Q:
What corrdinate system is used for the reports in the Structural Post Processing menu ?
REV 7.05
A: The reports in the Structural Post Processing menu are reported in the beam coordinate system. You can ask MOSES to tell you what this system is in the &summary menu with the BEAM command.
Q:
Why do the AISC LRFD results reported not match my
hand calculations?
REV 7.05
A: The problem is due to your model. The option
-SECTION
was used to change the area and inertias of the section (in particular, the
area was changed by increased by a factor of 85). The options -P_N and
-M_P were however, not used to change theses factors. According to the
documentation, if a value is not overridden with an option, the computed
value will be used. For this case, the Pn value used is probably 1/85
smaller than what you would like.
A good rule is that you should model as close to what you have as possible and override as few items as possible. I can think of no reason at all to override the area of a "prismatic" beam. My guess is that is it not prismatic at all but a shape that would be better modeled with a tube, box, or W.
Q:
Why does MOSES not follow the standard scaling rule for
damage?
REV 7.04
A: The standard scaling rule to which you refer says that if the damage for a stress concentration SCF1 is D, then the damage for a stress concentration SCF2 is D*(SCF2/SCF1)**M where M is the slope of the SN curve and the all of the contributions to the damage are within the curve. An important thing to notice here is that almost all SN curves have more than one slope and the above is valid only for a single slope SN curve.
All published SN curves all have limits, and what are we to do outside these limits? Before Rev 5.06, we actually computed the damage based on the curve. We found that this can be seriously wrong. In particular, if one has a beam with really large stresses, then the larger the stresses, the smaller the CDR! This is because as the stress becomes large, more of it is outside the range of the SN curve. We have now changed this so that if the probable maximum stress in greater than the first point in the SN curve, damage is accumulated as if it were all at the first point on the curve. This gives some discontinuities in the CDRs, but it is better than overly optimistic results.
Q:
Are the "Stress Cycles" reported with "counting"
peak to trough values or amplitudes?
REV 7.06
A: These results are the exception to the rule that nothing in MOSES is peak to trough. Stress Cycles are peak to trough to be consistent with the definition given in almost all sources.
Q:
Why did I not get an increase in the CDR of
A**m (m is the slope of the SN curve)
when I increased the SCF by A?
REV 7.02
A: Your approximation is only good provided that there are no cycles below the endurance limit or above the left most point in the curve.
Q:
Why do I get "Hydrostatic Check" results for a member that is not
in the water?
REV 7.01
A: MOSES interprets these checks to be checks on the hoop stress. Thus, cone shape members get hydrostatic collapse checks because they have hoop stress.
Q:
How can I get MOSES to resize stiffener spacing?
REV 7.01
A:
Everything is done via class. Create classes with different
spacings and resize.
Click Here for an example.
Q:
Why, if I use -USE_MEAN NO on an environment to check
only the dynamic load, do I get only positive values.
A:
Without a mean, the stress is cyclical with the same
positive peak as the negative valley. If you are
going to do a code check. You should make the negative
values with a combination; i.e.
Q:
It seems like MOSES reports
the CDR's for the W curve regardless what SN curve we define
with the -SN option on the BEAM_POST command. What is going
on here?
A:
In the past (before REV 6.02) the SN curve used for beam fatigue
was specified on the BEAM_POST command. It no longer is
and the -SN option on BEAM_POST is ignored.
Now one has an SN curve associated with both ends and at each change in
section. These SN curves are specified when the BEAM or PLATE is defined,
so you need to make sure the -SN option with the correct SN curve is
specified there.
Click here
to read the documentation (this link and the following
subsections) or
Here
to read what the REV 6.02 release document has to say on the
subject.
Q:
How does MOSES compute "Joint Crushing"?
A:
Click here and look in the Miscellaneous Topics section and then
the Joint Crushing subsection.
Q:
MOSES did not choose the correct chord for a joint,
how do I redefine it?
A:
MOSES finds the largest "thru member" and proclaims it the chord.
The only control the user has over this is to alter your model so
the largest thru member is the one you want.
Q:
Where on the section are the eight points at which
MOSES computes stresses (and fatigue)?
A:
For tubes they are spaced at equal angles around the
circle. For other shapes it is more complicated.
The first four are the "bending" points which are at
the points at the extremes; .i.e. (max y, max z), (min y, max z),
(max y, min z), and (min y, min z). Now, any non tube can be
viewed as being composed of several "boxes". Four points
will be added such that points 1 and 3 have the z coordinate
of the maximum and minimum z coordinate of the centroids of the
boxes and points 2 and 4 will have the y coordinate of the maximum
ad minimum y of the centroid of the boxes.
Q:
How do I know what SCF is used for determining
a fatigue CDR?
A:
The SCF used depends on the classification of the joint
which depends on the loads in the chord and the braces.
What MOSES does is to assign a fraction of each type of
joint to the joint in question; i.e. the joint is A K joint,
B T joint, and C X joint and A + B + C = 1. So the
SCF that MOSES uses is:
Q:
I have several members that are governed by "Hyd. Tension".
How can this be since the structure is out of the water?
A:
Your deck model includes cone elements. According to
RP 2A-WSD 21st Edition, 3.4.1.c Subparagraph 2, hoop stress
must be checked for conical transitions. This provides the
same comments in the code check as for hydrostatic collapse.
This only applies to MOSES Rev 6.02 and higher.
Q:
Do the API ratios obtained during a hydrostatic collapse check on a
beam include the Safety Factors defined by the API for Axial and Hoop
stresses (1.67 and 2.0)?
