NBSIR 77-1254

J FILE COpy

DO NOT REMOVE

Transportation, Handling and

Field Service Loads for Air

Mobility Shelter Systems

C. W. C. Yancey

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 1977

Final Report

Prepared for Civil & Environmental Engineering Development Agency

Tyndall Air Force Base, Florida 32401

NBSIR 77-1254

TRANSPORTATION, HANDLING AND

FIELD SERVICE LOADS FOR AIR

MOBILITY SHELTER SYSTEMS

C. W. Yancey

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 1977

Final Report

Prepared for Civil & Environmental Engineering Development Agency

Tyndall Air Force Base, Florida 32401

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'
TABLE OF CONTENTS

Page

PREFACE ii

ABSTRACT iii

SI CONVERSION UNITS ..... iv

1. INTRODUCTION 1 1.1 Background 1 1.2 Objectives and Approach 1 1.2.1 Objectives 1 1.2.2 Approach 1 1.3 Description of Information in the Literature 4 1.4 Organization of Report 7

2. RAILROAD ENVIRONMENT 8 2.1 Over-The-Rail-Transport 8 2.1.1 Random Vibration Excitation 11 2.1.2 In-Transit Shock Excitation 15

2.2 Coupling Operations in Switching Yards 17

3. ROAD ENVIRONMENT 27

4. AIR ENVIRONMENT 37 4.1 Airplane Transport of Cargo 37 4.1.1 Propeller-Driven Airplanes 37

4.1.2 Turbojet Airplanes 42

4.2 Helicopter Transport of Cargo 44

4.2.1 Cargo Transported Inside Helicopters ... 44

4.2.2 Cargo Transported Outside Helicopters 50

5. HANDLING ENVIRONMENT 57 5.1 Normal Handling Excitations 57 5.1.1 Lifting Inputs 57 5.1.2 Placement During Stacking. .... 57 5.1.3 Short Transport by Handling Devices 58

5.2 Abnormal Handling Excitations 58

6. FIELD SERVICE LOADS 60

6.1 Gravity Loads 60

6.2 Snow and Ice Loads 60

6.3 Wind Loads 60

6.4 Thermal Loads 61

6.5 Combination of Field Service Loads 62

7. SUMMARY AND CONCLUSIONS 63

7.1 Areas of Sufficient Data 63

7.2 Areas of Insufficient Data 64

8. LIST OF REFERENCES 66

i
PREFACE

This report was prepared by the Structures Section, Center for Building Technology, Institute

for Applied Technology, National Bureau of Standards, Washington, D. C. for the Air Force

Civil & Environmental Engineering Development Agency, Tyndall Air Force Base, Florida. The

preceeding Interim Report identified the deficiencies in the current design procedure for

military shelters. This report is an expansion of the Interim Report and its purpose is to present some of the dynamic load information—incident to transportation—needed to improve

the design procedure for shelters.

ii
Abstract

To fulfill one of the prerequisites for establishing design criteria for military

field shelters, transportation, handling and field service load data are presented. An

extensive literature search was conducted to determine the magnitude and frequencies of

shock and vibration responses produced by railroad, road and air cargo vehicles and by

devices commonly used in handling field shelters. Summary data, extracted from field

study reports, are presented in the form of acceleration versus frequency diagrams. The

acceleration values can be used to compute an Equivalent Static Force input for the design

and analysis of shelter models. The probabilities of occurrence of the acceleration amplitudes generated by the three transportation modes are indicated in cases where data reduction included statistical analyses. Sources of the dynamic load data included flatbed

trucks, propeller and jet airplanes, helicopters, railroad flatcars and forklift trucks.

Where possible, the shock and vibration data used in presenting the summary diagrams were

restricted to that obtained from tests involving military vehicles commonly used to transport

shelters. Recommendations are presented for the selection of static design loads to account

for gravity, snow, ice and wind effects.

