]

1.5... Most of Manitoba is above 1.7 kPa... and some areas of Canada are a lot higher...

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

-Dik

 

[Tomfh]

Personally I think a lot of the code designated load requirements are incredibly conservative. And then we go and safety factor them up again!

And many an engineer has been saved as a result!

 

[dik]

... and they still have the odd collapse. Our snow load is about 3' of snow... and, I've seen it. Snow loading will likely increase with Climate Change.

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

-Dik

 

[hetgen]

Thank you all.

It's interesting to note the wide range in the live load requirements, 0.25kPa to 1.7kPa [5psf - 35 psf]. How do they come up with these numbers?
I don't disagree that the added load help with redundancy as @Tomfh noted above, but it does look very high.

I'm reviewing an interesting roof structure. The structure won't make it if one considers live-load patten loading on the full panel as shown below.
The response I got from the designer is that they only consider 10m² [~110ft²] area service live-load for pattern loading, somewhat similar to what @MIStructE_IRE noted above.

The problem is that there are 100's of similar structures built for the same client and are performing well( see photo below).
So it is not easy to say " NOT CODE COMPLIANT! ". How would you approach such a situation?

 

[Settingsun]

See Note 3 from the Eurocode Table 6.10 you posted.

 

[MSUK90]

I'm reviewing an interesting roof structure.

Seems really an interesting roof(never encountered a similar one before).
Those ties from extended columns considered as supports to the other rafters?
Which program you are using for analysis and design approach?

 

[hetgen]

@steveh49, thanks. I did notice that, but I'm yet to see a national annex that follows that route. all I can find is a full area load

 

[hetgen]

@MSUK90... Yes, that is the idea. The struts extending from the columns are to support the main roof rafters.

 

[MIStructE_IRE]

What load combination are you failing in?

Is it failing in strength or deflection?

Is there any fixity at column/beam connections you can benefit from?

If the original designer only considers a patch load at any one time, presumably they have considered snow separately and combined it with dead + wind??

 

[Settingsun]

Then I would word your review along the lines that the loaded area the other designer used is 10sq.m as described in the model Eurocode, however this was changed by the national standards organisation so not compliant. That should head off the argument that the load is excessive. "Take it up with the code writers" since they've made a conscious decision to go another way.