Home and a Mall seismic

[Chster6]

Let's say there is a home and mall beside each other.

The home has typical short beam of 4 meters with let's say 50 Kn-m moment capacity at support while the mall has typical 10 meters span beam with say 300 Kn-m moment capacity at support. If the home has service load moment of 25 Kn-m and the mall has service load moment of 150 Kn-m. In the event of seismic movement, which of the beams would be stronger (we are just focusing on the beams and I know the column-beam joint and column size and foundation has a lot to do with seismic resistance but let's just focus on the beam support moment for the sake of discussion).

 

[JAE]

I don't understand what you are asking. Can you be more descriptive? When you ask about whether one beam is "stronger" than the other, what are you after?

 

[wannabeSE]

I don't think I understand the question
1. One beam as installed has a 50kN-m positive and negative moment capacity (strength).
2. Another beam as installed has a 300kN-m positive and negative moment capacity (strength).
3. Which beam is stronger.
Is this a trick question?

 

[Chster6]

Not a trick question. If the seismic load of the 300kN-m beam is larger.. and the seismic load or inertia from dead load of the 50kN-m beam smaller, then they would be similar in strength? Does any disagree with this?

 

[CELinOttawa]

You're confusing strength and stiffness... So yes, I disagree.

What's more you seem to be mixing the state-of-the-art displacement based design with current code. Be very careful, Structural Engineering is best considered to be about the correct and dutiful application of the State of Best Practice, not the State of the Art.

 

[Chster6]

If you create very strong columns braced, then the beams don't have to be strong.

If you create average columns not braced, then the beams have to be strong to accommodate bending in the columns, joints and beams.

I think you will agree with the above?

So is this the sense that stiffness is different from strength in that when the columns are very stiff, the strength of the beams in flexure don't have to be very strong?

I keep wondering this when I see the small house and the big mall across the street. How do you think each would behave when the same seismic wave hit them?

 

[CELinOttawa]

Okay, yes, I can see where you're going with this now...

Yes, fundamentally the weaker your columns are, the more rotation your beam must be able to handle... However modern seismic design is dedicated on strong column, weak beam, so as to preclude (or look to eliminate) the chance of pancake collapse....

Note that the two buildings are likely to behave more differently due to their periods rather than any difference n strength.

 

[Chster6]

Periods don't just have to do with the rotational restraint of the foundations and flexibility of the columns but the dead load above which causes more inertia if the load is more. So in the 50kN-m beam in the house, the dead load above is just little since it's residential. In the mall with 300kN-m beam, the dead load is heavier with more live load... hence during seismic activity, say Intensity 6, the house and mall would have the beam moment demand reach their peak at 50kN-m and 300kN-m so they should perform the same. Refute this.

 

[msquared48]

If you consider the "performance" measurement to be the deflection to time response curve, no way, unless they have the same fundamental frequency, which is highly unlikely. And that is not even considering the higher order responses.

Mike McCann
MMC Engineering

 

[cvg]

all fine, except that residential construction is generally not steel moment frame design, they are wood framed with shear walls taking the majority of seismic force and anchors preventing the shear walls from bouncing up and off the foundation. For the record, seismic forces are both horizontal and vertical. And the vertical component can be quite large, possibly affecting the beams as much as the columns

 

[CELinOttawa]

Okay, if your house and the mall happen to share the same period (very unlikely) as well as the same dampening (laughable) and the earthquake hits them from the same angle and with the same intensity (unlikely as there is an effect from the surrounding buildings) as well as being founded on identical soils with no variation due to the differing surcharges, yes... You're right. They perform the same, under at a minimum those conditions.

As a thought exercise this has some small merit, in practice you need to be more practical.

 

[Chster6]

About this vertical component of the earthquake. Rock has bigger shear wave velocity value. And shear wave is vertical component of the earthquake. Does it mean in rock, the vertical component move so fast or has high frequency making the building vibration like oscillator? But they say rock has better seismic resistance.. is it only in the horizontal or longitudinal component?

Cel, the period of the house is much smaller than a mall with more storey. So this is the context that the house would suffer smaller shake. But then all the base shear is concentrated on the ground floor. This means if the columns are made so stiff and strong. The beam won't need large moment capacity to accommodate the bending back and forth of the columns?

This also means that in high rise building, the lower floor beams don't bend at support as much as in the higher floor?

 

[cvg]

surface waves (especially Rayleigh waves) resemble ocean waves and I would suggest may be the most destructive

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/6/SeismicWaveBehavior_Building.pdf

http://www.windows2universe.org/earth/geology/quake_4.html

 

[Chster6]

Interesting sites... but rock having larger shear wave velocity value means they have larger or smaller movement? But it is often said rock has better seismic resistance than soil. Why is that.

 

[cvg]

we're talking surface waves, not shear waves. regardless if it is rock or soil, a surface wave will displace a building upwards by several feet when it passes underneath, just like a boat on the water.

 

[Chster6]

surface waves (especially Rayleigh waves) resemble ocean waves and I would suggest may be the most destructive

http://www.iris.edu/hq/files/programs/education_an...

http://www.windows2universe.org/earth/geology/quak...

cvg, there is a mistake in figure 1 in iriu.edu site, in "exclusively" P-wave, the house should move horizontal, not vertical. In "exclusively" S-wave, the house should move up and down, not horizontal, see animation of the wave motion at:

http://www.geo.mtu.edu/UPSeis/waves.html

In ubc, soil profile type is important with rock as the best.. how does this got to do with surface waves if they strike soil profile equally?

we're talking surface waves, not shear waves. regardless if it is rock or soil, a surface wave will displace a building upwards by several feet when it passes underneath, just like a boat on the water.

But surface waves are related to shear waves. see:

http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/notes/waves_and_interior.html

"Love waves are transverse waves that vibrate the ground in the horizontal direction perpendicular to the direction that the waves are traveling. They are formed by the interaction of S waves with Earth's surface and shallow structure and are dispersive waves."

 

[Chster6]

How many percentage usually is the effect of these so called "Seismic Zone Factors" and "Soil Profile Types" on the overall strength of the say beams. Would there be as much as twice more flexural bars for example for soft soil versus rock?

http://site.iugaza.edu.ps/sshihada/files/2012/02/Handout-11-12.pdf