Key Words: Acceleration; airplanes; cargo, dynamic loads; equivalent static force; gravity

load; handling devices; handling loads; military field shelters; probability

distributions; shocks, snow load; static load; trains; transportation; trucks;

vibrations; wind load.

iii
SI Conversion Units

In view of the present accepted practice in this country for building technology, common U.S. units of measurement have been used throughout this publication. In recognition of the position of the United States as a signatory to the General Conference on Weights and Measures, which gave official status to the metric SI system of units in

1960, appropriate conversion factors have been provided in the table below. The reader

interested in making further use of the coherent system of SI units is referred to:

NBS SP 330, 1972 Edition, "The International System of Units"

ASTM E380-75/IEEE Std. 268-1976 ASTM/IEEE Standard Metric Practice

Table of Conversion Factors to Metric (S.I. ) Units

Physical Quantity To Convert From To Multiply By

Length inch (in) meter (m) a 2.540 x 10 * foot (ft) meter (m) 3.048 x 10

_1

*

Area inch 2 2 2 meter (m ) 6.4516 x 10~4 *

2 foot 2 2 meter (m ) 9.290 x 10“ 2

Volume inch 3 3 3 meter (m ) 1.639 x 10" 5

foot 3 3 3 meter (m ) 2.832 x 10" 2

Force pound (lbf) newton (N) 4.448

kilogram (kgf) newton (N) 9.807

Pressure, Stress psi Pascals (pa) or

2 2 newton/meter (N/m )

6.894 x 10

3

Energy inch-pound (in-lbf) joule (J) 1.130. x

10"1

foot-pound (ft-lbf) joule (J) 13.558 x 10"1

Weight, Mass pound (lbm) kilogram (kg) 4.536 x 10

_1

Unit Weight pcf 3 3 kilogram/meter (kg/m ) 16.018

Velocity foot/sec (fps) meter/sec (m/s) 3.048 x 10

-1

*

Acceleration 2 foot/sec 2 2 meter/sec (m/s ) 3.048 x 10

_1

lieter may be subdivided. A centimeter (cm) is 1/100 and a millimeter (mm) is 1/1000 m.

* Exact

.

iv
1. INTRODUCTION

1.1 Background - The dynamic environment generated during the transportation and handling

of cargo consists of randomLy occurring dynamic forces. These forces have characteristics

which are categorized as periodic, transient or random. Combinations of these types of

dynamic forces result from transportation and handling events. In general, the forcing

function input to the cargo is complex in nature; furthermore, forcing functions are difficult to describe for some events because of a scarcity of recorded data upon which to base a definition.

The right hand side of figure 1.1 illustrates the point that dynamic loads can be defined

at several interfaces within a given transportation or handling mode. From an analytical

standpoint, it is desirable to describe the loads at the point of attachment of the shelter

to the support or tiedown system (i.e. interface A in figure 1.1). The dynamic loads so described would become the applied forces needed to analyze the shelter as a structural

system. The desired condition notwithstanding, forcing functions usually have not been

defined at the attachment point interface in test programs. Thus, the dynamic loads to be applied to shelters must be derived from dynamic response measurements made at other, more convenient interfaces. For the various transportation modes documented in the

literature, dynamic response has usually been described in terms of shocks and vibrations

on the cargo loadbed at locations directly adjacent to (e.g. fore and aft) the cargo.

In view of the fact that mobile shelters are placed into service in many parts of the

world and are transported by various modes, it is necessary to establish a set of design/

analysis criteria. Predicting transportation and handling loads is. one of the prerequisites

to establishing criteria. The description of the design loads must be compatible with the

available mfethods for analyzing structures such as shelters. While there have been several

efforts to gather and summarize transportation load data, no existing compilation fulfills

the objective of translating the data into a common format which can be used in developing

a design criteria.

1 . 2 Objectives and Approach

1.2.1 Ob j ectives - This study was conducted to establish design loads for mobile

shelters, which are incident to three modes of cargo transportation: railcar, truck

and aircraft. To provide the basis for a comprehensive design criteria, the study

also sought to define load inputs induced by handling operations and field service.

