Horizontal Trussing Threshold for Roof Diaphragm 3

[mes7a]

Hi,

How rigid must be the horizontal trussing in a roof before it can be considered a full fledge diaphragm? When the structure was constructed. They use 0.5x0.5 meter column from foundation all the way to third floor (it was designed for 4 storey).. but we decided now to build up to 3 storey only and plan to use thin metal roof to shed rain at gutter at side.

I know there must be a threshold in the horizontal trussing before the huge columns can become a diaphragm. For example. If you merely use a pole connecting the columns.. it can't be considered a diaphragm. Is it?

Also must the rafters be straight horizontal.. how about a bent gable rafter with apex at middle.. can a bent rafter create a diaphragm.. again what is the threshold of the rigidity? The pole is also rigid.. must it be certain strength compared to the huge columns?

Thank you.

 

[KootK]

I know there must be a threshold in the horizontal trussing before the huge columns can become a diaphragm.

I know of no such threshold.

Also must the rafters be straight horizontal.. how about a bent gable rafter with apex at middle.. can a bent rafter create a diaphragm

Non-horizontal rafters can support valid diaphragms. We do this all the time with residential wood trussed roofs.

I suspect that you would draw much better responses here if you posted some sketches of your situation. Is your diaphragm truly horizontal trussing with discrete members acting as truss webs? Or are you using the thin metal deck as your roof diaphragm?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

I know of no such threshold.

The functions of beams or rafters as diaphragm is to equalize the loads between all the columns? But if the columns are huge.. the small rafters may not even move them.

I suspect that you would draw much better responses here if you posted some sketches of your situation. Is your diaphragm truly horizontal trussing with discrete members acting as truss webs? Or are you using the thin metal deck as your roof diaphragm?

This is the layout.

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The roofing is just thin metal or plastic sheet to shed rain with gutter at sides.. so there is no roof diaphragm.. just rafters diaphragm and they only rested at the columns (where the perimeter beam that holds the wall would be solely connected). What kind of diaphragm action would this produce?

The rafter is made of HSS that is sized 250mm x 100mm and 8mm thick. Only 3 pcs as the picture shows. The slope is as follows.

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[KootK]

What is the vertical lateral system here? Are there shear wall or cross braces? Or is the lateral system just the columns cantilevered up from the floor below?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

What is the vertical lateral system here? Are there shear wall or cross braces? Or is the lateral system just the columns cantilevered up from the floor below?

No. It's purely special moment frames with no cross braces or shear wall at the transverse and longitudinal side. Yes, the lateral system are just the columns which started from the foundations all way to third floor.

The rafter is made of HSS that is sized 250mm x 100mm and 8mm thick. Only 3 pcs as the picture shows. The slope is as follows.

..

 

[KootK]

You don't need a diaphragm here for equilibrium. You'd only need it if you felt that differential lateral column movement might damage the roofing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

You don't need a diaphragm here for equilibrium. You'd only need it if you felt that differential lateral column movement might damage the roofing.

This is the whole building in 3D. Note the walls are only in the left and right side.. the middle front and back is open. The 9 columns are the sole lateral resisting system.

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If I don't have a roof diaphragm. I'm afraid the floor diaphragm below may be stressed. What do you think about steel deck. Must the roof be made heavier to transfer half the wall load to the roof diaphragm? Without such.. I think it would go down to the floor below just you stated in some threads here.

 

[KootK]

I stand by my previous post. Your walls can span up to your beams and your beams can span laterally over to your columns. If your metal deck had an established shear capacity, all the better.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

I stand by my previous post. Your walls can span up to your beams and your beams can span laterally over to your columns. If your metal deck had an established shear capacity, all the better.

What is the threshold to determine when the roofing would be cantilevered to the floor below and whether the roofing can serve as independent diaphragm.. when there is perimeter beams at top of the columns?

Also I'm reading a thread made in 2008:

http://www.eng-tips.com/viewthread.cfm?qid=219626

Msquared48 said:

"The "roof diaphragm" is the plywood membrane over the roof truss system, not the roof trusses themselves. Now the trusses have to be able to withstand wind and seismically induced loads applied in certain ways, but the structural diaphragm is the plywood. The roof trusses and intermediate eddge blocking serve to provide support to the diaphragm to stiffen it as necessary, but they are really not a part of the structural diaphragm per se."

So Kootk. Rafters are not considered diaphragm? But I read elsewhere where you said :

"Unless the top storey perimeter wall system is cantilevered from the floor below, which would be very rare, there pretty much has to be a roof diaphragm of some sort. That could include:

1) Roof deck acting as a diaphragm or;
2) Horizontal trussing in the roof plane acting as the diaphragm."

