Interesting situation in Etabs dead load moment diagram

[austinb98]

I am examining the moment diagrams of steel beams connected to reinforced concrete shear walls under dead loads. However, as seen in the figure, I connected two steel beams to a single point at the corner of the shear wall. As seen in the figure, the moment diagram of the marked beam under dead load appears incorrect. What might be causing this problem? Will connecting the beams with a "link" solve the problem?

 

[cliff234]

I don't know the answer to your question, however I am curious as to why are you moment connecting the steel beams to the shear wall, and why are you moment connecting the beams to the columns? Also, are the columns steel or concrete? They appear to be concrete. Perhaps there are regional differences in construction between where your project is located and where we do most of our work, but when we design steel-framed buildings with shear walls, the columns are steel. (Actually we usually use braced frames (versus shear walls) when using structural steel floor framing - but shear walls in steel-framed structures are not unheard of.)

 

[austinb98]

I don't know the answer to your question, however I am curious as to why are you moment connecting the steel beams to the shear wall, and why are you moment connecting the beams to the columns? Also, are the columns steel or concrete? They appear to be concrete. Perhaps there are regional differences in construction between where your project is located and where we do most of our work, but when we design steel-framed buildings with shear walls, the columns are steel. (Actually we usually use braced frames (versus shear walls) when using structural steel floor framing - but shear walls in steel-framed structures are not unheard of.)

Under earthquake effects, the frame system of the building experiences a tipping moment of about 20%. In other words, under an earthquake, the shear wall does not cover all earthquake effects. I do it this way to make the design safe. That's why some beams (steel main beams) are constantly connected to the shear wall. These beams run between the columns and the shear walls.

Coming to your question, do you think it would be better to connect the beams jointly to the column and shear wall? However, in this case, I cannot create a frame system under earthquake effects.

 

[cliff234]

It sounds like you are designing a fairly tall building. From my experience if you are trying to engage the steel framing with the shear wall core, you’ll need to do more than just moment-connecting the steel framing to the core at every floor. Depending on how tall the building is, we’ve added deep outrigger trusses at the roof (and perhaps other floors) to engage the dead load of the outer columns and reduce drift. Outrigger trusses will be more effective than moment-connecting the steel framing at every floor.

It sounds like seismic forces are governing the design (governing over wind). Is that true? It sounds like the seismic overturning moment (tipping moment) is putting net tension in the shear wall. Can’t you just design for that tension by adding reinforcing steel in the wall - or are you trying to control drift?

In any case I have strayed far away from your original question. It sounds like you are a younger engineer. I suggest that you ask the senior engineer supervising you for guidance. He’ll know more about the specifics of your project and the feasibility of various options. That engineer will be able to better answer your questions. It’s hard for me to give you answers without knowing more about the building, its location, design constraints, and the governing building code.

 

[austinb98]

It sounds like you are designing a fairly tall building. From my experience if you are trying to engage the steel framing with the shear wall core, you’ll need to do more than just moment-connecting the steel framing to the core at every floor. Depending on how tall the building is, we’ve added deep outrigger trusses at the roof (and perhaps other floors) to engage the dead load of the outer columns and reduce drift. Outrigger trusses will be more effective than moment-connecting the steel framing at every floor.

It sounds like seismic forces are governing the design (governing over wind). Is that true? It sounds like the seismic overturning moment (tipping moment) is putting net tension in the shear wall. Can’t you just design for that tension by adding reinforcing steel in the wall - or are you trying to control drift?

In any case I have strayed far away from your original question. It sounds like you are a younger engineer. I suggest that you ask the senior engineer supervising you for guidance. He’ll know more about the specifics of your project and the feasibility of various options. That engineer will be able to better answer your questions. It’s hard for me to give you answers without knowing more about the building, its location, design constraints, and the governing building code.

We can make the design safe by continuously connecting the steel frame with deep steel beams on each floor. It complies with earthquake regulations. Because it is a high-rise building, but not a building over 500m. İt is 250-280m high. In fact, the columns receive axial forces due to the frame effect, and 20% of the earthquake effects of the building are covered by the frame system. This is a situation we want. Additionally, my columns are concrete-filled steel embedded composite columns. In fact, the frame system absorbs some of the earthquake effects due to the tensile and compressive stresses that occur in the columns along with the frame effect.

 

[a_urbs]

Just because it complies with earthquake regulations does not mean it's the most efficient design, as I think Cliff is alluding to. I agree with Cliff, though. Is moment-frame action not a poor bang for your buck in terms of lateral performance? All your lateral resistance is concentrated at the connection in conjunction with your column stiffness.

To answer your original question, is the restraint on the north end of the beam free to rotate? That would explain the zero moment at that node. Also, just another observation, if you're providing moment connections for all these beams to the shear walls, I would expect some negative moment at the ends. Unless you've pinned them to oversize the beams and then will get to the connections later.

 

[austinb98]

Just because it complies with earthquake regulations does not mean it's the most efficient design, as I think Cliff is alluding to. I agree with Cliff, though. Is moment-frame action not a poor bang for your buck in terms of lateral performance? All your lateral resistance is concentrated at the connection in conjunction with your column stiffness.

To answer your original question, is the restraint on the north end of the beam free to rotate? That would explain the zero moment at that node. Also, just another observation, if you're providing moment connections for all these beams to the shear walls, I would expect some negative moment at the ends. Unless you've pinned them to oversize the beams and then will get to the connections later.

Both ends of the beam are permanently connected against rotation. (There is no pinned) At the north end of the beam, the positive moment value shown in Yellow occurs at very small levels, around 15-20kN-m. If you pay attention, a situation similar to the moment diagram occurs in the continuously connected beam between two columns on the left. I'm wondering what could be the reason for this.

 

[austinb98]

I defined a simple two-dimensional frame system. As you can see here, the moment values where the two end beams meet the column give positive results as 1-2kn-M. However, the results change towards the lower floors. Could this be due to altitude?

30 floor.


1 floor