Long Span Truss Live Load

[hotrock1423]

Hi.

I have been designing this 60meter span truss for a multipurpose sports arena.
But using the live load indicated in our local code (PH), it says that the roof live load is 0.6kpa.
With this, 0.6kpa load is not that big if the span and area is small, but using 0.6kpa to a 60m span and 7.5m truss bay spacing is huge and I really think that is not realistic when converted into a concentrated load.

I want to know what is your take in this? Should I ditch out the code and use a smaller and more realistic value? Thanks and more power.

 

[GeorgeTheCivilEngineer]

I would stick with the 0.6kPa.

Also - an allowance for solar panels in the permanent action is a good idea.

Assuming a symmetric truss, he wind uplift will likely control the design due to the buckling of the lower chord.

Assuming PH is Philippines, presumably snow load isn't a concern...

 

[CJLCivilStruct]

Can't say I have experience of designing a 60m truss, but I would suggest sticking to the code.. they exist for a reason! 6m bay spacing may be more appropriate?

On Georges point - I always think having some form of bracing / ties between bottom cords of large span trusses is a good idea.

 

[jayrod12]

Don't forget to build some camber into the truss to counteract the full dead weight of the truss itself plus a portion (maybe all) of the anticipated roof dead load. Also make sure that wherever the roof drains are located that the slopes accommodate the worst case truss deflection, otherwise you could end up with negative slope to drains and then ponding.

I don't think 0.6 kPa is an unreasonable load to design for. Where we are, it's usually snow that governs at 1.72 kPa or higher on larger roofs. In areas where the snow is less than 1 kPa, our code requires the use of a 1 kPa minimum.

Remember with trusses that depth is your friend. Increasing the size of the chords is a linear increase in section properties, i.e. Ad^2, but an increase in depth has a power curve increase.

 

[Lomarandil]

I've used 0.4kPa (from Eurocode) for certain inaccessible roofs with no maintenance requirements in Cambodia, but my default there was 0.6kPa. I'd probably stick with 0.6kPa for a sports arena.

I also agree with George -- unless your roof pitch is (uneconomically) steep, wind load will govern your design, regardless of your live load.

 

[JAE]

...Should I ditch out the code?

No - it's a minimum requirement.

0.6kPa isn't that much of a live load - appears that it is reduced from that used for smaller tributary areas.

Agree with others on wind uplift - especially if you are in the Philippines.

 

[lexpatrie]

Sounds a bit like what was it, Shaffer Arena in KC? Kemper? (this looks like some kind of High School report on the subject, but heck, why not?) What I'm getting at is live load deflection (or rain load) versus drainage can be a concern. Minimum live loads at least back when, for roofs, would produce a sort of implicit rain detention tolerance and/or rather ineffective and potentially unreliable ponding defense. Kemper had ponding and drainage issues in the original design, plus a hung system that appeared to be susceptible to fatigue on the A490 bolts, as I recall.

 

[hotrock1423]

Thank you for your inputs everyone.

Yes unfortunately the depth is low thats why the governing load combination is 1.2dl+1.6ll

And unfortunately since this is a govermenment project (if you know what I mean), I opt to use the most common steel available which is A36 and getting a large section HSS 14x14 tubular in the design thats why the self weight is huge too.. Yes, the suction of wind pressure is massive but self weight and the roof live load can easily counteract it.


Any advice to somehow reduce the loads, what steel grade would you recommend if government's supplier permits? Thank you very much

 

[BridgeSmith]

Yes, the suction of wind pressure is massive but self weight and the roof live load can easily counteract it.

I hope your statement is a wording/translation issue and you're not actually counting on roof live load to counteract wind uplift. For uplift, the critical roof live load is zero load. Only permanent loads can be counted to reduce net uplift.

 

[hotrock1423]

Yes i do not worry about that. Theres e load combination were dead load is reduced and live load is omitted to reduce the effect of the counterweight

0.9dl + windloas

 

[hotrock1423]

And still it does not govern

 

[JAE]

I opt to use the most common steel available which is A36 and getting a large section HSS 14x14 tubular in the design thats why the self weight is huge too.

Are you saying you are using A36 for tubes? That can't be right.

 

[lexpatrie]

Domestically (meaning the United States), A992 is more common for Wide flange and A500 Grade B is pretty common for HSS although there are other more recent options A1085?

See the FAQ here - 2022 - Are You Properly Specifying Materials, (Modern Steel Construction, June 2022)

 

[JLNJ]

In Mexico, they buy tubes from whomever. Could be of Asian origin. They are not necessarily rolled to A500 specs. I believe you can and do still get the equivalent of A36 material in tubes in Mexico.