×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

tube collectors between joist seats
3

tube collectors between joist seats

tube collectors between joist seats

(OP)
I am designing an occupancy category IV structure that is in a seismic design category of D. The structure is steel bar joists on CMU bearing-shear walls. Where the joists bear on top of the CMU walls, to transfer the diaphragm load to the top of the wall, I am debating whether to use tube collectors between the joists seats per the attached or utilize the joist seats themselves as a rollover load. The question I have is would I need to amplify the load on either the joist seat or the tube collector by the overstrength factor to satisfy the intent of section 12.10.2.1 of ASCE 7-05? Without the overstrength factor applied, the rollover load at each joist seat would be between 2,000 and 4,000 lbs.

RE: tube collectors between joist seats

That's already a pretty high rollover load for the joist seats. May want to look at a collector tube for that reason alone. I believe much above 2000 lbs and they start to need to reinforce the seats for rollover.

RE: tube collectors between joist seats

I would use the tubes rather than rely on joist seat rollover for load of that magnitude. Without the tubes I don’t see how you could justify the last few deck welds delivering the diaphragm shear to the seat.

I don’t have ASCE 7 in front of me, but if these tubes are between every joist (or nearly every joist) then I think they act as extensions of the shear wall and not collectors per se. You don’t have a singular point of load collection (and possible failure) nor are you “dragging” the force from one location to another. I would not use the Omega multiplier.

RE: tube collectors between joist seats

I like JNLJ's rationale for this and would apply over strength to neither the joist seat rollover option nor the lug option. Neither setup does enough collecting to warrant collector treatment in my opinion.

RE: tube collectors between joist seats

(OP)
12.10.2.1 Collector Elements Requiring Load Combinations with Overstrength Factor for Seismic Design Categories C through F. In structures assigned to Seismic Design Category C, D, E, or F, collector elements (see Fig. 12.10-l), splices, and their connections to resisting elements shall resist the load combinations with overstrength of Section 12.4.3.2.

So the consensus is that as long as the tube collectors are placed at every joist space, no overstrength amplification is warranted? With the rollover seat option, there is an edge angle running continuous over the joist seats so I'm not relying on the few deck welds near the seat to transfer the load. The diaphragm load will be transferred into the edge angle then into the top of joist seat directly with welds.

RE: tube collectors between joist seats

I'd be willing to make the same argument even with lugs at every other joist space.

RE: tube collectors between joist seats

Quote (smvk3)

So the consensus is that as long as the tube collectors are placed at every joist space, no overstrength amplification is warranted?

Unfortunately I don't believe there is a consensus among engineers on exactly where the diaphragm ends and the collector begins. I've seen both interpretations. A common argument among those who choose not to amplify is that the overstrength factor is meant to apply to discrete connections rather than multiple distributed connections, and that a loss of one distributed connector (in this case, one tube collector) would not compromise the integrity of the load path as a whole.

My concern with that argument is that if each connector is more or less uniformly loaded, failure of one connector essentially does result in failure of all connectors. I tend to base my decision on the ductility available in the connection under cyclic loading. If it's ductile, no overstrength; if it's not, overstength. This agrees with what I interpret to be the intent of the overstrength factor. See this article (and screenshot below) for some additional discussion: Link


RE: tube collectors between joist seats

Quote (Deker)

A common argument among those who choose not to amplify is that the overstrength factor is meant to apply to discrete connections rather than multiple distributed connections, and that a loss of one distributed connector (in this case, one tube collector) would not compromise the integrity of the load path as a whole.

Quote (Deker)

My concern with that argument is that if each connector is more or less uniformly loaded, failure of one connector essentially does result in failure of all connectors.

You didn't say so explicitly but I suspect that your logic there is based upon the classic, unzipping of brittle things. I feel that argument falls apart somewhat when enough of those brittle things are involved and are loaded by a common distribution element. A brittle anchorage failure mode, for example, will be based on the unfavorable tail end of a strength distribution curve. As such, when there is a large collection of such connections, most of them can be expected to have significantly higher capacities than the one that fails first at its design strength.

For example, if you have twenty connections in line and lose one, the demand in the remaining connections goes up about five percent and, in all likelihood, everything remains just fine. I suspect that the capacity at which any potential unzipping would be arrested in such a system is probably well in excess of an amplified demand. The trouble with this line of reasoning is quantifying it. I wait patiently for some reliability theorist to tell me if the magic number is five, ten, or twenty.

