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Re: staad modeling

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Arman,

In designing a self-supporting communications tower, what I usually do is model the columns as space frame members and model the diagonals as truss members. So in your starter command, you need to use Staad Space. The effect in modeling the diagonals as four members (connected at the mid-point) or as two members (no connection at each mid-point) is really very minimal (you can try these two options out).
The four-member option usually is used when you have widely spaced columns, because if you don't connect the diagonals at the midpoint, then you come with large members because of slenderness effects.

However, I'd like to add that STAAD is a dangerous tool when used by inexperienced engineers especially if used for structures located in UBC zones 3 & 4 because of its inability to implement the special seismic design provisions for high and very high seismic risk areas.
If used by experienced engineers, it reduces the productivity of the design team again because of its implementation of the code provisions for UBC zones 3 & 4. So when using STAAD, please make sure that you check your design for compliance to the provisions of the code.

If your projects are those for cell site transmitters such as those for Globe or Smart or Islacom, then you can use the two-member option for the diagonals since these towers are relatively slim.

With regards to the modulus of subgrade reaction, you need to specify in your request to the geotechnical engineer to provide the subgrade modulus.

Regards,

Roy


arman s rivera wrote:

> >In my earlier version of STAAD, it says that TRUSS can be used
> >for either a plane or space analysis. If all of your members
> >are only to be designed to carry axial loads the TRUSS command
> >seems best to avoid the extra work of entering releases. But if
> >you want to model the chords as continuous members, the SPACE
> >command would be better - but can't you do a "list" of members to
> >be defined as "truss" members, rather than entering end releases?
> >As others have noted, you make encounter member instability errors
> >when entering end releases, which require one end to have rotational
> >restraint.
>
> the "member truss" command would be correct if the member was pinned at both ends, such as in horizontal members of a communication tower.
>
> however, for the diagonals, in which the the two members are in an "X" configuration, usually in actual construction, one of the diagonal is one continuous member, but in staad model, this would still have to be two members because there would have to a joint at the intersection of the two members.  the continuous member cannot be defined as a truss because there should not be a release at the center.  thus we release only the ends that connect to the posts for moments.  for the other diagonals, it is ok to define them as member truss because they are in actuallity, one member that extends from the posts to the other diagonal.
>
> however, sometimes, some contractors, provide plates at the intersections, to which they bolt the four members of the diagonals.  in this case, the four members would be defined as truss members.  but most do not like this method as this is more tedious and expensive.  so they opt for the first method.  in which case, we must also ensure that our assumptions in creating the staad model is correct.
>
> so far, in my experience (and very little at that), the method of releasing the joints have not caused any instability errors.  what i am concerned about is if the principle and the way i use that principle in creating the staad model are correct.  to be honest, the engineers at my firm do not receive any formal training in staad (or on any other softwares we use, for that matter).
>
> >
> >> Given the allowable soil bearing capacity, how is the modulus of
> >> subgrade reaction computed?
> >
> >Generally there is not direct correlation, although Bowles does give
> >an equation for estimating purposes. In a previous thread on modeling
> >the modulus of subgrade reaction, it was suggested to "bracket" the
> >design by making runs with both a high and a low estimate of subgrade
> >reaction, such as 50 pci and 300 pci. Most decently compacted soils should
> >have a modulus in the range of 100 to 200 pci, so this seems reasonable.
>
> i have read that the value of (120 * allowable soil bearing pressure) is a good estimate of the modulus of sugrade reactiom, as recommended by bowles.  now the value of 120*qa would have a unit of pressure, such as kilopascal (kpa).  is the value of the modulus of subrade reactions (kfy) that staad expects from the user, should this also be in kpa?
>
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