Span tables & CSC Tedds

[frank999]

I have a calculation for a flat roof joist run on CSC Tedds:

50x175mm
C24
400mm centres
UDL: 0.75kN/m2
Dead Load 1.00kN/m2
3900mm span

Trada Span tables:
Table 2.36
Access only for
cleaning and maintenance
Imposed load 0.75 kN/m2;
C24
Dead Load [kN/m2 ]: More than 0.75 but not more than 1.00
47x170mm
This is giving me a max span of only 3690mm.

Any reason why the Software (CSC Tedds) is giving a greater span ?

 

[jeds]

It's your span table that's wrong. The current table gives span of 3850. Maybe you are using the old table?

 

[tony1851]

Neither is 'wrong'in the strict sense of the word. It depends on how much deflection is allowed for in the tables and calcs.
Whoever wrote the calculation software obviously allowed for more deflection than those who prepared the tables.

 

[frank999]

Your right, it could be an old table.

I assume tables would err more on the side of caution as they are more general, the software can give a greater span as you can be more specific ie such as with deflection ...

 

[jeds]

The newer tables are more relaxed compared to the old ones. The new Eurocodes are the same. I'm finding calculations based on similar loads coming out a serial size smaller in some cases. Incidentally, the standard deflection used in tables is 0.003xspan but not more than 14mm. All the softwares I know use the same standard.

 

[RonnyRaygun]

I thought I could answer this one with an Excel calc...bearing in mind TEDDS should give exactly the same answer.

So I set up the spreadsheet, and through trial and error found the maximum span governed by deflection, given the info above.

I keep getting 4170mm...

Now this seems long, so I'm wondering if I've made a mistake somewhere...can't see one though...

I did go to a wedding yesterday though and am feeling a bit groggy this afternoon

*Edit* Was using 175mm rather than 170mm as depth, but still getting 4050mm as maximum span...

 

[baxiengineer]

yep 7x2 in old money that will be half the span plus one inch gives you the 7
anything over 14ft goes to 3 inch timber