RE: tube collectors between joist seats

I see your point, but I think you rightly pointed out that the issue is quantifying it. Your approach could also be reframed as designing for the amplified demand but using a strength reduction factor of 1.0 (or something similar) in recognition of the expected strength (as oppposed to the minimum).

A repetitive connection will of course have more strength by virtue of the number of connections, but I contend that the total demand should still be based on some consideration of the the type of failure expected.

RE: tube collectors between joist seats

Quote (Deker)

Your approach could also be reframed as designing for the amplified demand but using a strength reduction factor of 1.0 (or something similar) in recognition of the expected strength (as opposed to the minimum).

That's right, so long as it's in recognition of the expected aggregate system strength.

Dialing it back to the decision to use joist rollover or not, for many fastening schemes I would consider joist rollover to be ductile. More ductile than most things really.

RE: tube collectors between joist seats

Yes, the aggregate system strength, although I believe that there used to be precedent in the code for using phi=1.0 with amplified seismic load combinations even when checking individual elements. Unfortunately I can't recall where I've seen it.

Philosophically, I don't have as much of an issue being liberal with the strength reduction factor (which I have done myself on occasion) as I do with neglecting to consider the overstrength factor altogether. The effect you described above wouldn't be enough to make up for a 300% shortfall in demand--possibly more depending on the SFRS and how it was proportioned by the designer--unless the demand capacity ratio was kept low to begin with, in which case one could argue that the system already was designed considering amplified loads.

I wish I could offer an informed opinion on available ductility of joist rollover, but where I practice it's routine to use collectors / blocking between the joists.

RE: tube collectors between joist seats

Quote (Deker)

I wish I could offer an informed opinion on available ductility of joist rollover, but where I practice it's routine to use collectors / blocking between the joists.

I'm sure that your opinion would be as informed as anybody's. Even for those of use who utilize joist rollover it basically comes down to one's judgment regarding the situation shown below: metal plates forming plastic hinges with no real propensity for local buckling.



RE: tube collectors between joist seats

In my high seismic region (SDC D by a good margin), adding the HSS collector tubes is common and we do not rely on joist rollover in any situation.
I would not "omega"fy the HSS tube collector design, but I would record omegafied loading to the joist girder top chord and then any connection of the joist girder top chord to the lateral force resisting element.

S&T - www.re-tug.com

RE: tube collectors between joist seats

Great picture KootK, thank you. When relying on rollover, do EORs typically specify whether the joist seat will be stiffened or unstiffened? I imagine that would impact the decision on whether it can be considered ductile.

Quote (sticksandtriangles)

I would not "omega"fy the HSS tube collector design, but I would record omegafied loading to the joist girder top chord and then any connection of the joist girder top chord to the lateral force resisting element.

Not that I agree or disagree, but what is your rationale behind not amplifying the load? Is it the the distributed strength argument? Granted, it's probably a moot point for this particular condition given that the amplified diaphragm demand is trivial compared to the shear transfer capacity of a tube collector with nominal welding.

For those that subscribe to the unamplified demand / distributed strength argument, do you also disagree with the article I posted above that shear studs in a composite deck should designed for overstrength when delivering diaphragm shear to collectors?

RE: tube collectors between joist seats

Quote (Deker)

Not that I agree or disagree, but what is your rationale behind not amplifying the load?

Unfortunately I do not have great rationale for why, I deem the joist girder to be the start of my "collector element" per 12.10.2.1 and design accordingly. Anything above that, being the HSS tubes and deck attachments gets the load to my collector element.

Quote (Deker)

For those that subscribe to the unamplified demand / distributed strength argument, do you also disagree with the article I posted above that shear studs in a composite deck should designed for overstrength when delivering diaphragm shear to collectors?

I have not designed shear studs in composite deck for overstrength either... not sure if that is correct. I also do not design chords and their connections for overstrength (last bullet of your table Deker), I only design collectors and their connections for overstrength.

S&T - www.re-tug.com

RE: tube collectors between joist seats

Quote (Deker)

When relying on rollover, do EORs typically specify whether the joist seat will be stiffened or unstiffened? I imagine that would impact the decision on whether it can be considered ductile.