Msquared48 said roof truss system are not diaphragm.. you said it is. Can you please elaborate what is really the case?

I think diaphragms is to simply distribute the forces to the vertical lateral force resisting system. Why do the 2nd floor need slab diaphragm yet the roof doesn't need steel deck diaphragm? Unless you mean if the 2nd floor doesn't need to have slabs.. diaphragm are not necessary?

 

[KootK]

You're making this a good deal more complicated than it needs to be mes7a. And you're clipping quote from threads that pertain to substantially different scenarios. Think of it this way:

1) If you don't have an equilibrium satisfying load path without a diaphragm, then you need one (usual case in steel).

2) If you do have an equilibrium load path without a diaphragm, then you don't need one (this case).

3) Even if you don't need a diaphragm, you may want one for the sake of structural efficiency (as with a concrete floor).



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

You're making this a good deal more complicated than it needs to be mes7a. And you're clipping quote from threads that pertain to substantially different scenarios. Think of it this way:

1) If you don't have an equilibrium satisfying load path without a diaphragm, then you need one (usual case in steel).

2) If you do have an equilibrium load path without a diaphragm, then you don't need one (this case).

3) Even if you don't need a diaphragm, you may want one for the sake of structural efficiency (as with a concrete floor).

Key is the word equilibrium. I can't find the word equilibrium on references on diaphragm, for instance. Diaphragm is defined in one source as:

"Diaphragms are horizontal elements that distribute seismic forces to vertical lateral force resisting elements. They also provide lateral support for
walls and parapets. Diaphragm forces are derived from the self weight of the diaphragm and the
weight of the elements and components that depend on the diaphragm for lateral support. Any roof, floor, or ceiling can participate in the distribution of lateral forces to vertical elements up to the limit of its strength.

In the above context. What do you mean by equilibrium? You simply mean to distribute the lateral forces? But why does steel not considered as equilibrium satisfying load path? Are you talking of steel column (noting my column is RC). Also noting my rafter is made of HSS steel. why does steel column not equilibrated compared to RC columns?

Thanks so much.

 

[KootK]

What do you mean by equilibrium? You simply mean to distribute the lateral forces?

Sort of. I mean a path that gets lateral loads from their point of origin down to the foundation.

why does steel column not equilibrated compared to RC columns?

Steel columns are often pin supported at diaphragm levels. Concrete columns almost never are.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

1. There is nothing unusual about using cantilevered concrete columns.
You just have to have horizontal members which deliver the load to the columns, and the columns need to be designed for the loads.

2. Steel roofing is to shed water, not to serve as a diaphragm.

 

[mes7a]

Sort of. I mean a path that gets lateral loads from their point of origin down to the foundation.

Ok. In your structures.. do you use steel deck as roof or thin metal or plastic roof to shed rain? Why would anyone use the heavier steel deck. They offer no resistances to falling airplanes too.. unless it is to be able to walk on the steel deck?

 

[hokie66]

When steel deck is used, the roofing is of a different type than metal (or plastic).

Falling airplanes? Is that a design condition for you?

 

[mes7a]

Kootk, I need to know something. First. Have you ever designed building like the following where there is no shear wall nor is it braced frame.. but entirely special moment frame? not only that.. but all the walls are located at the sides. The middle is open (just glass at front and back).. meaning the heavy tranverse side is entirely resisted by the 9 columns during seismic lateral movement.

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The building was originally designed for 4 storeys with concrete deck with all the seismic load combinations. But I lessen the actual building to just 3 storey with light roofing metal sheet.. I do this because you can't expect the actual construction to match every thing in the design or theoretical frame analysis.. therefore cut it one storey short to be on safe side (at least my philosophy).

The following is the column details for them. Only the 0.5x0.5 column is at center.

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Now in the book Design of Concrete Structures (14th edition), there is this paragraph:

"The typical shape of a column interaction diagram shown in Fig. 8.10 has important design implications. In the range of tension failure, a reduction in axial load may produce failure for a given moment. In carrying out a frame analysis, the design must consider all combinations of loading that may occur, including that which would produce minimum axial load paired with a given moment (the specific load combinations are specified in ACI Code 8.10 and described in Section 12.3). Only that moment of compression that is certain to be present be used in calculating the capacity of a column subject to a given moment."

Now going back to the structure which is designed for 4 storey with concrete roof and only actually 3 storey built with light roofing. What is your experience about reducing actual storey? Have you ever done it? Generally, you may think the structure is even stronger with lesser floor actually built.. but in the tension part of the column, a reduction in axial load may not have the compression to make the tension part stronger (or below the balanced point in the interaction diagram). What do you make of this? What structure is sensitive to this?

