Loading combination: Basic question

[Soco]

Dear All,


I am currently checking the work done on a project in Singapour (calculation done with UBC 97 and British standards.
The earthquake seismic combinations used are not the classical 1.2 D + 0.5 L + Ex + 0.3 Ey
Instead the design office uses 1.44 D +.55 L + 1.1 Ex

Has somebody see such a combination??
I am very amazed and want to diaprove this formula.
Any idea where this formula comes from?

Thank you in advance for your help.
Alan

 

[JAE]

When you say "UBC 97 and British standards" I guess that raises a flag with me - UBC 97 is a USA product - how does it even mix with "British standards" I can't imagine.

Perhaps the combination is a British one? I'm in the USA and I've never seen those numbers (1.44, 0.55,etc.)

 

[NewImhotep]

Alan
It's really very weird combinations. !
Seismic load can be applied in the Load combination of British standards as the wind load.

1.4 DL+ 1.4 WL.
1.2*(Dl+LL+WL).

where WL is wind load or seismic load.

Because British standards clear of any method to calculate seismic load.u can use UBC to calculate the base shear force only
but don't use UBC load combination if ur general code is BS.

Ahmed

 

[Soco]

Thank you very much for your answers.

Now I am totally confused by the paragraph "Exceptions" of section 1612.2.1 of UBC 97.
In this section the basic load combinations 1.2 D + 1 E + 0.5 L are defined.

In the exception it is writen " Factored load combinations multiplied by 1.1 where load combinations include seismic forces"

Does it mean that when I am designing a concrete structure I need to take into accout:
1.32 D + 1.1 E + 0.55 L
If somebody can clarify this exception, I would be very grateful.
Thank you in advance and thank you for the help already given to me. It's very helpful

 

[JAE]

Does it mean that when I am designing a concrete structure I need to take into accout:
1.32 D + 1.1 E + 0.55 L

Yes, for non-seismic load combinations your are directed to section 1902.2 for concrete (per Exception 1)

For seismic combinations you must multiply the two seismic combos in 1612.2.1 by 1.1 for concrete and masonry.

 

[Soco]

Thank you very much. I do not understand the physical meaning of this 10% ponderation and this 1.1 coefficient seems to have disapear in IBC 2003.

Anyway, thank you so much for your help.

Alan

 

[JAE]

In the USA, the concrete design was previously governed by load combinations found within ACI 318. These were generally:

1.4D + 1.7L
1.05D + 1.3L + 1.3W

and were tied to specific phi factors (strength reduction factors).

The ACI 318 has now revised its load combinations and phi factors to match that of the IBC. When the UBC 97 was published, ACI was still under the older combinations so the UBC had to adjust the Chapter 19 combinations to account for the difference.