Difference of SFRS and LFRS in US code 2

[TTTKAO]

Hello All,

I am a newer to US design code. May i ask some experience and advice regrading to difference of seismic force resisting system(SFRS) and lateral force resisting system(LFRS)?

1. what's the difference between these two system, are they same once to resist seismic forces? somebody said LFRS is part of SFRS?

2. What's the SFRS including of ? my understanding is including Diaphragms, vertical braces and columns connected by vertical braces(take OCBF as an example).

3. if diaphragms should be included in the SRFS, For a industry platform(flexible diaphragm for most of case), all the horizontal braces and beams connected by horizontal braces should be part of diaphragm? and part of SFRS as well?

Thank you for advice in advance.

Regards!

 

[JoshPlumSE]

1. what's the difference between these two system, are they same once to resist seismic forces? somebody said LFRS is part of SFRS?

They're pretty much the same thing. We might say SFRS when we're specifically talking about a structure where seismic governs the design and there are specific seismic detailing rules regarding the connections.

2. What's the SFRS including of ? my understanding is including Diaphragms, vertical braces and columns connected by vertical braces(take OCBF as an example).

Usually we mean the shear walls, braces, moment frames, drag struts and such. But, technically diaphragms are part of it too.

3. if diaphragms should be included in the SRFS, For a industry platform(flexible diaphragm for most of case), all the horizontal braces and beams connected by horizontal braces should be part of diaphragm? and part of SFRS as well?

Yes and no. Any horizontal braces would be considered part of the "diaphragm". So, in that respect, they are part of the seismic force resisting system. But, they're not part of the Special Moment Frame, Ordinary Braced Frame or such. That's where most of the seismic detailing provisions for connections and such come from.

 

[driftLimiter]

The difference between these two abbreviations is nuanced. All buildings need a LFRS to resist either wind or seismic loads. The SFRS is just a type of LFRS. SFRS requires additional detailing requirements beyond just the strength of the system in order to meet US code. ASCE 7-16 provides engineers with the ability to check if SFRS detailing is required, and how much of it based on the site location and usage category etc.

That should help answer your item 1.

Item 2. Any and all elements needed to get the seismic loads into the soil. Generally speaking, Diaphragm (including collectors and chords), Shear walls, or braced bays. For braced bays, all of the elements, beams columns and braces are part of the LFRS.

Item 3. This question I think is a loaded one. Horizontal braced systems are not the same as a diaphragm. And when you say industry platform, are you talking about a smaller mezzanine type structure within a building? These are subject to different requirements and required their own lateral system. And yes if Horizonal braces are used to distribute the lateral forces to the LFRS, then they too are part of the LFRS.

 

[JoshPlumSE]

driftlimiter -

In industrial structures, you'll frequently have a "floor area" that is covered either by metal decking or steel grating. The steel grating is NEVER sufficient to act as a diaphragm. But, the metal decking might be.

Regardless, it is common to add horizontal bracing to create a clear load path to the LFRS. Sometimes this is just at locations where we have heavy equipment anchored. Sometimes, it's the only way to get the lateral forces into the LRFS. Regardless, we often look as this as a "diaphragm" in terms of design forces because it is performing the same basic role as a diaphragm would.

At least that's how I interpret his "mixed use" of those terms.

 

[driftLimiter]

Yea I understand its often called a diaphragm when you have a system of H-braces. The definition of Diaphragm in the IBC does include horizontal braces So yes I agree.

 

[TTTKAO]

JoshPlumSE & driftlimiter ,

Thank you so much for the explanation.

the diaphragm in question 3 I tried to say the covered grating floor for heavy equipment with braces to transfer lateral load.

Regards!