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Dynamic Design Analysis Method


The provided input spectrum is based on units of pound-force, inches, and seconds for force, length, and time. All analyses that use this input spectrum must be performed with these units. (The other units are not restricted.) Perform the natural frequency analysis on this unit system as well.

Navy developed analytical procedure for evaluating the design of equipment subject to shock loading. The student will learn to use the NX Nastran DDAM solution (SOL 187) through a presentation of lecture materials and the completion of example problems.

The method illustrated here for generating these spectra avoids the common combinatorial techniques that lead to unrealistic design values, and yet provides design values that are still conservative. The NX Nastran DDAM Analysis (NX) course offers training in the performance of the Dynamic Design Analysis Method using NX Nastran.

DDAM description

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This evolution began with attempts to use earthquake engineering practice, circa 1950, in dealing with hostile environments created by modern weaponry. It became necessary to develop new theories that went beyond the then current status. This led to research programs that went back to basic physics and engineering principles that resulted in a sound technique for naval applications.

DDAM description

DDAM simulates the interaction between the shock-loaded component and its fixed structure. The free motion of a vessel in water produces a higher shock spectrum than a heavy structure would on ground. DDAM takes this effect into account in relation to the weight of the equipment, mounting location and orientation of the equipment on the vessel. The DDAM analysis processor uses the Naval Research Laboratory (NRL) summation method to combine the peak responses from all mode shapes into overall displacements and stresses.

DDAM description

The DDAM simulates the interaction between the shock-loaded component and its fixed structure as the free motion of a naval vessel in water produces a higher shock spectrum than a heavy structure would when mounted to a terrestrial surface. The DDAM takes interaction into account in relation to the mass of the equipment, its mounting location, and the orientation of the equipment on the vessel.

These equations demonstrate how shock spectrum data are applicable for calculating deflections, stresses, etc., for structures subjected to transient excitations. The concept of design shock spectra is discussed, and an example shows how design spectrum data might be applied to a particular case. Engineers have experienced difficulty when attempting to use enveloping or high fiducial limit statistical techniques for defining the shock environment by shock spectra. A procedure is described for generating shock design values for a simple class of structure subject to transient motion excitation. The shock design values for each mode are those that approximate as a single number each of the Duhamel integrals present in the normal mode equations that are used for performing stress and deflection calculations.

DDAM description

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Control simulations on the discrete model are carried out to demonstrate the effectiveness of the method. Experimental work is shown to demonstrate a final verification. The normal modes and natural frequencies of a structure are determined from a knowledge of the normal modes and natural frequencies of its components parts. Only linear elastic structures resting on an immovable base are considered. The total structure is mentally broken down into a primary structure (part including base points) and one or more substructures.

DDAM description

The class is focused on NX Nastran and most of the material applies independently of pre- or postprocessor. However, additional material is available for demonstration of use with NX.

This paper examines the theory of shock spectra, Fourier spectra, and mechanical impedance to determine qualitatively the effect upon shock spectra of the dynamic reaction of structures. The concept of shock spectrum dip is presented, explained, and experimental verification exhibited.

DDAM description

The article also examines those cases where a new hull is derived from an original hull by the linear scaling law. The solution of the shock response is well known when the internal equipment has also been linearly scaled.

Compliance standards for DDAM simulation and analysis software are maintained by the Naval Sea Systems Command (NAVSEA). After performing a natural frequency analysis to determine the mode shapes and natural frequencies, a DDAM analysis is performed using an input spectrum of shock design values (displacements or accelerations).

  • The DDAM takes interaction into account in relation to the mass of the equipment, its mounting location, and the orientation of the equipment on the vessel.
  • This article presents scaling rules developed to predict the response of submarine equipment subjected to underwater chemical explosions.
  • One important example is the structural interaction effects between a vehicle and large equipment structures by means of a newly defined quantity called modal effective mass.
DDAM description

All mission-essential equipment on board naval ships and submarines must be qualified for shock loads caused by underwater explosions. This course is intended for finite element analysts who need to perform shock analysis per the Navy’s DDAM procedure. DDAM must reference a DDAMDAT Bulk Data entry to perform DDAM analysis. The research purpose is to control the vibration and motion of the shaking table by using the reduced order modeling technique and LQ and LQI control theory.

The elements of the technique were theoretically based and confirmed by laboratory and large scale field testing. One important example is the structural interaction effects between a vehicle and large equipment structures by means of a newly defined quantity called modal effective mass. Another example led to the discovery that attaching a vibration generator to a structure in an effort to find the frequencies useful for foundation motion response analysis was guaranteed to produce failure. Although familiar in US and international naval circles, it is not well known by persons other than naval engineers. Many myths and misconceptions have grown during this period, so some of the major ones are addressed.