I don't specify it but, at the same time, I have every expectation that it will either work unstiffened or we'll be doing something else (lugs etc). The cost differential between stiffened and unstiffened joist seats can be shocking in my experience. This is third hand information at this point but a very knowledgeable person here suggested to me once that the cost is less about the addition of the stiffeners themselves and more about messing up the ability to nest joists together for efficient shipping.

Quote (Deker)

Granted, it's probably a moot point for this particular condition given that the amplified diaphragm demand is trivial compared to the shear transfer capacity of a tube collector with nominal welding.

Much depends on the situation. OP's situation is CMU shear walls by the sound of it. In my neck of the woods, that would mean that the tube lugs would get welded to small embeds in the CMU which would then become the brittle link in the chain and affect this calculus:

1) If I can make a go of it with joist rollover, I need to do that to stay competitive.

2) If I can make a go of it with a tube lug every other joist space, I need to do that to stay competitive.

Quote (Deker)

For those that subscribe to the unamplified demand / distributed strength argument, do you also disagree with the article I posted above that shear studs in a composite deck should designed for overstrength when delivering diaphragm shear to collectors?

I wouldn't say that I disagree with it, per se but, rather, that it's a bit of a surprise to me and it moves the conservatism needle a bit further over than I usually put it. Some thoughts:

3) I noticed that concrete to concrete shear friction is in the ductile category. That surprised me a bit. I've no doubt that grinding the interface down to a fine powder and then picking up some truss action probably does give you a decent hysteris loop, at least for a few cycles. But then, in a way , it still kind of is a version of unreinforced concrete shear.

4) In many ways, I see well distributed composite studs as being just another version of shear friction, albeit a lesser version between dissimilar materials. Moreover, the very procedure that we use to calculate the flexural strength of composite beams assumes some ductility in the connections between the studs and the slabs. So, in these respects, yes, I was surprised to see composite studs on the list.

5) In many ways, I see well distributed composite studs as being just another version of shear friction, albeit a lesser version between dissimilar materials. Moreover, the very procedure that we use to calculate the flexural strength of composite beams assumes some ductility in the connections between the studs and the slabs. So, in these respects, yes, I was surprised to see composite studs on the list.

6) Obviously my general argument about distributed connections would apply to this as well.

7) The article mentioned that the authors and some of their external associates were using this, more conservative approach despite it not really being codified. I have mixed feeling about that:

a) One the one hand, I celebrate a group going above and beyond what is codified based on their judgment. The rare, "race to the top" as it were.

b) I feel that it harms are profession a bit when you get stuff like this floating around that suggests that what many engineers are doing is incorrect / less correct without that stuff having made it into codes and/or widely accepted design guides yet. In sending out mixed messages to the design and construction community, we undermine ourselves I feel. Collectively, we seem wishy washy in a way that diminishes confidence in what we do. If it were up to me, we'd keep this kind of stuff close to our chests until a consensus is reached and disseminated. That said, I doubt that many contractors are reading Sabelli articles or identifying there reasons for relatively slight differences in connection strategies on EOR drawings.

RE: tube collectors between joist seats

Quote (Deker)

The effect you described above wouldn't be enough to make up for a 300% shortfall in demand--possibly more depending on the SFRS and how it was proportioned by the designer--unless the demand capacity ratio was kept low to begin with, in which case one could argue that the system already was designed considering amplified loads.

That bit about the 300% caught my eye and I'm hoping that you can elaborate. My general thoughts:

1) It sounds to me like you're pitching true capacity design for diaphragm connections rather than the over strength approach. I see an inherent logic in that but, at the same time, to my knowledge we are not enforcing that level of rigor and performance elsewhere where things are non-ductile and the overstrenght approach is used ubiquitously.

2) The whole thing is complicated by the fact that, in multistory buildings, the forces that we use for diaphragm boundary elements are not the same forces that we use to design the VLFRS. As I understand things, the diaphragm forces are higher to reflect higher local accelerations resulting from higher mode effects. So, in this sense, the simple model where we assume a thing to continue attract force until a mechanism forms in the VLFRS is a bit murky. So is using over strength for a force level not actually matching VLFRS mechanism formation in my opinion.

3) Across the board, I suppose that I don't really understand how an over strength approach deals with with the DCR issue that you've tabled in ELF design. To me, it would seem to be a much weaker version of capacity design than is the real thing.

