topic Re: Automatic Load Combinations using service load factor instead of ULS Factor in Robot Structural Analysis Forum

Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Tue, 14 Sep 2021 18:31:27 GMT

I am trying to build custom load combination to match all of the load types defined in ASCE 7-10 or ASCE 7-16. To start I am trying a few simple modifications to try and learn what is being done in the back end.

I have found that for some reason the automatic combinations keep using the service load factor for the ULS load combinations in some instances. see ULS/2 in the attached screenshot:

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Tue, 14 Sep 2021 18:33:52 GMT

I would like to replicate these load types and combinations:

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 22 Sep 2021 19:25:19 GMT

bump....not having any luck tracking down why it using the service load factor in some instances.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 20 Oct 2021 17:33:33 GMT

bump...have not been able to figure out why it appears to be using the service load factor sometimes.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 24 Nov 2021 13:58:20 GMT

@Krzysztof_Wasik are you able to provide any assistance on this, we really need the service factor for wind to be 0.6 otherwise the automatic SLS combinations cannot be used.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

okapawal — Wed, 24 Nov 2021 16:49:31 GMT

This formula (20) was designed mostly for EC code SLS frequent combinations. It is not documented well but it indeed use gamma_s factor.

You can use instead formula (45) and psi_0,3 factor for SLS combinations.

Actually why do you want to define 0,6 factor for wind in LRFD SLS combinations?

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 24 Nov 2021 17:07:38 GMT

@okapawal we want define SLS as 0.6 because the ASCE 7 wind definition is an ultimate condition and in lieu of defining a separate 10yr MRI wind load for service checks it is fairly standard practice in the States to apply the 0.6 ASD factor when checking deflections.

Thank you for the information and suggestion to use formula 45.

Would it at all be possible for the ASCE 7 combinations to be updated to include the full combination and load definitions outlined in message 2 in a future Robot update or release? We run into needing to account for earth, ice and fluid loadings fairly frequently.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 24 Nov 2021 18:32:48 GMT

Actually it would be better to define the load combinations per the IBC as that is typically the governing code.
CHAPTER 16 STRUCTURAL DESIGN, 2018 International Building Code (IBC) | ICC Digital Codes (iccsafe.org)

Re: Automatic Load Combinations using service load factor instead of ULS Factor

okapawal — Thu, 25 Nov 2021 15:56:07 GMT

Hi @DonBAE

I will update ASCE combination rules and share with you for checking.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Thu, 25 Nov 2021 16:00:44 GMT

@okapawal Very much appreciated!

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Tue, 12 Apr 2022 12:56:51 GMT

@okapawal

Was hoping to have seen the correct combinations in Robot 2023 but it still contains the same combinations inclusive of the wrong SLS factor on Wind load and missing several of the load natures defined in IBC/ASCE 7? Are the full IBC/ASCE 7 load natures and combinations coming in a future hotfix/patch?

Re: Automatic Load Combinations using service load factor instead of ULS Factor

okapawal — Tue, 12 Apr 2022 15:16:28 GMT

Hi @DonBAE

I'm having more time now after the release. I will prepare IBC code combination and share with you.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Tue, 12 Apr 2022 16:30:22 GMT

@okapawal Thank you very much, honestly you can take your time I'm not in a hurry just a reply noting it will be worked on is enough for now. I imagine there was some major crunch time for the release, hopefully the team is able to get some relaxation time.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

wassim_said_said — Wed, 01 Jun 2022 11:29:23 GMT

Dear @okapawal ,

Any news concerning the update to correct the ASCE 7 Automatic load combinations?

It will be great to have automatic and corrected load combinations without need to manual editing specially when we have big number of combinations.

Thank you in advance.

Wassim Said

Re: Automatic Load Combinations using service load factor instead of ULS Factor

okapawal — Wed, 17 Aug 2022 14:24:36 GMT

Could you help me this time?

How do you interpret OR in the combination formulas according to ASCE and IBC

Is it OR(inclusive) or OR(exclusive)? I think English is missing this subtle difference.

In other words, can Snow and Rain loads act together?

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 17 Aug 2022 14:33:38 GMT

They are or(exclusive), the snow loading computations contain provisions for including rain-on-snow surcharge which is to be included in the snow (S) loading for combinations.

Re: Automatic Load Combinations using service load factor instead of ULS Factor

DonBAE — Wed, 17 Aug 2022 14:40:33 GMT

I'd encourage you to use IBC as the primary reference for preparing the load combinations as that is typically the governing code:

https://codes.iccsafe.org/content/IBC2018P5/chapter-16-structural-design#IBC2018P5_Ch16_Sec1605

Re: Automatic Load Combinations using service load factor instead of ULS Factor

okapawal — Wed, 17 Aug 2022 14:47:36 GMT

Thanks @DonBAE

That's what I'm working on now.