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've racked my brain over this Mes7a. However, I cannot think of a good, simple way to repair your building. My advice would be to somehow engage the services of a structural engineer that you trust and have them perform a thorough analysis of the situation.

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.

 

[BAretired]

@KootK,

I haven't followed this thread all the way from the start but I am somewhat familiar with the problem. Do we know that a problem exists? It seems to me that the column in question will simply form a "soft spot" at the location of the epoxy filled void, a partial hinge so to speak. For gravity load only, moments will tend to redistribute to the top of the column and if that is not sufficient, to the beam which is applying the moment. We don't have any information on the analysis by the Engineer of Record so we don't really know how tightly he designed it. Perhaps he has a little spare capacity in other parts of the structure which could alleviate some stress redistribution. For seismic loads, perhaps the building can be rendered safe without reliance on this column at all.

I agree that the problem must be reviewed by a competent structural engineer. Why not ask the Engineer of Record to revisit the issue and provide an opinion. The building is occupied now so it is not something which can be put off indefinitely.

BA

 

[mes7a]

I haven't followed this thread all the way from the start but I am somewhat familiar with the problem. Do we know that a problem exists? It seems to me that the column in question will simply form a "soft spot" at the location of the epoxy filled void, a partial hinge so to speak. For gravity load only, moments will tend to redistribute to the top of the column and if that is not sufficient, to the beam which is applying the moment. We don't have any information on the analysis by the Engineer of Record so we don't really know how tightly he designed it. Perhaps he has a little spare capacity in other parts of the structure which could alleviate some stress redistribution. For seismic loads, perhaps the building can be rendered safe without reliance on this column at all.

I agree that the problem must be reviewed by a competent structural engineer. Why not ask the Engineer of Record to revisit the issue and provide an opinion. The building is occupied now so it is not something which can be put off indefinitely.

The engineer on record said this problem is common in construction.. it occurs even on columns anywhere of 50-storey buildings. Once the epoxy company injected about 1 meters high of epoxy to a joint of a high rise. The designer on record said he forgot the concept of stress strain so just couldn't comprehend what I was saying. I'm not making this up. He said compressive test shows it is tested at 10,000 PSI. so no problem.. but when I said this stress occured at strain that is over 8 times that of concrete. He said he didn't want to discuss theoretical subject as he forgot them in school. When I visited other structural engineers. They said they also do it. So I have no one to turn to locally in my country. Hence seeking international multidisciplinary experts. Only left for me to let my voice be heard is to write an article explaining it all in the Structural Society newsletter. This is the only way for me to reach out to the contractor and designer who is foreign to concept of interaction diagram. He hired 20 newly graduates to operate the ETABs software which output the bars and that's what they gave to clients. When I tried to discuss with them. They said their only job is to output rebars numbers and it is up to the contractor to execute things properly. But the contractor is very incompetent.. so only thing left is to write a newsletter about this all so my voices would be heard.

About moment redistribution. What you mean by soft spot when you mentioned "It seems to me that the column in question will simply form a "soft spot" at the location of the epoxy filled void, a partial hinge so to speak. For gravity load only, moments will tend to redistribute to the top of the column and if that is not sufficient, to the beam which is applying the moment.". Won't the soft spot simply have maximum moment enough to put the bars in yield at the compressive bars and break the column. Why would it distribute the moments to top of column? I use a straw or tube to visualize what you all have been describing.. I cut a void in the straw and when I bend it.. the void just bends more.. there is no redistribution. I need to be familiar with this concept so if the newsletter readers used this same explanation. I can understand what he meant and can discuss it. Thanks a lot.

[Edit: maybe you meant the void section turn the column from fixed to pinned.. could that happen"]

 

[BAretired]

Yes, your edit is roughly correct. I don't believe it turns it into a complete pin, but under stress, the epoxy material will allow more rotation at that point, reducing the stiffness of the column. What this means is that the column is not as stiff as the designer assumed in his analysis, so the moment in the column from gravity loading is not as high as he calculated but other members meeting at the top of that column will be affected so the issue is not confined to a single member.

In the case of seismic loading, the story is a little different. The added flexibility of the lower column will not be a problem if the remaining columns are capable of resisting lateral forces by themselves. The partial hinge in this column will tend to prevent it from contributing significantly to ground accelerations which cause compression on the exterior face of this column. That means that the rest of the columns, stairwells etc. must be capable of resisting seismic forces without much contribution from the column with the epoxy filled void.

Perhaps the Engineer of Record could accommodate you by doing another computer run with a reduced stiffness at the void location. Seems to me to be the least he could do.

BA

 

[mes7a]

Yes, your edit is roughly correct. I don't believe it turns it into a complete pin, but under stress, the epoxy material will allow more rotation at that point, reducing the stiffness of the column. What this means is that the column is not as stiff as the designer assumed in his analysis, so the moment in the column from gravity loading is not as high as he calculated but other members meeting at the top of that column will be affected so the issue is not confined to a single member.