What am I missing in the logic there?

RE: tube collectors between joist seats

Quote (KootK)

Much depends on the situation. OP's situation is CMU shear walls by the sound of it. In my neck of the woods, that would mean that the tube lugs would get welded to small embeds in the CMU which would then become the brittle link in the chain and affect this calculus:

1) If I can make a go of it with joist rollover, I need to do that to stay competitive.

2) If I can make a go of it with a tube lug every other joist space, I need to do that to stay competitive.

Thanks for pointing that out, I overlooked that this was occurring directly above the wall.  In that case, the demand may not be trivial relative to the capacity of the connection.  For anchorage into CMU it's more likely that overstrength should be applied.

Quote (KootK)

b) I feel that it harms are profession a bit when you get stuff like this floating around that suggests that what many engineers are doing is incorrect / less correct without that stuff having made it into codes and/or widely accepted design guides yet. In sending out mixed messages to the design and construction community, we undermine ourselves I feel. Collectively, we seem wishy washy in a way that diminishes confidence in what we do. If it were up to me, we'd keep this kind of stuff close to our chests until a consensus is reached and disseminated. That said, I doubt that many contractors are reading Sabelli articles or identifying there reasons for relatively slight differences in connection strategies on EOR drawings.

I mostly agree, but I would prefer a solution that doesn't result in over-codification.  I think most engineers recognize code as a minimum standard and have no qualms about exceeding that when they have a good rationale for doing so.  You actually touched on one of the main reasons I participate in this forum...aside from how much I learn from others and enjoy reciprocating where I can, I think it's important to hash out issues like the one we're discussing in the hopes of reaching a broader consensus so that we don't have so much variation in how we interpret code requirements / intent.

Quote (KootK)

2) The whole thing is complicated by the fact that, in multistory buildings, the forces that we use for diaphragm boundary elements are not the same forces that we use to design the VLFRS. As I understand things, the diaphragm forces are higher to reflect higher local accelerations resulting from higher mode effects. So, in this sense, the simple model where we assume a thing to continue attract force until a mechanism forms in the VLFRS is a bit murky. So is using over strength for a force level not actually matching VLFRS mechanism formation in my opinion.

Interestingly, we used to design collectors for Ω0Fx and compare to Fpx prior to ASCE 7-10 when the code started requiring that the overstrength factor be applied to the diaphragm forces, as well. A less conservative approach than what the code now requires might be to amplify the first mode response only (reduced by R) and add the elastic response of all higher modes. Another approach is to use the peak response from a NLRH procedure (obviously way beyond what is considered practical for most buildings). In any case, it's clear that ELF diaphragm forces on their own are generally not large enough to preclude inelasticity in diaphragms and collector elements.

Quote (KootK)

For example, if you have twenty connections in line and lose one, the demand in the remaining connections goes up about five percent and, in all likelihood, everything remains just fine. I suspect that the capacity at which any potential unzipping would be arrested in such a system is probably well in excess of an amplified demand. The trouble with this line of reasoning is quantifying it. I wait patiently for some reliability theorist to tell me if the magic number is five, ten, or twenty.

Quote (KootK)

1) It sounds to me like you're pitching true capacity design for diaphragm connections rather than the over strength approach. I see an inherent logic in that but, at the same time, to my knowledge we are not enforcing that level of rigor and performance elsewhere where things are non-ductile and the overstrenght approach is used ubiquitously.

I'm not pitching a true capacity design approach, but I am pitching a design that's informed by capacity design principles.  Under seismic load there is going to be much more overlap between the load and resistance probability curves than is typical for other types of loading.  If you're designing this connection for code level forces and not considering overstrength, there's a high likelihood that you're underestimating the load that the system will experience by 3x to 6x (see the last part of my post in this thread), thereby shifting the load curve to the right. The effect of this shift would far exceed the benefit gained by the effect you described above where the net system strength is greater than the sum of the minimum strength of each individual element. To my knowledge, even the maximum tested values for most elements do not exceed the code nominal strength values by that wide of a margin.

Quote (KootK)

3) Across the board, I suppose that I don't really understand how an over strength approach deals with with the DCR issue that you've tabled in ELF design. To me, it would seem to be a much weaker version of capacity design than is the real thing.