The format for presenting the load data was to be consistent with an equivalent static

force analysis of typical shelter conformations. The formulation of load combinations

and the development of design criteria were not within the scope of this study.

1.2.2 Approach - As shown in figure 1.2, a cycle of shelter use may involve loads

incident to transportation, handling and field service. The transportation and

1
ELEMENTS OF SYSTEM

I MFID SHELTER

2 TIEDOWN FIXTURE

A LOAD INPUT TO SHELTER

3 TRANSPORTATION MODE

(TRAIN TRUCK. BOAT,

AIRPLANE. HELICOPTER)

B B B B

B LOADS AT LOADBED

OF TRANSPORT VEHICLE

4 TRANSPORTATION MEDIUM

(RAIL ROAD. WATER. AIR)

5 HANDLING DEVICES (CRANE.

FORKLIFT. ELEVATOR. ETC.)

I 1 f I

C FORCING FUNCTION INPUT FROM TRANSPORTATION MEDIUM

D LOADS FROM HANDLING

DEVICES

Figure 1.1 - Schematic of transportation and handling systems and loads

2
3

Figure

1.2

-

Principal

direetions

of

cargo

bed

response
handling loads are primarily dynamic inputs. Th6y are discussed in this report as two separate categories of dynamic loading. The transportation loads are subdivided

according to the type of carrier: railcar, truck, fixed-wing aircraft and rotarywing aircraft. Activities such as lifting, short transport and stacking are the sources of handling loads. Field services loads due to snow, gravity, wind and thermal

gradient are treated separately because they are virtually static in nature and they

may be applied to different shelter configurations than exist during transportation

and handling operations. It should be noted that the temporary storage of shelters

in a stacked configuration is considered a field service condition in that the same static loads are considered. On the other hand, the stacking operation - involving

lifting and placement - is considered a handling activity in that it involves dynamic

loads . »

Each source of loading which may affect the design/ analysis of field shelters is

discussed independently; the selection of the applicable transportation and handling

modes is considered a design decision. The dynamic load information is presented in

the form of acceleration versus frequency diagrams to facilitate its use in an equivalent static force method of analysis.

The design force is to be computed by multiplying the shelter's mass by an acceleration

magnitude selected from the appropriate acceleration versus frequency diagram. The

design frequency is to be established on the basis of the natural frequency (known

or estimated) of the shelter or shelter component. This simplified analytical approach

is analagous to the equivalent static force formulation commonly used to analyze the

seismic resistance of building structures. Also, the probabilities of occurrence of

the various transportation and handling loads are presented when such information is

available. Consequently, a risk-oriented design approach is made possible. A typical

risk-oriented design decision is as follows: Equipped with the knowledge that a particular high-amplitude acceleration is caused by an event of low probability of

occurrence, a design acceleration of lower magnitude may be selected, consistent with

the stated risk of damage.

1.3 Description of Information in the Literature - A comprehensive literature search was conducted to determine the magnitude of dynamic loads generated by railroad, road and air

transportation vehicles and by devices typically used to handle field shelters. Complete

transportation and handling load information must, in addition to quantifying the dynamic

loads, establish frequencies at which the loads occur and indicate the probability of occurrence of the loads. Toward the objective of establishing reliable statistical load

information for the above mentioned transportation and handling media, applicable data

have been extracted from reports of several extensive field measurement programs. These

studies were conducted to quantify the shock and vibration loads experienced by the cargo

floor of railroad flatcars, tractor- trailer combinations, flatbed trucks, airplanes and

helicopters. Where possible, the load information presented herein is based on data

4
obtained from the loadbed of military vehicles commonly used to transport cargo such as field shelters. Data from commercial vehicle counterparts have also been included in some

cases. The load information is presented in both graphical and tabular formats to facilitate its use in an analysis of a shelter in its shipping configuration.