In the case of seismic loading, the story is a little different. The added flexibility of the lower column will not be a problem if the remaining columns are capable of resisting lateral forces by themselves. The partial hinge in this column will tend to prevent it from contributing significantly to ground accelerations which cause compression on the exterior face of this column. That means that the rest of the columns, stairwells etc. must be capable of resisting seismic forces without much contribution from the column with the epoxy filled void.

Perhaps the Engineer of Record could accommodate you by doing another computer run with a reduced stiffness at the void location. Seems to me to be the least he could do.

The Engineer of Record is the president of the structural firm. Since they design 7 projects at a time.. half of them high-rise. The president didn't check all details.. for example.. he didn't know how many bars in my beam. But ok. I'd try to explain it to their design leader. Before the ground was covered with slabs and filling. I told the contractor to retrofit the column base with epoxy. The design leader looked at it and just ok it for peace of mind.. not necessarily to make the void section fixed. Have you designed any retrofit like the following?

picture shows epoxy section surrounded by rebars to confine the section.

concrete poured:

It won't make it fixed.. would it.. note there is only 2 stirrups (20mm) around the top of the section. Should the stirrup yields, the concrete confinement would no longer be effective?

In any case.. It looks like it may act like a pinned column base on a pedestral. Isn't it. Do you know of foreign firm who can run manual computations of my structure. In my country. Almost all use computer software so most doesn't compute manually anymore (it's labor intensive and time consuming for them).

 

[BAretired]

No, I can't recall ever designing a retrofit like yours. A concrete sleeve around the column. Vertical bars projecting above the sleeve, probably cut off flush after the pour. It is not clear how well they are attached to the foundation at the bottom; are they drilled and epoxied in?

2-20M stirrups? Are you sure? They look smaller than 20M to me.

The sleeve may provide some confinement, hard to say how much good it does. I would not rely on it to do very much.

Not many foreign engineers would attempt to analyze your structure manually. Computers are almost universally used nowadays to analyze structural frames. To engage a foreign engineer would likely be costly and communication would be difficult.


BA

 

[mes7a]

No, I can't recall ever designing a retrofit like yours. A concrete sleeve around the column. Vertical bars projecting above the sleeve, probably cut off flush after the pour. It is not clear how well they are attached to the foundation at the bottom; are they drilled and epoxied in?

2-20M stirrups? Are you sure? They look smaller than 20M to me.

The sleeve may provide some confinement, hard to say how much good it does. I would not rely on it to do very much.

Yes. They are 20mm stirrups..the footing floor is drilled so the 20mm bars can be inserted. see the following pics showing them in more details:

Note the red arrow showing 20 means it's 20mm bars.

The idea being that if the orginal column crushes and yet the particles are confined inside the column, it can still resist much load.. it's like the concept of cans filled with marbles or stones.. any load imparted inside will be resisted by the materials inside via call confinement. So using the same concept. But it's not meant to make fixed.. but to confine possibly crush concrete.

Btw.. about this colume base fill with epoxy acting like pinned boundary condition. Well. In both sways of the seismic (front and reverse).. there will be one where the void will be in tension.. one where it will be in compression. The one where it will be in tension means the compressive side is fully concrete hence making the column take lateral resistance and can act fixed. while at another cyclic where the void become like compression, then it's like pinned.. so you have a vibrating condition where half cycle it is fixed half cycle it is pinned.. what would this be doing to the columns or any unwanted effect?

 

[BAretired]

The sleeve is more heavily reinforced than I originally thought. It should add considerable stiffness to the base of the column. For gravity loads, I don't believe there is a problem. For seismic loads, I'm not sure. It's hard to say how the combination of column and sleeve will behave. I think I would be inclined to leave it as it is rather than attempt further remedial measures.

BA

 

[mes7a]

The sleeve is more heavily reinforced than I originally thought. It should add considerable stiffness to the base of the column. For gravity loads, I don't believe there is a problem. For seismic loads, I'm not sure. It's hard to say how the combination of column and sleeve will behave. I think I would be inclined to leave it as it is rather than attempt further remedial measures.

BA

Not only that. On top of the sleeve are added 4 inches of slab with 10mm mesh poured right around the column (without any gap or membrane used), perhaps causing further fixity to the column. See:

My hope 2 years ago was to fixed the column right above slabs so moments would start above slabs avoiding the void from bending.

The 2 pcs of 20mm stirrups has confinement (shear-like) capacity of 2 pcs * 2 legs *314*470MPA = 600 kN. I wonder how much moments it can suppress. Any way to convert it to values of moments it can resist?

If the void really becomes fixed. What may happen (based on what you are saying about pinned) is that if
the sleeve and slabs break from the moments in the fixed base. Then the column converts to pinned?

Whatever, you are right that repairing it now means breaking the slabs and breaking the sleeve and the tenant won't allow so my remedial repair is to lessen seismic base shear by making the building lighter.

By the way. In our place. We don't use seismic membrane between column and ground slabs with foundation about 1.5 meters below. I'm worried about other columns. In your experience. Have you encountered the slabs (without seismic gap) compressing on the column causing shear failure of some kind? fortunately, the ground slabs are 3000 psi compared to columns 4000 psi.