It's not perfect, and that's why it's important for designers to be aware that design decisions made when proportioning the SFRS may impact the amount of force that is delivered to other elements in the load path. A strict capacity based design approach may not be practical, but any element in the seismic load path that is not expected to behave ductility should be designed for some level of force higher than that used to design the SFRS. To me, the idea that I can throw 100 little non-ductile connections in the load path and say that I can design them for the same level of force as my SFRS just because there are so many of them isn't really compatible with that philosophy.

RE: tube collectors between joist seats

Alright, sign me up for TEAM OVER STRENGTH then. I'm substantially convinced.

Taking the drill down a bit further, another place that I see inconsistency is in the calculation procedures normally used to determine the demand in distributed diaphragm connections. Designers will normally make the shear panel assumption and assign all of the diaphragm flexural demand to discrete chords. In reality, you almost always have distributed chords which could amplify the peak shear connection demand by 50%. This, in reference again to the potential unzipping of brittle things.

So, tallying some of this stuff:

1) You've got a real world overstrength factor that might be as much as double the ASCE values.

2) You've got DCR ratio problems only loosely accounted for.

3) You've got diaphragm forces different, and higher than, VLFRS forces in multistory structures.

4) The procedures for calculating distributed shear connection demand are gross approximations.

5) There will be a significant redundancy benefit even if it's difficult to pin down and likely less than 600%.

6) In my experience, many common diaphragm to VLFRS connections are non-ductile. Certainly that is the case with CMU.

Quote (Deker)

To me, the idea that I can throw 100 little non-ductile connections in the load path and say that I can design them for the same level of force as my SFRS just because there are so many of them isn't really compatible with that philosophy.

I agree. But, then, I think that one can be forgiven for not getting overly excited about accuracy given all of these things. In general, and with seismic design especially, I still view structural design as little more than the somewhat intelligent, rough proportioning of things. But, yeah, maintaining a consistent design philosophy is important, even when great precision is not available to be had.

It is still my understanding that, even in high seismic events, failures of these kinds of distributed diaphragm connections in shear are few and far between. What is more often observed is:

1) Failures of collector connections where there is a far greater concentration of diaphragm load and;

2) Failures where out of plane forces on heavy wall systems tear potential VLFRS elements away from the diaphragm in such a way that connections to the diaphragm for tension, rather than shear, is the issue du jour.

RE: tube collectors between joist seats

Glad to have you on board! Couldn't agree more with your comment about proportioning things. If you'll notice, I'm not as hung up on the EXACT force to be used as I am about making an effort to ensure that collectors and shear transfer elements are proportionally stronger than the SRFS.

RE: tube collectors between joist seats

Quote (Deker)

If you'll notice, I'm not as hung up on the EXACT force to be used as I am about making an effort to ensure that collectors and shear transfer elements are proportionally stronger than the SRFS.

Acknowledged. That's precisely what I was alluding to with this statement:

Quote (KootK)

But, yeah, maintaining a consistent design philosophy is important, even when great precision is not available to be had.

Quote (Deker)

Glad to have you on board!

It may be short lived. I'm now thinking that:

1) Connection ductility is largely irrelevant and;

2) Every collector connection, whether ductile or not, should be designed for over strength (or whatever is meant to capture a degree of capacity design).

I'll post a sketch when I've got it worked out in my head to my satisfaction...

RE: tube collectors between joist seats

Quote (KootK)

2) Every collector connection, whether ductile or not, should be designed for over strength (or whatever is meant to capture a degree of capacity design).

Certainly if it's a discrete collector connection in the traditional sense, which is spelled out clearly in the code. And ideally it would be applied to all distributed shear transfer and collector elements, although that bit is open to interpretation, i.e. where does the diaphragm end and the collector begin? For those who wish to consider these distributed elements as part of the diaphragm, at a minimum the failure mode should be considered when choosing whether or not to amplify the loading.

Looking forward to your sketch...

RE: tube collectors between joist seats

Quote (KootK)

In reality, you almost always have distributed chords which could amplify the peak shear connection demand by 50%. This, in reference again to the potential unzipping of brittle things.

I am not following this train of thought, care to explain? I've heard of distributed chords before, but I am not seeing how this could increase peak shear.

