beam-column joints in concrete frames

[jfmann]

I am interested in discussing the design of beam-column joints with any structural engineers who design reinforced concrete building frames. I am thoroughly familiar with the design issues...........but am seeking to find out if, and under what conditions, practicing engineers are using the design recommendations in ACI 352 for buildings that do not have to be designed for seismic loading. <br>
<br>
Specifically...........are the provisions for joint shear in ACI 352 used to size the columns??.........are designers considering shear reinforcing even though it is not recommended by the standard??

 

[HochwaltPE]

In the 10+ years that I have practiced structural engineering in Maryland, Ohio and Washington I have yet to run across an engineer who designs joint reinforcing in non-seismic regions. Generally ties in the joint region are provided in accordance with the ACI Detailing Manual which I believe requires the provision of minimum column ties (per ACI 7.10) up to the underside of the slab unless beams frame into all four column faces.<br>
<br>
I am not aware of any reason to do anything more than this in non-seismic regions.

 

[jfmann]

HochWaltPE.......Thanks for your reply. In most cases, as long as the column is sized to be deep enough to anchor the top flexural steel. for negative moment, the joint shear will not be excessive. The Code requires a check on "development of moment transfer" in Chapter 11 (and references ACI 352) but it seems that there should be a much more clearly stated reason......as explained in 352 with diagrams. I have been attempting to find out if building design engineers run a check on joint shear as required by this provision......admittedly ambiguous. So far all responses have been in the negative!!...........

 

[bongiorno]

I am a structural engineer with a large firm involved in the design of tall buildings. This past Friday I gave an in-house presentation on the &quot;Analysis, Design and Detailing of Concrete Connections and Joints&quot;. the lecture dealt with beam-beam and beam-column connections under extreme conditions that I've dealt with on just about every large project. The topics covered were

A. Deep beams framing into shallow beams or beams of equal depth

B. Wide shallow beams supported by Narrow columns(bm width = 8ft./ bm. depth = 3ft., col. width = 3ft.)

C. Deep beams framing into small columns(Bm depth= 8ft. col. depth = 3ft.)

I was pretty suprised that only a few of my coleauges had ever addressed the design and detailing issues that I was presenting. The ACI does not in fact adress these conditions. Most current textbooks present some of these issues with regard to Strut and Tie Analysis, but even they fall short of giving engineers any adequate direction. The basic underlying premace behind all of these cases is:

1. Conventional concrete design assumes that forces travel from the load point to the support from Top to Bottom. Therefor, any loading condition that produces a reaction at the bottom of a concrete member must first be translated to the top of that member. Such is the condition at beam-beam connections and wide beam - narrow column connections, where the support shear falls outside of the column width and is therefor reacting at the bottom of the beam adjacent to the column.

2. Proper anchorage must be a satisfied at all locations in a joint. Developing a top bar with a straignt bar anchorage in a deap column means nothing if the joint shear mechanism , at ultimate, is a single struct reacting at the top back face of the column joint.

3. Stability and confinement of a joint are crucial to developing the beam and column moments framing into it (Joint Efficiency)

4. The column bars that extend into the joint do not necessarily have the benefit of an axial load that the column itself (below the joint) was designed for. Equilibrium would not be satisfied within the joint.

5. With proper detailing and consideration of displacements, engineers can manipulate the laws of compatibility and design connections for the load paths of their choosing (Ex. designing a monolithic beam-beam or beam-col. joint as a pinned condition)

 

[Gourile]

ifmann
As a professional engineer, I personally don't do analysis related to the joint strength. I beleive most of the times the ACI recommendations are pre-satisfied if the whole design process is preformed correctly. In the case of suspicion, it would be a good idea to chech the conditions for a few critical joints. Practically I don't know anyone to do special design for concrete joints.

 

[IJR]

I agree with all pals above in all respects, but please allow me to emphasize on only one point

In regions of high seismicity, critical joints must be detailed properly. Since there are usually a few such joints in a typical building, this should not be very difficult. By critical joints, I mean joints that seem to attract large beam moments induced by lateral seismic loads.

Since seismic code philosopy is to keep this joint ductile it is imperative that good confinement be provided. As beams reach their plastic capacities(as required), large shear is induced in the joints. Shear failures are known to be brittle. And you dont want cracks to propagate very fast in these regions.

Further reading: T.Park and R.Paulay's classic on RC

Regards

IJR