stirrup hook at the bottom of a cantilever beam

[chekre]

hi guys,

during an inspection on site, i saw that for all the cantilever beams, the hook for the stirrup was at the bottom of the beam not at the top. When i discussed why it was done that way with the foreman, he told me that is because a cantilever beam. I couldn't find anything in the ACI regarding this subject.

Any thoughts whether he is right ?

 

[Structureslover]

1. Top of the cantilever beam reinforcement should extend well past the face to meet development length criteria.

2. Bottom of the cantilever beam should also have reinforcements, but its mainly due to formwork (scaffolds and the sorts)

Item 2 is further clarified with the following diagram.

 

[chekre]

Thanks structuresolver but the above is well known. i am talking about the stirrups hook in a cantilever beam.
any reason for the hook to be at the bottom of the beam not at the top ?

 

[KootK]

I'll assume open stirrups until you tell us otherwise. If one considers the hooked end of an open stirrup to be the less well anchored end, I suppose that it would be sensible to try to keep the hooks embedded in the compression zone of the beam. That being said, I wasn't aware that this was a requirement or a standard practice. You would think that it would compromise the constructibility improvement associated with using open stirrups.

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.

 

[JAE]

Some older engineers in the past used to do that. There's nothing in ACI that requires it - not sure about British, Canadian, etc. codes.



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[chekre]

Thank u jae.
Kootk, closed stirrups but the hook (the one required to be 6 db) is at the bottom face of the cantilever beam.

 

[hokie66]

KootK has given the reason, I think. But I am surprised that it is actually being done somewhere in the world. Where are you located, chekre?

Anchorage of hooks is best achieved on the compression side, but this would include all negative moment regions, not just cantilevers. There was at one time a clause in the Australian code recommending this, but as it was not followed, I think the clause has been removed. Just not practical, and without evidence of actual adverse performance, the requirement is unnecessary. It is still important for designers to consider how stirrups are anchored, especially in shallow beams.

 

[nonplussed]

Hokie is correct with this. The clause in the australian standards was removed in 2009 but said:
8.2.12.4 End anchorage of bar
Bars used as shear reinforcement shall be anchored to develop the yield strength of the bar,
at any point in the legs, transverse to the longitudinal flexural reinforcement.
This requirement for the anchorage of stirrups and ties shall be deemed to be met provided
the following requirements are met:
...
(b) A fitment hook should be located preferably in the compression zone of the structural
member, where anchorage conditions are most favourable. Such an anchorage is
considered satisfactory, if the hook consists of a 135° or 180° bend with a nominal
internal diameter of 4db plus a straight extension of 10db or 100 mm, whichever is the
greater.
(c) Where a fitment hook is located in the tension zone, the anchorage described in
Item (b) is deemed to be satisfactory provided the stirrup spacing calculated using
Clause 8.2.10 is multiplied by 0.8 and the maximum spacing specified in
Clause 8.2.12.2 is also multiplied by 0.8.

Cracking in a tension zone runs perpendicular to the main steel, therefore it runs in the same plane as the stirrups and may reduce the strength if the stirrups are lapped as opposed to hooked. I assume this is why it is good practice to lap in the compression zone. But the clause was removed in 2009...

 

[chekre]

i am located in Angola and the foreman is an old chinese man