You can use DDAM to analyze the shock response at the mountings of shipboard equipment, such as masts, propulsion shafts, rudders, exhaust uptakes, and other critical structures, due to underwater explosions. All mission-essential equipment on board surface ships and submarines must be qualified for shock loads, such as from depth charges, mines, missiles, and torpedoes. Attempts to combine shock spectra, by envelope of fiducial limit types of analysis, to produce curves useful for design purposes have led to design shock spectra which are extremely overconservative. It was noticed that this technique assumes that the impedance of any structure at its foundation for its fixed base natural frequencies is very low, and therefore unable to affect the foundation motion. This was in contrast to test evidence that a dip would appear in the regions of these frequencies in the spectrum plot.

Dynamic Design Analysis Method Data Set Selection

Several theorems regarding shock and Fourier spectra are advanced. The necessary distinction between shock spectrum values at fixed base natural frequencies of a structure and at natural frequencies of a structure? The potentially extreme overconservatism in design resulting from incorrect usage of shock spectra is pointed out.

DDAM description

NX Nastran DDAM Analysis with NX (G2H)

DDAM description

It is emphasized that the test sections were short and devoid of typical equipment present in a real compartment. The results, nevertheless, provide trends and ratios in shock design values, not necessarily absolute design numbers. The approach taken in developing these scaling rules could be useful for enhancing field data that may exist for a given class of boat to allow greater usage of these data for different equipment subject to a variety of charge weights, attack geometries, and other boats.

DDAM description

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Results can be viewed for each mode shape and the resultant in the Results environment. configuration of the structural system in order to provide more useable space in a building, and (3) reduce the cost of construction due to fewer and/or smaller structural elements. Traditionally, TMDs have only been used to control perceptible and excessive motions from wind loading and crowd movement. As such, they have not been used in laboratory and other sensitive spaces due to equipment criteria which specify vibration levels far below perceptibility.

The displacement of the total structure is expressed as a series expansion of the scaled normal modes of the primary structure. The scaling factors involved are determined from the equilibrium equations of the primary structure with the secondary structures removed and replaced by their internal forces of connection.

It makes the ships vulnerable and they can lose the ability to accomplish their missions. This study presents a shock analysis, using the dynamic design analysis method (DDAM), of a naval ship stern ramp subjected to a non-contact underwater explosion. The objective is to evaluate the performance of the ramp subjected to a shock load, identify areas for structural improvements and recommend design changes. The DDAM in the commercial software ANSYS is used in the evaluation of the ramp. The structural response to the shock load is estimated by combined modal and response spectrum analyses.

DDAM description

With the advent of a new dashpot damper design, it is now possible to use TMDs to control imperceptible motions as required in sensitive occupancies. This paper presents the results of controlled testing of these new dashpot dampers and a full-scale implementation of TMDs with this component in a long-span structure. Keep in mind that the displacement results are relative to the ground excitation (input spectrum).

This article addresses the degree of success that may be achieved by using simple equipment–vehicle models that produce time history responses whose equipment fixed base modal maximum response values are equivalent to those found in the U.S. The criteria of success is measured by a comparison of the results with typical interim shock design values. A study has been made of the influence on shock spectrum curves of the interaction between a superstructure and its nonrigid foundation. Such interaction causes the spectrum values of interest to lie in the valleys corresponding to the superstructure's fixed base natural frequencies, while the peak spectrum values occur at the combined system natural frequencies. Normal mode theory for lumped mass systems is reviewed briefly and the necessary equations are presented.

The input spectrum values are provided automatically by the software, based on data from unclassified U.S. Navy documents (primarily Naval Research Laboratory Report NRL-1396). Optionally, you can provide user-defined coefficients, which can be for an alternate unit system, or classified coefficients. The security of classified coefficients is maintained through the ability to run the DDAM portion of the analysis on a secured computer. Shock load caused by underwater explosion in naval battles can lead to malfunction of the equipment on-board naval vessels.

DDAM description

The shock spectrum dip effect, for a structure subject to transient excitation, is investigated, with use being made of controlled laboratory shock experiments. A rough understanding of the phenomenon is provided by observing that a structure on a non-rigid foundation tends to act as a dynamic vibration absorber for frequencies corresponding to its fixed base natural frequencies. The two mathematical models used provide information helpful in distinguishing between ordinary shock spectra and design shock spectra, and thus in developing shock design values for future structures. This article presents scaling rules developed to predict the response of submarine equipment subjected to underwater chemical explosions. A simplified model of a hull section was used to contain frame-mounted single degree of freedom equipment.