How to overcome lack of anchoring length in Reinforced Concrete Beams
How to overcome lack of anchoring length in Reinforced Concrete Beams
(OP)
HI,
So i have a Reinforced Concrete Structure with beams, columns and slabs.
Columns and shear walls are 25cm thick, therefore you can understand that at locations such as edge beams there is lack of anchoring length for the RC beam reinforcement.
Please see picture for examples of edge beams with lack of anchoring.
Lack of anchoring results in the bars not being able to utilise their full bending resistance capacity.
Whereas the provided reinforcement is adequate for the bending moment in middle the bending resistance of the bars at the edges is considerable reduced due to lack of anchorage.
I have seen many houses being erected in the past year and none of the engineers used any kind of measure to increase anchorage resistance at edge beam locations.
Now that I am actually doing my first structural analysis and design for an RC Frame I have come across this issue and i am just wondering how did the other engineers overcome this?
or could they have not seen it or could they have just ignored it?

So i have a Reinforced Concrete Structure with beams, columns and slabs.
Columns and shear walls are 25cm thick, therefore you can understand that at locations such as edge beams there is lack of anchoring length for the RC beam reinforcement.
Please see picture for examples of edge beams with lack of anchoring.
Lack of anchoring results in the bars not being able to utilise their full bending resistance capacity.
Whereas the provided reinforcement is adequate for the bending moment in middle the bending resistance of the bars at the edges is considerable reduced due to lack of anchorage.
I have seen many houses being erected in the past year and none of the engineers used any kind of measure to increase anchorage resistance at edge beam locations.
Now that I am actually doing my first structural analysis and design for an RC Frame I have come across this issue and i am just wondering how did the other engineers overcome this?
or could they have not seen it or could they have just ignored it?







RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Here are my questions?
1. How do you actually decide if you can use a pinned or fixed support at a joint?
2. Is this depended on the actual construction of the structure?
3. For example; in this type of construction
a. The column is cast first at beam height,
b. Then the beams and slab are poured together on top of the column, therefore we have a monolithic element.
4. Since the columns, beams and slabs have not been poured all together does it mean that we can actually use a pinned support?
Please let me know if i am complete out of the subject.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
In seismic conditions there is always a positive moment, it may not be of the same value as the negative though.
How do you actually insert the beam reinforcement inside the column since the column is cast in place first? This is one way to solve this issue but i have never seen it before.
Also this is not what they do actually do in practise here in my country, and they cant be all wrong.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
If the beam doesn't have sufficient height to do the rebar anchorage entirely inside its height, the beam top rebar (already properly bent) is placed alongside the column rebars (creating a top rebar lap at 1/3 span or similar). This is my standard detail.
If you look in the IStructE manual "Standard Method of Detailing Structural Concrete - 3rd Edition", they have another option which envolves the placement of U-bars after the column casting. It simplifies construction but I really don't use this detail for primary seismic elements as the column reinforcement is not fully mobilized and the anchorage doesn't seem to work as well.
Ultimately, it's your call, regardless of the practice of your typical contractors. I'm just trying to help you take an informed decision.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Dik
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Thats the reason why i am here, I need advice on this. However its not the contractors choice how the bars are installed but the engineers. I suppose what i see on site its the engineers design.
Well i think the intensity of the seismic load i use is pretty high, i am designing for a factor ag = 0.25g.
Also from the design response spectrum analysis to EC8, all of the beams get +ve and -ve moments at the supports.
I will check on that but i havent seen this before anywhere, I know its one solution but why dont they use it?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Can someone discuss this anchorage example? Do you think this is sufficient for earthquake resistant structures?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Some of you have suggested using pinned supports, therefore my question now is:
In reality, in an RC frame where the beams and slab are poured together on top of the already cast column The beam-column joints are:
1. pinned
2. fully fixed
3. something inbetween
4. Depends on how you carry out the analysis & design (if you use more steel reinforcement in the joint then the joint will be able to resist a bigger moment),
similar to the steel frame joints, where you actually choose what connection to use.
Are there any articles to read on these? maybe some experiments?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
3. something inbetween
4. Depends on how you carry out the design, similar to the steel frame joints, where you actually choose what connection to use.
when you make a model you have to choose one idealized extrem: pinned or fixed. For some advanced models you may use a spring like support with a stifness that is something between fixed/pinned but you have to know what you are doing. I dont do that...
in real world its rarely one extrem or another. But based on how you will detail connections / reinforcement details it will act more towards one extrem than another.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
so many hours of studying and i still didnt know this, noone explained this clearly.
i knew that the RC beam-column connections are neither pinned neither fixed, however I totally missed out on the fact that
the behaviour of the connection can be adjusted through the reinforcement, similar to the steel frame connections.
I have one more question for you mats12, when designing an RC frame house (2 storeys) do you usually use both fixed and pinned
beam-column joints around your model? and how do you decide on the pattern so that you have a stable frame?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
If you want your frame to be horizontaly stable (in both direction) you either have to have:
-fixed base + fixed or pinned connection between columns - beams (you have to reinforce joints/connections accordingly)
-fixed base + pinned connection between columns - beams (large moments at the base of columns - I wouldnt do that for structures with more than one storey)
- you have pinned base but in this case you have to have fixed connection between columns - beams.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
I have an issue with insufficient anchorage length at almost all corner/edge beams due to very low thickness of columns/walls (25cm)
and due to the high seismic forces.
How can i decrease the moments at beam-column joints for these beams so that i do not need so much anchorage?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Are you dealing with a project that is half way done (photos you posted) and now you are concerned about details or are you asking for structure that hasnt been designed yet?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
I think i have two options here:
1. Use a fixed beam-column joint and Find a way to increase the reinforcement at the joint in order to provide more anchorage.
or
2. Find a way to decrease the moment at the beam-column joint so that less anchorage is required.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
I will have to check and see how much the bending moment decreases at the supports when using 0.15(MEd at span) compared to the bending moment at the supports with a fixed support.
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
1) Use smaller bars and more of them (shorter dev. length).
2) Provide more As than As req'd (dev. length x As req'd / As provided).
3) Extend beam past column if possible.
4) At top column, bend outside bars down into top of beam and provide req'd lap with beam hooked top bars. Provide hooked bars for column inside bars and hook beam bottom bars if req'd.
5) Consider using A706 bars welded to anchor plate and lapped with beam bars if req'd.
6) A haunch will sometimes provide additional length req'd for development. Depending on the size, it may significantly increase the bending moments.
Just a few ideas but I am sure there are others........
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams
Therefore I decided to follow another approach
1. I changed the supports of every edge/corner beam (where there is anchorage issues) from fixed to pinned and carry out the analysis again.
In some cases I only changed the support at only one side of the beam, since the other side had no issue due to the continuity of the beam
2. I then carried out the design of these beams and the anchorage problem was solved, the depth of the beam was enough to provide the required anchorage length.
3. I also took into consideration the clause EC2 9.2.1.2(1) which states that even when using pinned supports, the supports should be designed for a bending moment
arising from partial fixity of at least b1 of the maximum bending moment in the span. whereas b1 = 0.15.
Do you think there could be an issue with this approach?
RE: How to overcome lack of anchoring length in Reinforced Concrete Beams