Good day,
I am currently modelling a steel structure that employs the use of tension cable for lateral bracing in addition to an eccentrically braced frame. I have a few queries as it relates to the specification for the cables, which I have listed below:
1. Is it sufficient to use a tension member specification rather than a cable specification to achieve accurate results from the analysis.
2. Which type of analysis procedure would be best to achieve achieve accurate answers for this type of system (cable bracing + eccentrically braced steel moment frame).
3. Under the cable specification which command should I employed, Initial tension or Unstressed length. Also, what value should I use for the option I select, whether it be the initial tension or the Unstressed length.
I have included the model as a reference to the issue.
Thank you. ECFH-Soufriere + Bracing.zip
For defining cables which would serve as braces in tension, it should be enough to specify these as MEMBER TENSION. You do not need to define these as cables as such. The cross section can be defined using the Define option within the Property - Whole Structure dialog box. This would ensure that the cable stiffness is accounted for correctly when these members are subjected to tension. These are going to be switched off when subjected to compression which is the expected behavior. You can use the PDELTA 30 ANALYSIS as you have used in the model. Just to answer your third question, unstressed length can only be used with advanced nonlinear cable analysis. Otherwise you can specify initial tension. The value has to be decided by the engineer as the cable stiffness will be affected by it.
ECFH-Soufriere + Bracing (2).zipThank you for your suggestion, I do however have few more queries as it relates to using the Tension command and the member releases I have employed in my Models. I have been receiving the following warning message (multiple times) and would like to know its significance as it relates to my analysis.
*WARNING- ZERO STIFFNESS IN DIRECTION 2 AT JOINT 205 EQN.NO. 31
LOADS APPLIED OR DISTRIBUTED HERE FROM ELEMENTS WILL BE IGNORED.
THIS MAY BE DUE TO ALL MEMBERS AT THIS JOINT BEING RELEASED OR
EFFECTIVELY RELEASED IN THIS DIRECTION.
Does this command mean that the loads from these members are not transferred to the other members within the structural frame? As my intention is to transfer only axial and shear forces, but this command seems to imply that no forces will be transferred. Also, does this mean that the stiffness of these elements is not considered for the displacement and drift calculations?
I have included an updated file which contains the cables specified with the tension member command rather than the cable command for your reference.
At nodes 134, 201, 203 & 205 you applied full releases to beams framing in from both directions. The continuous beams ( 177, 429 … ) should not have releases within the span.
Also you should avoid releasing MX when applying member releases as that allows members to spin about their own axis which is not realistic.
The PERFORM ANALYSIS and CHANGE commands after load cases 9 and 10 are redundant and can be taken out.
A modified file is attached for your reference.
ECFH-Soufriere + Bracing_modified.std
Answer Verified By: Gilbert Fontenard