S&T - www.re-tug.com

RE: tube collectors between joist seats

Just stepping back a bit and looking at this structure holistically, the out-of-plane anchorage for top of CMU wall will be critical. The code explicitly amplifies these anchorage loads based on observations of poor performance in earthquakes, especially with flexible diaphragms. These OOP anchorage forces may govern the connection design and supersede the in-plane shear forces. Seems like the more important consideration from a collapse prevention perspective.

RE: tube collectors between joist seats

@S&T: you know how the peak shear stress in a rectangular cross section is [1.5 V / A] rather than just [V / A]? Similar thing here. Let me know if that doesn't do the trick.

RE: tube collectors between joist seats

Quote (Deker)

where does the diaphragm end and the collector begin?

Yeah, that. Maybe I can work with that instead of a sketch for now. Humor me.

In your opinion, what is it about a diaphragm that justifies it being designed without over strength? If the answer is "ductility", then what, specifically, is it about ductility that justifies not amplifying the diaphragm load?

RE: tube collectors between joist seats

Excerpt from FEMA P-1026 - Seismic Design of Rigid Wall Flexible Diaphragm Buildings: An Alternate
Procedure


Quote (FEMA P-1026)

Observation of earthquake damage to the in-plane rigid shear walls or the main flexible roof diaphragm has been rare, except for collateral damage from the out-of-plane wall anchorage issues. The perimeter shear walls often consist of largely solid wall portions with relatively few penetrations, resulting in inplane lateral strength significantly in excess of that required for seismic forces. This inherent overstrength of the shear walls transfers the inelastic building behavior into the diaphragm. It is important to consider that the out-of-plane detachment of the heavy walls from the diaphragm in the past may have protected the diaphragm from experiencing in-plane seismic forces which could have led to global failure. The overstrength of the walls, combined with the code required higher wall anchorage and collector force levels, could potentially make diaphragm yielding, foundation rocking or sliding, and global response more critical for RWFD buildings in future earthquakes.

RE: tube collectors between joist seats

Some further relevant discussion from the FEMA Design Guide:

The main point here is that RWFD-type structures' yield mechanism in the diaphragm. So the boundaries need to be super strong to permit a more distributed development of inelastic behavior deeper into the interior regions of the diaphragm. In my mind, it's clear that the boundary connections should be designed with a substantial overstrength factor. Especially given the fact that metal deck yielding is not a well-defined mechanism and thus actual boundary forces in a seismic event could significantly exceed the design forces we come up with using R-factors.

Quote (FEMA P-1026)

4.2.1 Encouraging Distributed Inelastic Behavior
Analytical studies have demonstrated that the performance of a diaphragm during strong earthquake shaking can be improved if yielding is spread over a large portion of a diaphragm’s span instead of focused at the boundary. The spread of diaphragm yielding is improved if the location of initial yielding is shifted away from the boundaries of the diaphragm. Surprisingly, this can be achieved either by intentionally weakening a portion the diaphragm’s interior areas below current building code levels, or by increasing the strength-to-demand ratio of the diaphragm near its boundaries.

Distributing the inelastic behavior deeper into the diaphragm also requires that the diaphragm connectors that yield first exhibit sufficient positive post-yield stiffness behavior. In other words, once the critical connectors begin to yield, they need to resist increasing load rather than maintaining the initial yield load so connectors elsewhere in the diaphragm also reach their yield load. The connectors that yield must also have sufficient post-yield deformability so the diaphragm as a whole has adequate post-yield deformability to provide the required collapse resistance. The overall diaphragm deformability increases if connector yielding spreads over a large portion of the diaphragm. Also, the spread of yielding reduces the deformation demand on individual connectors. The hysteretic responses of the nail connectors in the wood structural panel diaphragms have sufficient positive post-yield stiffness to effectively move the location of first yield away from the diaphragm boundaries and spread the yielding over a large portion of the diaphragm’s span. In the case of steel deck diaphragms, insufficient data is available to make that determination. Thus, the recommendations for spreading diaphragm yielding are limited to wood structural panel diaphragms at this time. An alternative for steel deck diaphragms could involve simply using a smaller Rdiaph until sufficient data is obtained to justify an approach that distributes inelastic behavior. An appropriate Rdiaph for steel deck diaphragms where the spreading of inelastic behavior is beyond the scope of this report.

