Moment connection for R.C structures 2

[186]

How do we ensure moment connection (beam column joint)for a sway frame concrete structure ? For a 4-storey structure, is moment connection really necessary? Please help !

 

[ishvaaag]

You say "sway" frame, then if the structure is to be called engineered of course it needs have moment resisting joints. Structures are made to upheld the buildings to proper structural and other safety standards, and it is no ideal then to rely in the infills against lateral forces. Er, what supported shouldn't be being supporting (it does many times and especially at destructive stages).

For 4 stories tall, proper analysis of the infills acting as braces may also show that stability is warranted enough; then a clause of inamovability of the critical infills must be issued with the design.

Other fact is that in buildings of the past we find colums bearing mainly vertical loads only because the general stability against lateral forces is warranted by either masonries or some braced frames. In the last case we would be talking of "non sway" structures relying on braced frames and floor diaphragms to warrant stability, which is an acceptable way of designing today.

 

[186]

So for edge columns, does this mean, the beam reinforcement need to be anchored sufficiently into the columns - in the case "sway" frames.

 

[ishvaaag]

It is clear that many things are not properly known respect joints.

It is also clear that if you design your 4 stories tall building as a sway frame (we dimension that way RC frames in Spain) but you infill the building with attached masonries the building in the infilled stories won't be working as a sway frame at least for wind, but contrarily as a braced one.

In any case the sway frame design will ensure that strength and serviceability (take here lateral displacement) will be warranted till the conditions implied the design.

Proper detail of joints is better made at some works and countries than others.

It is clear that many of the details being used at least here in Spain maybe won't be warranting even the design assumptions, i.e., significant numbers of joints are being badly detailed.

I have a good photo of a exterior joint failed by the sway movement supported in one earthquake. External joints fail, for sure, in sway mode, so proper anchor of the rebar in the beams unto the edge columns is required whenever the area is assigned a high probability of having to stand a big earthquake.

Why are the joints not being detailed so exactingly? Well, in Spain there are scarce sites where the probability of a big earthquake is great, so most are familiar only with the far more ordinary ways of the less engineered details. Also, in fact the detail exigence is different of course for different seismic zones.

So if your work is in a high risk seismic zone, yes, you design for the earthquake and within it detail properly for the alternate sway movement, with proper anchors in the columns.

It is clear that damage to the building is to be expected under such earthquake.

So there may be more effective ways to deal with earthquakes, and specially for low structures, shearwalls for example. In any case, the choice may be not yours, sometimes happens.

 

[JAE]

For a reinforced concrete moment frame (sway structure) you must analyze the frame for the applied gravity, wind and seismic forces. This produces numerous shears and moment envelopes across the beam and column lengths. From these, you must detail reinforcing to resist accordingly.

For beams, this usually results in positive and negative moments at the beam/column intersection for which the top and bottom reinforcing must be developed into the column or through the column into a beam on the opposite side of the column.

In addition, for seismic condition, the volume of concrete in the beam/column joint must be specially detailed to properly confine the reinforcing.