A:
Of course they include the safety factors - without them
it would not be a code check.
Q:
How can I check a joint between two wide flange
sections?
A:
In MOSES, we use joint only for a tubular joint, not a connection
of two non tubes. For beam fatigue, MOSES checks both ends and
any section change. The ends are what you are calling the "joint".
The reason for checking the intermediate points is that you have
SCFs for changes in OD and T, for cones, etc.
Q:
Why when I check a cylindrical member subjected to combined
hydrostatic pressure, axial tension and bending manually do I
get a different RP2A unity ratio than MOSES?
A:
According to RP2A, the following equation should be satisfied
in this case:
Q:
We cannot find certain joints in the fatigue report. Does MOSES
print out fatigue results for all joints, or just those joints
with damage to report?
A:
If there is no damage on a joint, it does not get reported.
Now, we print the Cumulative Damage Ratios to the 5th decimal
place, so if this happens, there really is no damage to report.
Q:
Why do we obtain 8 CDRs for each end of beam of a prismatic
member?
A:
Why should you "get only 4"? In some cases the maximum stress is not
at the extremes of the section, so we compute stress at 8 points
that will normally govern: 4 extreme points, the intersection of the
two neutral axes, and the 3 most extreme points on a neutral axis.
Q:
Why when I analyze the same model using the old and
new Moses version I get big differences in CDR values?
A:
When you run the new version of the program you get small differences in
stress values due to program improvements.
One should keep in mind that fatigue damage is approximately proportional to
stress raised to the power of the inverse slope of the SN curve. Since CDR is
a sum of ratios of the applied number of cycles and the number of cycles for
which the given stress range would be allowed, then small changes in stress
result in much greater changes in CDR value. For example, just a 5% stress
difference gives a 20% to 23% difference in CDR depending on the type of the
SN curve.
Q:
How can I resize beams and
change only the wall thickness?
A:
When beams are resized, the possible sizes are defined
by a selector. Thus, define a selector that only
changes the thickness. For example:
Q:
As per API joint check criteria, the Fy of the chord should be
2/3 of the tensile strength or the Fy of the member, which ever
is lower. Does MOSES take care of this or do I have to manually
change the Fy of the chord member to whichever is the lower value?
A:
Of course MOSES treats this correctly. Be sure, however, that you have
defined the tensile strength correctly. The default is to use a fraction
of the yield. We know that this is not correct, but it prevents bogus
joint checks for 50 ksi material when people have not altered the
tensile strength from the default.
Q:
Why is it when I do my code check, there are 3 member reported to be
overstressed, but when I do the pictures for member IR > 1.0,
the plots show more than 3?
A:
Actually, your statements are not correct! According to your
data you asked for:
Q:
How can I determine the deflection of the barge in a certain
environment?
A:
You can use the JOINT DISPL command from STRPOST menu.
However, to get something that makes sense you need to
look at deterministic load cases, not spectral ones.
To generate deterministic cases,
you could use either the LCASE -TIME, LCASE -PROCESS, or
CASES -TIME command.
Q:
There are supposed to be three unity ratios reported
in the Hydrostatic Collapse Check.
Why in some cases is one of them missing?
A:
The code has three sections: hoop stress, tension, and compression. For
some cases, the can be no tension (or compression). In other words,
the axial load in the beam is such that the maximum normal stress
(axial stress +- bending stress) is always of the same sign. In this
case one of the check cannot be made and we print no results.
Q:
Which stresses are considered for the fatigue
analysis of structural components?
A:
For beams we use the normal stress and for plates we
use Von Mises stress.
Q:
How can I get zero for the allowable punching shear?
A:
In some cases with high chord stress, Qf can become
negative. When this happens, vpa is set to zero.
Q:
What is "Mom AMF" in the beam check summary output?
A:
This is the bending moment modifier, in a slightly more
usable format:
Mom AMF = 1/[Cm/(1-fa/F'ey)].
REV 7.01
&data env
env wave -sea ......
end_data
cases -spectral wave
cases -combine c_wave wave -1.
beam code -load c_wave
REV 7.00
REV 7.00
REV 7.00
REV 7.00
REV 7.00
SCF = A * SCF_k + B * SCF_T + C * SCF_x
The Joint SCF Summary provides the SCFs for all 3 joint
types and for Punching Shear the factors A, B, and C
are printed. For Fatigue, however, this is simply too
much information to present. The only way you
really know which SCF is being used is if you use -cls_mean yes.
Otherwise, you get a new joint classification for each force response
operator.
REV 7.00
REV 7.00
REV 7.00
REV 6.02
A**2+B**2+2*ν*|A|*B≤1 (3.3.3-1)
Since the equations 3.3.1-2 and 3.3.3-1 check the same thing we take a
square root here in order to be consistent and get unity ratios closer
to those of 3.3.1-2. when a zero hoop stress is applied. Notice that this
change does not effect whether or not a member satisfies the code.
REV 6.02
REV 6.01
REV 5.10
REV 5.08
&SELECT :LEG -SEL P60@
~LEG TUBE 60 1 -RED :LEG
will resize using only the tube shapes in the table which
are 60 inches in OD.
REV 5.08
REV 5.08
beam code -summary 1 1e30 0 1
The manual clearly states that what you get here is the largest
code check for each class. Thus, your output says that
there are three classes overstressed (not members). Your pictures
are telling you that there is more that one member failure
for at least some of the overstressed classes.
REV 5.08
REV 5.07
REV 5.07
REV 5.03
REV 5.03