RE: tube collectors between joist seats

I'd not meant to target the RWFD typology in this discussion but I think that I can use it to make the point that I've been thinking of from another angle.

For energy dissipation, all buildings designed conventionally for seismic require some degree of inelastic, inter-story drift between the ground and the significant mass sources. In a RWFD building, that happens within the diaphragm. On a 30' tall big box building, that drift my be on the order of 6" at the middle of the diaphragm. And it's that capacity for significant ductile displacement that allows us to cap the forces developed within the structure at roughly the VLFRS mechanism formation values.

Conversely, there aren't many ductile, diaphragm boundary connections schemes that could tolerate even 1" of ductile displacement without being torn apart. As such, it seems to me that ductility cannot be assumed to cap the forces developed within the structure near the VLFRS mechanism formation values.

I'm not saying that ductility in the boundary connections is bad. Ductile stiff is almost always an improvement over brittle stuff. I simply question whether the ductility available in these kinds of connections provides enough inelastic drift capacity that one can consider it to effectively aid in the capping of seismic force development. If not, then I feel that boundary connections ought to receive overstrenght treatment even if they are nominally ductile.

RE: tube collectors between joist seats

Quote (KootK)

I'd not meant to target the RWFD typology in this discussion but I think that I can use it to make the point that I've been thinking of from another angle.

Understood. I was making a conscious effort to steer the discussion towards this specific typology, since I think the OP's detail used mostly within that classification of structures. Given that RWFD buildings response to earthquakes is fundamentally different, I thought it important to put the discussion of boundary/collector overstrength into that context.

I agree with everything you stated in your last post Koot. Given the nature of the connection details we have for metal deck to CMU or tilt up walls, there's not much ductility to be had. That's why I say overstrength is the only real way to go in this situation. Not only for in-plane shear, but for the out-of-plane anchorage as well.

RE: tube collectors between joist seats

Quote (KootK)

In your opinion, what is it about a diaphragm that justifies it being designed without over strength? If the answer is "ductility", then what, specifically, is it about ductility that justifies not amplifying the diaphragm load?

I don't think it is justified in the spirit of traditional capacity design where we are expected to limit all yielding to the vertical elements. But I think it's implied by the design requirements of ASCE 7 that some diaphragm inelasticity is expected and is perhaps even beneficial in terms of total energy dissipation and force reduction by way of period elongation. However, it does present issues with inconsistency between the R values used for vertical elements and the available ductility in the diaphragm. There's a lot of work that's been done recently to address these issues, with FEMA P-1026 that bones206 referenced being one resource. There's also the alternate diaphragm design procedure that was introduced in ASCE 7-16, the 2015 NEHRP Recommended Provisions Resource Papers, and a ton of research that's been taking place as part of the Steel Diaphragm Innovation Initiative.

As for the logic behind designing a distributed shear transfer element without overstrength, I think it goes something like this: If some diaphragm inelasticity is expected to result from normal use of the ASCE 7 provisions, that inelasticity is likely to occur at high shear regions near vertical elements / collectors. If there are ductile distributed shear transfer elements at the same location and directly downstream from the point of peak shear in the diaphragm, it doesn't matter if yielding at that location occurs in the diaphragm or in the shear transfer elements. Yielding in either will limit the force that can be delivered to the other, and the global behavior of the diaphragm would be the same in either case.

I don't necessarily agree with that design strategy, but I can't say it's unreasonable compared to standard practice.

RE: tube collectors between joist seats

Quote (Deker)

If there are ductile distributed shear transfer elements at the same location and directly downstream from the point of peak shear in the diaphragm, it doesn't matter if yielding at that location occurs in the diaphragm or in the shear transfer elements.

I agree with this as a hypothetical concept, but in practice the available ductility of these shear transfer elements is very limited. Maybe someone will invent some innovative rigid shear wall to metal deck seismic connectors that absorb tons of inelastic energy, but I think the current reality is that ductility at the boundaries is inadequate for significant earthquakes. Therefore you gotta go overstrength.

RE: tube collectors between joist seats

Quote (kootk)

1) If I can make a go of it with joist rollover, I need to do that to stay competitive.

2) If I can make a go of it with a tube lug every other joist space, I need to do that to stay competitive.
I have been out the real world for a number of years now (government employee), but is omitting the tube or going with half the number going to make much difference in the overall cost of a building? Just curious.

