Hi Sumant,

if you want to use equivalent diagonal strut model, analys must nonlineer. Because of features to equivalent strut you defined (compression and tension)

Hi,

the problem is due to the elastic properties that you assign to struts. In Sap200, if you use one strut (suggested only for pushover in one direction), you’re assigning the elastic properties through a certain section and a certain elastic modulus. If you wanna use 2 struts, you should subdivide (based in the number of struts that you put) the elastic properties, by inserting lower sections. You need of this, also because the software doesn’t know that the struts should not have the tensile behaviour for the masonry infills.

Sergio

I find different aspects of the problem here:

1) You said “Also the modal periods of both models are different”. Modal analysis, being a linear analysis, doesn’t work as expected with compression-only struts. Do modal with only one set of diagonal struts – not the crossing arrangement – either all zig, or all zag is the good option.

2) Pushover is non-linear, and therefore both the sets of diagonal struts can be included, but “with tension limits set to zero for struts”, as you have said, included. If both sets of diagonal struts are used, the problem is, since you are extracting mode shapes with one model, but then 'pushing over' another model, the mode shape parameters from the former will have to be manually input in to the latter

3) If your time history is non linear -- I presume it is, as it should be if to be compared to pushover -- both the sets of diagonal struts should be included, but “with tension limits set to zero for struts”, as you have said, included.

Hope I’m clear.. Dear peers, please point out if anything problematic/incorrect in the above.

Rahul Leslie

You should define the strut as tension/compression member, and assign tension/compression limits to it.

I am drawing my experience from use of STAAD Pro. Is it possible to define one of the cross bracings as inactive? Several analyses may have to be run and some trials needed, if the model is complex.

1. To perform linear analysis (static or dynamic) in SAP2000, you must model the infill (masonry) wall with 2 diagonal pined frame elements (equivalent diagonal strut model) with sections of half area properties (1/2 Axial Stiffness, i.e. Aw/2). The two diagonal struts are connected to the column-beam end-joints. The strut section area Αw is equal to w*teff. where w is the strut width and teff is the strut thickness. This way of modelling results to some differences on the Axial forces of the exterior columns but are small relative to the Axial Forces due to the vertical loads, so they can be neglected when the diagonal struts are connected to the end-joints of frame elements (column, beam). To calculate w, you can use a combination of the Later and Mainstone (1971) and Asteris methods (w = [0.175 * L * λh -0.4] * λ) or use a standard value of 0.15*L, i.e. Αw=0.15L*teff, as the Greek Code of interventions (KANEPE) propose for all performance levels.

2. To perform non-linear analysis, you must also use the previous diagonal strut model, but this time each strut has its full area section properties (Aw) and also a compression NL law only (only compression non-linear behaviour, no tension). In Sap2000 this can be achieved, first by providing a full (symmetric) NL behaviour law for compression and tension and second by assigning (to the diagonal struts) tension limits equal to zero. In this way, during NL analysis in SAP2000, If the strut tries to go into axial tension, it will lengthen without any stiffness, i.e. the axial (elastic) stiffness will be zero and the strut will not carry axial tensile force. The NL behaviour law is characterized by the following values when a Hinge model is used (tri-linear Νy–ΔLpl diagram with vertical drop of strength at ultimate):

ΔLy=ey*L

ΔLu=εud*L

ΔLpl=ΔLu-ΔLy plastic shortening

Νy=Νu=εy*(EwAw)=fwcs*Aw axial compression force at yield and ultimate

Nres=0 remaining strength after ΔLpl

where L is the strut clear length, ey = fwcs/Ew is the strut yielding deformation, eud is the design ultimate deformation of the strut and fwcs is the strut axial strength. For masonry infills, eud is given in the Greek Code of interventions (KANEPE).

Athanasios Bakalis has given an excellent description on how to manage diagonal struts with his section (1) when finding modal parameters (time period and mode shapes), and section (2) when performing a non-linear analysis.

In fact the method described in section (1) is more balanced for determining the mode shapes than the ‘all zig or all zag’ method mentioned (in my earlier posting).

With the method described in section (2), one can even do a very neat response spectrum analysis for models involving diagonal struts: With both the diagonal struts in full section provided,

- One first inactivates all the diagonal struts to do analysis for the gravity load cases.

- They are then activated, assigned only the ‘compression-only’ property (which just activates or inactivates the struts based on whether they are in compression or tension, without other non-linear hinge properties required only for pushover approaches).

- One then manually applies the mode based seismic lateral floor loads – one load case for each modal load – run all the load cases, and

- combine the results using the SRSS combination (available with most packages), and

- the combination combined with the gravity load case results, as per the design codes..

These are required because, with response spectrum analysis, struts that are active with one mode based loading may not be so with loading based on another mode. And the one can’t be sure whether the automated response spectrum method built into any given software package takes this aspect into account. The only issue here is that CQC is never found to be a combination option. But that is not an issue if the modal frequencies are well separated.

Rahul Leslie