force & shear in seismic 1

[hocho]

can any confirm that the only reason we compute or determine story forces in seismic is to calculate for base shear or story shear and the forces themselves are not used in any other calculations (such as shear capacity of columns or beams)?

can you treat a building as cantilevered from ground and the seismic forces are like distributed load.. and if you focus story forces to a single floor.. it's like a concentrated point load and the responding differences in the shear diagrams between the 2 load cases?

 

[KootK]

can any confirm that the only reason we compute or determine story forces in seismic is to calculate for base shear or story shear and the forces themselves are not used in any other calculations (such as shear capacity of columns or beams)?

It depends. If your building is stabilized laterally by moment frames, then story forces will absolutely impact your beam and column shears. If your building is stabilized laterally by shear walls alone, then the impact on beams and columns may be limited to simply the forces generated by the the imposed drift.

can you treat a building as cantilevered from ground and the seismic forces are like distributed load.. and if you focus story forces to a single floor.. it's like a concentrated point load and the responding differences in the shear diagrams between the 2 load cases?

You can treat a building as cantilevered from the ground. In fact, you don't really have any other choice. The seismic inertial effect will be distributed up the height of the building and will spike at the levels of the floor diaphragms where mass is concentrated. As an approximation, engineers often consider all of the mass to be lumped at the floor levels.

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.

 

[hocho]

It depends. If your building is stabilized laterally by moment frames, then story forces will absolutely impact your beam and column shears. If your building is stabilized laterally by shear walls alone, then the impact on beams and columns may be limited to simply the forces generated by the the imposed drift.

I know. But the only reason we compute for the story forces is to compute for the column (and some beam) shears? The forces is not used in computing for moment of the building or others except shear only?

Also can a story have the same story shear but stronger column shear? Here's an example. Consider a 10 storey building with forces distributed at each floor (100,000 lbs each). Imagine all the forces are just focus on the 5th floor laterally (500,000 lbs alone). Even though the former has total 5th floor force (and shear) of 500,000 lbs too (by adding all the forces above). The one with concentrated force of 500,000 lbs at 5th floor only will have stronger shear even if the shear is the same. What terms do you use to distinguish for story shear and actual column shear?

 

[TLHS]

Are you asking if the distribution of seismic forces to floors is used in calculating your base moment? It generally is, but refer to your code.

 

[hocho]

Let me illustrate it again:

For 10 stories. Each storey forces is say 100,000 lbs.

Now compare that to a 5 storey building and the upper floor has concentrated forces of 500,000 lbs. This can happen if the seismic mass of the floor is 5 times... so even though the 5th floor storey shear of both building (5 and 10 storey) is the same.. at 500,000 lbs. There is more concentrated force at the one with 5 storey only. What term of principle do you use to distinguish between storey shear and concentrated force shear?

 

[KootK]

I'm afraid that I still don't understand the question Hocho. Perhaps it would help to post a sketch illustrating the concepts that you're interested in discussing. Seismic loads are applied at the diaphragms and the effect that those loads have on the various parts of the structure are determined through structural analysis.

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.

 

[hocho]

I'm afraid that I still don't understand the question Hocho. Perhaps it would help to post a sketch illustrating the concepts that you're interested in discussing. Seismic loads are applied at the diaphragms and the effect that those loads have on the various parts of the structure are determined through structural analysis.

Please see illustration below:

At the middle storey of the building. The first one has summation of storey shear of 1000kN x 5 = 5000kN

The second building to the right has a concentrated storey shear also of 5000kN.

Will the floors of both buildings that has similar storey shear of 5000kN have *same* shear in the columns?

 

[KootK]

Thanks for the sketch Hocho.

Will the floors of both buildings that has similar storey shear of 5000kN have *same* shear in the columns?

You bet.

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.

 

[hocho]

You bet.

So in the roof top with small swimming pool and open to the skies with surrounding walls cantilevered to columns and floor.. what is the point of creating a roof diaphragm to transfer the forces of half the wall above when you end up with the same storey shear anyway?

 

[KootK]

The roof diaphragm would be for the benefit of the walls rather than the lateral force resisting system.

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.

 

[hocho]

The roof diaphragm would be for the benefit of the walls rather than the lateral force resisting system.

Thanks for this.

In your experience.. how many percentage do walls contribute to seismic lateral load of a building.. if the walls is as thick and heavy as the slabs.. could it be as high as 30%.. in your estimation?

If so, then lightweight wall would be very advantageous... do engineers take advantage of this in decreasing the lateral load of a building?

 

[KootK]

Usually, on taller buildings, light weight wall systems are chosen because they tend to be economical in their own right. The fact that such wall systems are seismically advantageous is just a bonus. I'd say 5-30 percent would about bracket the range of possible wall seismic contributions.