RE: tube collectors between joist seats

Quote (Gopher13)

I have been out the real world for a number of years now (government employee), but is omitting the tube or going with half the number going to make much difference in the overall cost of a building? Just curious.

I wouldn't say it makes a substantial difference. However to the more frugal owners/contractors, every dollar matters. So if engineer A calls off the welded collector all the time everywhere, and engineer B only calls it off when absolutely necessary, those people are going with engineer B 100% of the time. And most extremely profitable clients are the frugal ones who pay attention to the small things.

My grandpa used to say "Worry about the nickels and dimes, the dollars will take care of themselves." Sometimes I look at my bank account statements and wish I listened to him even more.

RE: tube collectors between joist seats

Thanks jayrod12!!! Makes sense.

RE: tube collectors between joist seats

@Deker: thank you for your detailed explanation. For now, I think that I'll just have to accept that as the limit to my understanding when it comes to reconciling ASCE design philosophy with general, capacity design philosophy.

Quote (Gopher13)

I have been out the real world for a number of years now (government employee), but is omitting the tube or going with half the number going to make much difference in the overall cost of a building?

My answer matches Jayrod's:

1) Let's say we're talking $20K worth of cost on a big building to install those lugs on top of masonry wall embeds.

2) That $20K is probably nothing in terms of the overall cost of the development. Like, < 0.5% of the cost kind of "nothing".

3) That same $20K might buy four used cars for the kids of four of the contracting firm's principals.

So it's that constant juxtaposition between what is a small number for the owner and what is a large number for the contractor. I can tell you that, unless the owner has also been the contractor, I've never had an owner call me up to bitch at me because my deck angle is bolted to a block wall at 24" oc rather than 32" oc.

RE: tube collectors between joist seats

Quote (KootK)

unless the owner has also been the contractor
Which is one reason I hate doing Design-Build projects. So much of that bitching it makes the work not worth the hassle.

RE: tube collectors between joist seats

Would be nice if you could make the tubes continuous, with shop welded studs. Then offset them enough to clear the joist seats and run by. Then the masons just have to set them on top of the wall and grout. Way easier and cheaper than having a field welder up there measuring and welding a thousand little welds for a week.

RE: tube collectors between joist seats

Quote (kootk)

kids

Soft spot. ya got me. I will rely on joist rollover from now on.

RE: tube collectors between joist seats

Wouldn't offsetting it to miss the joist seat just put a whole bunch of additional eccentricity in the wall to change that design? Most joists need a minimum of 2 1/2" of bearing, if centered on the wall to minimize the eccentricity to the minimum value then that really only leaves you with 2 1/2" from face of CMU to edge of joist seat. Not really a whole ton of room. I guess you could put a 4x2HSS LSV there, but your studs you were talking about would never make it into the cores.

It's one of those damned if you do, damned if you don't type scenarios. Welding costs more, or your wall does. There's no free lunch.

RE: tube collectors between joist seats

Thanks for the star!!! Pretty sure it's my first one. Fitting it comes on a non-technical comment.

RE: tube collectors between joist seats

jayrod - eccentricity in what sense? I wouldn't think in-plane shear delivered eccentric to the wall centerline would cause an issue.

RE: tube collectors between joist seats

I meant either the joists would need to get shifted inwards on the wall, causing eccentric loading, in order to have your continuous tube be able to be anchored into the cores. Or you need to have some other form of connection between the collector and the wall if you want your joists concentric.

RE: tube collectors between joist seats

I see what you mean now. There’s probably a way to do it with some creative detailing, but I agree that in the final analysis that creative detailing will probably end up only lining the contractor’s pocket.

RE: tube collectors between joist seats

Quote (bones206)

I agree that in the final analysis that creative detailing will probably end up only lining the contractor’s pocket.

I wouldn't let that deter any innovation. I see the evolution of it being something like this:

1) The savings do just wind up in the contractor's pocket.

2) The contractor starts to prefer your design services, in part because of your awesome, cost saving detail.

3) Everybody in EOR land starts to adopt your awesome, cost saving detail because they have to in order to stay competitive.

4) The savings do start to wind up in the owner's pocket and, more importantly, come to represent efficiencies that make us more efficient as a society and a species.

It's the holiday season now so messages of hope like this are important.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close