 

[KootK]

Since your columns have the same reinforcing pattern full height, I would not expect this to give you any trouble. When your building was four stories, you had a roof level condition where there was moment but little axial load. Now that your building is essentially three stories, you should have an almost identical condition at the third floor.

It's actually a fairly common occurrence for top floor columns to require the most reinforcing for exactly this reason. Out of curiosity, is the book that you referenced this one: Link? "Design of Concrete Structures" is a rather common textbook title. I guess structural engineers suck at product differentiation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

Since your columns have the same reinforcing pattern full height, I would not expect this to give you any trouble. When your building was four stories, you had a roof level condition where there was moment but little axial load. Now that your building is essentially three stories, you should have an almost identical condition at the third floor.

But if the building is fully 4 storey. The tension part of the column of say the second storey would be stronger than if there is less axial load (from 3 storey). But then, for the compression side. Lesser axial load means more moment capacity (exactly the basis of the interaction diagram). You are saying it's more of a problem if the columns were tapered or made smaller as it reaches the roof? Since we use 12 to 14 meters bar length.. there is no splice from foundation to 3rd floor. The beams used were mostly 300 width by 500 depth, the beam bars were not spliced either because the building width is 12 meters side to side.

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Just imagine the distance between the column is 6 meters each. Right now I'm trying to put walls and light roof to the third storey (presently it's just 2 storey with concrete roofdeck (same beams as below, designed for a floor). It's so hard to decide whether to use plain hollow block walls or lightweight precast.. or even just very light PIR insulation. I want the lightest wall to even have more safety allowance. Anyway the walls are just on left and right side.. no wall at front or back. This would focus wall seismic mass on transverse side. When the wall is on left side, and there is seismic movement to the right.. Can you say the floor diaphragm would restrain the wall seismic loads or would the columns mainly on the left restrain the walls lateral movement? And in special moment frame building without shear wall or not braced frames.. what is the componenents that make up the diaphgrams? slabs and beams?

I also am deciding between lightweight thin metal/plastic roof or heavier steel deck. But the latter seems much more expensive. However, a heavier steel deck would produce more axial load and perhaps strengthen the column-beam joint below it (?)

It's actually a fairly common occurrence for top floor columns to require the most reinforcing for exactly this reason. Out of curiosity, is the book that you referenced this one: Link? "Design of Concrete Structures" is a rather common textbook title. I guess structural engineers suck at product differentiation.

Yes. It's that book where I memorized from cover to cover and my favorite structural references especially on manual computations.

 

[KootK]

You are saying it's more of a problem if the columns were tapered or made smaller as it reaches the roof?

Or, more likely, if the reinforcement was made smaller as the columns reached the roof.

Can you say the floor diaphragm would restrain the wall seismic loads or would the columns mainly on the left restrain the walls lateral movement?

All columns would share the load in proportion to their stiffness due to the presence of the diaphragm. As you mentioned above, one of the important diaphragm functions is load distribution.

a heavier steel deck would produce more axial load and perhaps strengthen the column-beam joint below it (?)

None of the roofing options that you've suggested would add enough compressive load to affect column flexural strength in a meaningful way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mes7a]

Or, more likely, if the reinforcement was made smaller as the columns reached the roof.

Why didn't you mention about the lower floor column tension strength. Perhaps you are saying that in the lower floor, the axial load is always on or above the balance point of the interaction diagram due to more axial load carrying the upper floors? But if the column is too massive like one meter across.. it is possible even all the load above would still make the lower floor column axial load below the balance point of the interaction diagram.. isn't it? For example. If you have only 2-storey building and your column is one meter in size. The axial load is below the balance point (of the interaction diagram) at the ground floor. What is the effect of this? Maybe lesser moment capacity but still enough for the ground floor??

All columns would share the load in proportion to their stiffness due to the presence of the diaphragm. As you mentioned above, one of the important diaphragm functions is load distribution.

In moment frames.. many structural engineers design the perimeter columns as moment frame while the center is just gravity frame (do you also do this?). But in my structure, the center column is the strongest (being 0.5x0.5 meter in size compare to 0.5x0.4 meter at the sides). Do you think center columns can be a main lateral resisting system?

None of the roofing options that you've suggested would add enough compressive load to affect column flexural strength in a meaningful way.

Ah. You mean it has only to be another floor to affect the lower floor column compression load in meaningful way. But I'm nervous following full 4-storey because you never know what the contractors do when you are not observing it. The following is the foundation plan of the building anyway (it's suggested by BARetired ) It's combined footing and we sized it 3 times larger to handle any trace of overturning moments etc.).

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