Expert: Arshad Khan - 12/5/2008

Question
QUESTION: Dear Sir,
   I have 3 questions to ask, i will be grateful if you could answer the questions.
   1.Staad gives the Mz and My moments, for column members, For designing the columns for biaxial moments, and axial loads, do i have to consider Mz as Mx,or what?
   2.At the beam and column junction, for the space frames, rebars are at both top and bottom. Is it necessary, for the negative rebars at the end of the beam members, have to go deep inside the column, taking a L shape, please advice!
  3. If the support assigned for the space frame columns, are pinned, can it be a normal footing pad, without any special hinged or pinned support design, Please Clarify!
 






ANSWER: Hi Ahmed,

Here are your answers:

1. Yes, consider both Mx and My
2. Yes you need to ensure they have good anchorage to far side of column and bent down into it.
3. No, you will need to either X-over the rebar so that it does not generate too much moment, or have a special detail, or as is normal practice you can assume that an isolated pad footing will rotate a little and hence the support can be considered pinned.

---------- FOLLOW-UP ----------

QUESTION: Dear sir,
Thank you so much for a prompt list of answer, Can you please elaborate the 3 answers, for me,
1. Do i have to take Mz values  as Mx values (as Staad does not give Mx value).
2. Is the anchorage sufficient for the beam depth, as the RC columns will be casted till the beam depth?
3. I do not understand X- over the rebar, please make it clear.

Thanks

Answer
Hi Ahmed,

1. Yes, follow the convention that is given in the help menu of STAAD.

2. In most cases that is sufficient, but you will need to check the bond stress at the end of the anchorage bar to make sure. In some cases one has to have the bar cast further down into the column, but that makes construction more complicated.

3. This means that the bars on the outer and inner faces are bent back towards the centre of the column, then cross over and are bent back down to the opposite faces. You will need to put nominal T12 bars in the 4 corners, though, in order to contain the links. However, the only way to ensure a proper pin is to use steel plates and form one by use of cleats.