Two-way slab as moment frame

[Robbiee]

Hello,
Reviewing an existing concrete building for lateral loads and noticed that the building has shear walls in one direction only. The slabs are similar to waffle slabs but differ in that the ribs are not continued to the next column line as seen in the attached. Was this system, i.e. the slab/columns, consider a moment frame? In my sketch I drew a section at the top and on the plan the slab at the bottom columns is the same as indicated ablove. Thanks for the input.

 

[mijowe]

Although I have never seen the structural system you have shown, yes it is common to use a two way slab system for a moment frame. We do a lot of slab checks so we get to see a lot of old building drawings. Most of the buildings we see in the 8-10 story range do not have shear walls in either direction. Some drawings clearly show lateral steel, ie they schedule additional steel to be put in the typical slab at each floor, with the amount of steel going up as you work down the building. Most of the drawings show no indication of additional steel for lateral.

 

[Robbiee]

Thanks mijowe. when you said "it is common to use a two way slab system for a moment frame" I am assuming you meant two-way slabs with no supporting beams between columns, because that is the typical moment frame.

 

[mijowe]

Yes, drop panels at the columns (sometimes) but no beams.

This reminds me of way back when I used ADOSS, That program allowed you to add lateral moments into your design strip.

 

[PicoStruc]

In my country, Canada ! The only way to use not standard seismic force resisting (SFRS) system, is to proove by testing that the system develop adequate resistance and that the dissipation mechanism is ductile and adequate. This is never done in real life.

Only university does that kind of researh !

I can assure you that a two-slab equivalent frame is not a standard SFRS ! You need beams and column system clearly defined. 10" slab arent beams ! Did the engineer add shear reinforcement in the "Beams", does the joint capacity checked ?

** Of course, this only apply to region where seismic design is required !

 

[Robbiee]

Thanks for the input. I agree with you that typically a moment resisting frame is columns and beams. But, is it allowed for the beams to have different sections along their lengths? The answer is yes like in pre-engineered steel buildings. Then, can we look at the three ribs in my sketch as a triple T-beam that changes its section to a 10’x10”rectangular?

 

[mijowe]

What are you trying to check? Are you adding too the building, or checking to see if it is adequate as is?

 

[Robbiee]

Checking for adequacy as is without adding any loads or changing anything.

 

[Forensic74]

No, these types of systems fail in punching shear when lateral loads are applied. I witnessed a full scale test of a multi story flat slab and saw what happens. Think about it, you get a hinge in the slab at the face of the column because the stiffness of the slab is so much less.

 

[JAE]

I've designed a six story two-way flat slab system before using lateral wind loads in the system. As long as you check the punching shear at the column, with the lateral slab-column moment included (see ACI chapter 11 and 13) you can make it work. This is in accordance with ACI and is used for these systems quite frequently.

MainMan10 is correct that the concern is with the punching shear which is sometimes significantly affected by the addition of the moment.

 

[Robbiee]

MainMan10,
The slab that makes a drop panel is 30" thick. probably you're talking about thin flat plates.

 

[Robbiee]

JAE,
I am trying to fit this system into what the code calls a moment resting frame to calculate the time period and then the seismic loads. Then -and based on that- check the system's adequacy.
Can I call this system a moment resisting frame?

 

[JAE]

If there's nothing else, then yes, it is a moment resisting frame.

 

[hokie66]

Looks like a typical waffle slab. The 30" deep ribs would be your "beams". I would guess that the column bending capacity would control.

 

[PicoStruc]

I would guess that the column bending capacity would control.

Be sure that at your joint, your beam are weaker that your column !

 

[rapt]

This slab would not qualify as a Waffle slab to most design codes. To be a waffle slab and therefore be treated as a flat slab in normal code design rules, there are stringent upper limits on rib spacing.

This is a 2 way rib slab which is not a normally accepted two way system system according to most codes. The designer would need to do a full grillage analysis to determine design actions rather than assuming a flat slab action as is assumed for a waffle slab.

 

[Forensic74]

My bad, didn't see the 30" section around the column....

 

[ahmedhegazi]

How high is your building? ,what is the seismic zone it is located in?