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Rõ ràng, để tính được tải trọng động đất thì cần phải tính được chu kì và dạng dao động của công trình, mà các kết quả này phụ thuộc rất nhiều vào "Mô hình khối lượng" hay "Mass Modeling".
Mình tìm mãi trong EuroCode 8 cũng chỉ thấy đoạn này:
"1.18.3.2 Mass Modeling
The natural frequencies and mode shapes of a structure are the
primary parameters that affect the response of a structure under
dynamic loading. The free vibration problem is solved to extract
these values. Since no external forcing function is involved, the
natural frequencies and mode shapes are direct functions of the
stiffness and mass distribution in the structure. Results of the
frequency and mode shape calculations may vary significantly
depending upon the mass modeling. This variation, in turn, affects
the response spectrum and forced vibration analysis results. Thus,
extreme caution should be exercised in mass modeling in a
dynamic analysis problem.
In STAAD, all masses that are capable of moving should be
modeled as loads applied in all possible directions of movement.
Even if the loading is known to be only in one direction there is
usually mass motion in other directions at some or all joints and
these mass directions (“loads” in weight units) must be entered to
be correct. Joint moments that are entered will be considered to be
weight moment of inertias (force-length2 units).
Please enter selfweight, joint and element loadings in global directions with the same sign as much as possible so that the “masses” do not cancel each other.
Member/Element loadings may also be used to generate joint
translational masses. Member end joint moments that are
generated by the member loading (including concentrated
moments) are discarded as irrelevant to dynamics. Enter mass
moments of inertia, if needed, at the joints as joint moments.
STAAD uses a diagonal mass matrix of 6 lumped mass equations
per joint. The selfweight or uniformly loaded member is lumped
50% to each end joint without rotational mass moments of inertia.
The other element types are integrated but roughly speaking the
weight is distributed equally amongst the joints of the element.
The members/elements of finite element theory are simple
mathematical representations of deformation meant to apply over a
small region. The FEA procedures will converge if you subdivide
the elements and rerun; then subdivide the elements that have
significantly changed results and rerun; etc. until the key results
are converged to the accuracy needed.
An example of a simple beam problem that needs to subdivide real
members to better represent the mass distribution (and the dynamic
response and the force distribution response along members) is a
simple floor beam between 2 columns will put all of the mass on
the column joints. In this example, a vertical ground motion will
not bend the beam even if there is a concentrated force (mass) at
mid span.
In addition, the dynamic results will not reflect the location
of a mass within a member (i.e. the masses are lumped at the
joints). This means that the motion, of a large mass in the
middle of a member relative to the ends of the member, is not
considered. This may affect the frequencies and mode shapes.
If this is important to the solution, split the member into two.
Another effect of moving the masses to the joints is that the
resulting shear/moment distribution is based as if the masses
were not within the member. Note also that if one end of a
member is a support, then half of the that member mass is
lumped at the support and will not move during the dynamic response."
Cái đoạn bôi màu đỏ đại ý nói thế này:
Hãy nhập tĩnh tải bản thân, các tải trọng nút và phần tử theo các hướng và cùng dấu nhiều nhất có thể để các "khối lượng" không triệt tiêu lẫn nhau
Mình quen dùng StaadPro để chạy nội lực. Trong phần tính toán tải trọng động đất theo Eurocode 8, STAAD nói rằng Tất cả các khối lượng có khả năng dao động đều được kể vào như là tải trọng tác dụng trong tất cả các hướng dao động có thể xảy ra (In STAAD, all masses that are capable of moving should be modeled as loads applied in all possible directions of movement.)
Như vậy có thể hiểu, mình khai báo tải trọng bản thân và lấy một phần hoạt tải. Nhưng một phần hoạt tải là bao nhiêu phần trăm nhỉ?
Ý kiến các bạn thế nào?
Mình tìm mãi trong EuroCode 8 cũng chỉ thấy đoạn này:
"1.18.3.2 Mass Modeling
The natural frequencies and mode shapes of a structure are the
primary parameters that affect the response of a structure under
dynamic loading. The free vibration problem is solved to extract
these values. Since no external forcing function is involved, the
natural frequencies and mode shapes are direct functions of the
stiffness and mass distribution in the structure. Results of the
frequency and mode shape calculations may vary significantly
depending upon the mass modeling. This variation, in turn, affects
the response spectrum and forced vibration analysis results. Thus,
extreme caution should be exercised in mass modeling in a
dynamic analysis problem.
In STAAD, all masses that are capable of moving should be
modeled as loads applied in all possible directions of movement.
Even if the loading is known to be only in one direction there is
usually mass motion in other directions at some or all joints and
these mass directions (“loads” in weight units) must be entered to
be correct. Joint moments that are entered will be considered to be
weight moment of inertias (force-length2 units).
Please enter selfweight, joint and element loadings in global directions with the same sign as much as possible so that the “masses” do not cancel each other.
Member/Element loadings may also be used to generate joint
translational masses. Member end joint moments that are
generated by the member loading (including concentrated
moments) are discarded as irrelevant to dynamics. Enter mass
moments of inertia, if needed, at the joints as joint moments.
STAAD uses a diagonal mass matrix of 6 lumped mass equations
per joint. The selfweight or uniformly loaded member is lumped
50% to each end joint without rotational mass moments of inertia.
The other element types are integrated but roughly speaking the
weight is distributed equally amongst the joints of the element.
The members/elements of finite element theory are simple
mathematical representations of deformation meant to apply over a
small region. The FEA procedures will converge if you subdivide
the elements and rerun; then subdivide the elements that have
significantly changed results and rerun; etc. until the key results
are converged to the accuracy needed.
An example of a simple beam problem that needs to subdivide real
members to better represent the mass distribution (and the dynamic
response and the force distribution response along members) is a
simple floor beam between 2 columns will put all of the mass on
the column joints. In this example, a vertical ground motion will
not bend the beam even if there is a concentrated force (mass) at
mid span.
In addition, the dynamic results will not reflect the location
of a mass within a member (i.e. the masses are lumped at the
joints). This means that the motion, of a large mass in the
middle of a member relative to the ends of the member, is not
considered. This may affect the frequencies and mode shapes.
If this is important to the solution, split the member into two.
Another effect of moving the masses to the joints is that the
resulting shear/moment distribution is based as if the masses
were not within the member. Note also that if one end of a
member is a support, then half of the that member mass is
lumped at the support and will not move during the dynamic response."
Cái đoạn bôi màu đỏ đại ý nói thế này:
Hãy nhập tĩnh tải bản thân, các tải trọng nút và phần tử theo các hướng và cùng dấu nhiều nhất có thể để các "khối lượng" không triệt tiêu lẫn nhau
Mình quen dùng StaadPro để chạy nội lực. Trong phần tính toán tải trọng động đất theo Eurocode 8, STAAD nói rằng Tất cả các khối lượng có khả năng dao động đều được kể vào như là tải trọng tác dụng trong tất cả các hướng dao động có thể xảy ra (In STAAD, all masses that are capable of moving should be modeled as loads applied in all possible directions of movement.)
Như vậy có thể hiểu, mình khai báo tải trọng bản thân và lấy một phần hoạt tải. Nhưng một phần hoạt tải là bao nhiêu phần trăm nhỉ?
Ý kiến các bạn thế nào?
Ghi chú