Concrete Beam End Moments
Concrete Beam End Moments
(OP)
Hello,
I'm evaluating an existing structure that has a 1-way slab supported by concrete beams and concrete walls. The existing documents indicate a Live Load for the room. I'm using the concrete beam reinforcement schedule to double check the beams for some new equipment in the room. I have a 3 span beam condition. Unfortunately, I don't meet the beam length or uniform loading requirements of 8.3 (ACI 318-08) to use the moment calcs in 8.33. I put my beams and loading into RISA to get load diagrams. From the reinforcement in my beam, which is an end beam, the only way I can get the positive moment reinforcement to work is if I fix the end of the beam at the wall. The reinforcement at the top of the beam at the end is hooked into the wall and there is enough reinforcement to develop the end moment assuming a fixed end. Is this a valid assumption, or do I need to determine the rigidity of the beam and use that for developing a moment at the end?
I look forward to the discussion.
I'm evaluating an existing structure that has a 1-way slab supported by concrete beams and concrete walls. The existing documents indicate a Live Load for the room. I'm using the concrete beam reinforcement schedule to double check the beams for some new equipment in the room. I have a 3 span beam condition. Unfortunately, I don't meet the beam length or uniform loading requirements of 8.3 (ACI 318-08) to use the moment calcs in 8.33. I put my beams and loading into RISA to get load diagrams. From the reinforcement in my beam, which is an end beam, the only way I can get the positive moment reinforcement to work is if I fix the end of the beam at the wall. The reinforcement at the top of the beam at the end is hooked into the wall and there is enough reinforcement to develop the end moment assuming a fixed end. Is this a valid assumption, or do I need to determine the rigidity of the beam and use that for developing a moment at the end?
I look forward to the discussion.






RE: Concrete Beam End Moments
Are-you sure that the negative moment (fixed in wall) is right. By experience, it should be A LOT higher that the positive moment and generally, not enough top steel are present !!
Anyway what you are doing is called Plastic Design, soo check the requirement for that type of design in you local code.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
So when I assumed anything less than fixed at the beam end the reinforcement at the middle of the beam is not acceptable for the listed Live load on the drawing. The building was built in 66 and I am using 40ksi steel which was backed up by the specs (can't believe they still had the spec book from then). I'm not sure how to move forward with the study if I can't even verfy the existing loading.
RE: Concrete Beam End Moments
The RISA model you are creating would want to use the wall as a member element in the model and if you use the proper stiffnesses (beam and wall) you should get fairly close to the true moments in the beam.
Also - per code you should alter the live loading since you have a continuous 3 span situation. Full dead load on all spans plus the following live load combinations:
1. Odd spans loaded only
2. Even spans loaded only
3. Spans 1 and 2 loading
4. Spans 2 and 3 loaded
5. All spans loaded
The beam and wall stiffnesses can be calculated in two ways -
Option A - use ACI suggested stiffnesses per section 10.11.1 where beams are 0.35 x Ig and cracked walls are 0.35 x Ig. If there is a lot of axial load in the walls (say from a wall above) then walls could be 0.7 x Ig.
Option B - analyze the beams and walls using second order effects and altering their EI values to account for cracking - in other words - use 1 ft. segments of wall (additional joints along the wall) and step-by-step calculate the Ie (see ACI 9.5.2.3) of each 1 ft. segment to model the cracking/stiffness response of the wall.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
Might be why you have problems with this one.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
Dik
RE: Concrete Beam End Moments
Many times we do have similar situation - a beam framing into wall with a sizable top reinforcing. However, in most cases the wall is not wide enough(thickness) to develop the beam top bars. (fy 60 ksi)
You may also check with gross properties (1.0 Ig) for all the elements as a starting point. Then do refinement with cracked properties.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
Have you checked the effective width of the slab to take advantage of a "T" effect? Sometimes this helps for positve bending. And have you tried using hand calculations rather than RISA?
As far as section 8.3, I never use it. I'd prefer just running my own numbers then using those.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
However, for "vintage designs" (love that phrase) usually were not conservative for shear.
T-beam design is also a good suggestion if you aren't doing that for the positive moments.
Also - for vintage designs, the analysis was probably done by hand using manual calculation techniques (think moment distribution) so my earlier post regarding wall moment participation probably wasn't used in the original design effort.
RE: Concrete Beam End Moments
1966 was in the transition period from Working Stress Design to Ultimate Strength Design. ACI318-63 was the first to formally introduce USD, and was included in the engineering curricula then. That was also in the transition period from Grade 40 to Grade 60 reinforcement, so even though the specification said 40, it could well have been built with Grade 60. Sorry if that adds more complexity to your problem...
RE: Concrete Beam End Moments
You beat me to it.
"older designers"? What the heck kind of talk is that? I was actually pretty young then, designing in concrete, nonetheless. As for "vintage designs," like vintage wines and engineers they get better with age. And, as for conservative designs, the rebar people got all uppity every time we called out 5 & ½ - #8s, so we almost always called out 6 - #8s in that case. Never had any shear problems either, undoubtedly sheer luck. We did start using Ultimate Strength Design at about that time, I actually taught/helped teach courses in USD at about that time at the U of MN. And, my buildings are still standing and functioning just fine, all 20 some stories of them, and the whole design process was a good deal less cumbersome in those old, vintage, days.
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
RE: Concrete Beam End Moments
Even now, a beam end supported on a wall is not "fully fixed". The concrete doesn't know whether it has been designed by a computer or not.
RE: Concrete Beam End Moments
I have thought about replacing the LL on the floor with the actual. However, there is a pretty big boiler and cooling tower in the room as well. I back calculated from my concrete beam the max LL on the floor to get the positive moment reinforcement to work I had to reduce my LL by about 60%. So, at this point, I'm less concerned about adding the new load and more concerned about the original design.
RE: Concrete Beam End Moments
Moonshine never lasted long enough to age, and I'm not that big a fan of wine, so I wouldn't pay a great premium for some fancy aging, can't taste the difference. Never did the moonshine thing myself, too young, but I did know where a couple old stills had been, out in the woods, where I grew up in northern Wisconsin. I did know a few older guys (born early 1900's) who did make moonshine though. Otherwise, I are damn near perfect, despite the aging, just ask me.
Acarnah2:
The original designer would have had a 100#/sf LL, or some such, where ever he didn't have boilers and cooling towers on that mech. room floor. Obviously, the 100#/sf LL does not apply under the footprint of your new mech. equip., so the new added load is less than the total operating weight of the equip., maybe (100#/sf)(foot print) less. You could also put the new equip. on skids, or some such, which would apply the new load more favorably. The walls will take some end moment as a function of their stiffness and reinf'g., they are doing that now; otherwise I agree with Hokie, I probably would not have designed for a fixed end beam at the wall, but would have still shown some reinf'g. for an end moment, to avoid/minimize cracking. ASCE has pages and pages on this, but my 100#/sf LL is a transient LL, and is different than the fixed LL of a large piece of mech. equip. The Tee action of the slab and beam is also worth considering if needed. The original design was working so why are you concerned about that? Your last sentence sounds bass-acwords to me.