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.

 

[hocho]

..

 

[hocho]

Kootk.. thinking about it some more:

The storey shear of the columns may be similar in both buildings with the 5000kN total seismic force. But did you notice the beam in the building with concentrated 5000kN can deflect more. So beams seem to be affected.

In the related question about putting a roof diaphragm for wall that you stated whose benefit is not for the seismic lateral force resisting system. But the beams below would deflect less (even if not directly connected to the wall but perpendicular to the joint that carries the wall) if half of the wall would be carried by the roof diaphragm.. so it can help in the seismic lateral force system by distributing the forces instead of focusing in one floor (as far as wall is concerned and let's ignore for now the flexibility and force reduction scenarios for sake of this discussion)?

 

[hocho]

Kootk

The above is the exact situation. It's from top view. When you increase the loads (height) of the walls at the left sides perpendicular to the seismic lateral force (left arrow) going to the right. The beam at center doesn't have significant changes in moments or swear (as shown in my frame analysis). It's as if the column at left restrain it. In this case.. is it really true that the column at left mainly take care of the wall? So the effect of not creating a roof diaphragms to distribute the loads of the wall from the floor is not significant in the traverse direction? Can you add a comment or two about this? Thank you.

 

[KootK]

You're showing walls and columns that cantilever above a concrete roof slab, right? I'm going to assume so.

The beam at center doesn't have significant changes in moments or swear (as shown in my frame analysis).

I would expect the beam to absorb some load assuming that it is rigidly connected to the column. If the beam is long and slender and the column is short and stocky below the roof slab, it makes sense that most of the moments and shears would be drawn to the column rather than the beam.

It's as if the column at left restrain it. In this case.. is it really true that the column at left mainly take care of the wall?

Yes. The column really doesn't have much choice in the matter in this scenario.

So the effect of not creating a roof diaphragms to distribute the loads of the wall from the floor is not significant in the traverse direction?

I imagine that the effect must be pretty significant for the columns which will be working pretty hard to support the wall seismic load via cantilever action.

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.

 

[hocho]

Kootk,

You're showing walls and columns that cantilever above a concrete roof slab, right? I'm going to assume so.

By cantilever is meant the walls above concrete roof slab is connected entirely to columns and not on the beams? Or by cantilever you also meant both connected to the columns and beams? I'd like to know your context in the previous message above.

Anyway. Right now. Let's take the case of a typical diaphragm in middle storey. Here the wall (shown in red) is typically connected to beam above and below. Now let's have a seismic lateral force directed towards the right. Since half is connected to the floor beam (below wall).. and the floor beam is connected by diaphragm (shown in grey) to the right beams and columns. Would the wall seismic load be transferred to the diaphragm towards the right or would the column on left restrain it? Let's say the column at left is strong.. then the wall forces would only go to the diaphragm a little and mostly resisted by the left column? Now let's say the column at left is weaker.. then the wall forces would mostly go to the diaphragm more?

Thanks for all this. You are the light amidst the darkness

 

[KootK]

You're very welcome Hocho. By "cantilever", I meant that the beams and columns would be laterally restrained at their bottoms but not at their tops.

Walls that are tied into high and low diaphragms and adjacent columns will deliver some portion of their seismic load to all of those elements. Don't let yourself get confused about the fundamentals of load distribution simply because we're talking seismic. Recognize, however, that the wall load delivered to the columns still eventually ends up in the diaphragms that laterally restrain those columns.

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.

 

[hocho]

Walls that are tied into high and low diaphragms and adjacent columns will deliver some portion of their seismic load to all of those elements. Don't let yourself get confused about the fundamentals of load distribution simply because we're talking seismic. Recognize, however, that the wall load delivered to the columns still eventually ends up in the diaphragms that laterally restrain those columns.

How thick should the floor be before you call it a diaphragm? How many 4 inches of slabs? Structural engineers in my place only design 4 inches slabs.. they don't have confidence with thicker slabs.. this is to avoid overloading the beams.. they also typically design beams 4 meters length only.. once I talked with a structural engineer about 6 meter beam design.. he is so nervous because he said our construction quality is so poor that columns are almost always honeycombed and can't take the load of more than 4 meter beam span.

Anyway. If the floor is just 4 inches slab. Then it's call a flexible diaphragm? I understand a diaphragm redistributes moments so the columns move together.. but if the slabs are thin or so flexible.. the columns may not move together anymore? is there a scenario where the diaphragm just break apart because of very unequal column loadings pulling the slabs apart?

 

[KootK]

Any thickness of concrete deck usually gets you a rigid diaphragm and 4" ought to be plenty. ASCE7 has criteria for assessing diaphragm rigidity based on the ratio of deck rigidity to shear wall / braces frame rigidity.

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.