Friday, April 10, 2009

RE: Beam-column joint details


All the outside column bars need to be developed is a distance represented by the beam depth( less cover),
and that can be accomplished by standard hooks or headed bars, if there is not enough depth available then
you will need smaller diameter bars, additional transverse reinforcement, and
or a deeper beam.This is the same as the beam bars being developed in the column say in a
middle floor, just rotate the joint 90 degrees

Happy Friday

Tarek Mokhtar, SE
Laguna Beach

Since I didn't get any responses, I'll expand on my thoughts: Where negative moments occur at a wall corner, the outside horizontal bars are typically spliced for moment continuity. Similarly, where a fixed base exterior wall is externally loaded, the outside vertical wall bars should be spliced to the bottom slab bars for outside moment continuity (if no footing extension). The same concept should apply to moment continuity at a roof beam connection to an exterior column, but I often don't see such rebar details addressed in such applications.
ACI 352R-02, "Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures" should be the best reference to address this condition - but it does not address it as well as I would like.  It actually contradicts my approach to some extent as it only addresses beam bars with standard hooks or with headed bars into the top of column (lacking bar continuity around the corner) - and then it shows a tremendous amount of transverse reinforcement in the joint and discusses its use for ductile seismic applications.  It does not appear to address simpler spliced longitudinal bar details without a mass of transverse reinforcement for low-seismic applications.
I've questioned a design I was asked to review but the originator doesn't want to change their details.  I'm searching for references that help justify what detailing should be used, but I'm finding very little in available literature for this particular case.
Bill Sherman

From: []
Sent: Thursday, April 09, 2009 1:15 PM
Subject: Beam-column joint details

Question: How do you detail longitudinal beam and column rebar at a monolithic beam-column joint, at than exterior column at roof level?
Addl info for current consideration:  Low-seismic region; two-story structure; rigid concrete frame resists lateral loads; concrete roof beam frames into top of exterior concrete column; typical beam details show top and bottom bars with standard hooks extended into columns; and standard hooks are shown at tops of longitudinal column bars.
Most references show a detail at an intermediate floor level where the column is continuous above - standard hooks at top and bottom beam bars are reasonable for that configuration.  But at the top of column, hooks at beam bars and at column bars do not appear to provide sufficient continuity for moment transfer around the outside face of the beam and column.  Nevertheless, drawings frequently show "typical beam details" for these columns as well - and the design engineers argue that this meets the code. 
Any thoughts?  Any references to justify your approach?
William Sherman - CH2M HILL / Denver
Principal Structural Technologist - Water Business Group

Tarek Mokhtar, SE
TMM Structural Engineers, Inc
31645 S. Coast Hwy
Laguna Beach, CA., 92651
949-499-2777 Fax