The loadbed dynamic response data are usually presented in the form of acceleration versus frequency diagrams. The acceleration ordinates indicate either zero-to-pdak acceleration,

root-mean-square (rms) response, or mean-square acceleration per frequency band. The

approach used to reduce the data is largely dependent on the type of vibration excitation

caused by the events, the relative amount of data collected and its intended use. For

example, random vibrations are commonly analyzed by passing the electrical analog of the

vibration measurements through a series of narrow bandpass filters (i.e. a spectrum analyzer).

The filtered data then essentially become single frequency sinusoidal vibrations whose

amplitudes are random variables. The output from each bandpass is the mean-square value,

from which the mean power spectral density can be obtained. The complete power spectral

density distribution then is determined by changing the frequency bandpass.

An alternate format used in some recent reports of random vibration data employs the concept

of a cumulative probability distribution. This approach usually involves the filtering of

the vibration measurements into narrow frequency bands and counting the number of occurrences

of various amplitudes. Thus, the fraction of time (i.e. the probability) that a given

amplitude will be below a specified value is computed within the frequency band or across

the entire frequency spectrum. Such an analysis can be performed for each transportation

event included in the study and for each of the three principle directions (see fig. 1.3).

This statistical method of data reduction requires a relatively large volume of data and

usually requires the aid of a computer. Because of these requirements, this method has

had limited use in transportation studies until recently.

For those transportation and handlipg events which cause shock response^ (e.g. railcar

coupling or humping), the data are usually presented in the form of shock spectra. Thus,

the accelerations experienced by a series of single-degree-of-freedom dynamic systems with

different natural frequencies, are given. The damping ratio (C/C^) is varied to generate

different spectra. This format gives information on the energy levels associated with

various frequencies and indicates the relative damage potential of different transient

inputs. To derive "worst-case" data, shock spectra obtained for several transportation

events (e.g. a truck crossing railroad tracks, cattle guards and rough road) and accelerometer

locations are enveloped. The envelope curve is comprised of the maximum accelerations

obtained for each frequency. Therefore, the envelope is not a continuous curve, but rather

is a number of data points connected by straight lines. This type of curve accounts for

the combined effects of various operating conditions, traveling speeds, accelerometer

1 / Shock is defined as a sudden and severe transient excitation of an object.
MANUFACTURE

INITIAL TRANSFER 'Handling (Chap. 5 )

5 )

MEDIUM & LONG

DISTANCE TRANSPORT

Truck Helicopter

Train Airplane

Ship

Transportation (Chap. 2 , 3 , 4 )

Figure 1.3 - One cycle of shelter use.

6
locations, weight of the carrier and the cargo weight. In some reports, the envelope

curves represent the absolute worst case in 'that directional effects are also combined.

All of the above-mentioned forms of data presentation will be exhibited in the remaining

chapters of this report.

1.4 Organization of Report - The remainder of the report is organized as follows. Both

over-the-rail and railroad switching yard environments are discussed in chapter 2. The

normal events comprising the over-the-rail environment are described and the critical

frequency spectra are presented for these events. Also, data are presented to show the

probability of occurrence of various coupling speeds. Shock spectra are presented for

speeds having a low probability of occurrency and for those having a high probability of occurrence.

Chapter 3 presents data helpful in predicting the environment associated with medium and

long distance truck transportation. Vibration measurements obtained from the deck of a flatbed truck, operating under "normal" and "abnormal" road conditions provide the bases

for the description.

The dynamic loads experienced in the cargo areas of typical military cargo aircraft

are discussed in chapter 4. The shocks and vibrations are separated into two classes:

(1) those caused by propeller-driven and turbojet airplanes (i.e. fixed-wing aircraft)

and (2) those produced by helicopters (i.e. rotary-wing aircraft). Recognizing that

cargo may be transported either inside or outside a helicopter, both of these air

transport modes are discussed.

Handling excitations are discussed in chapter 5. The primary operations of interest

to shelter handling are: (1) lifting by forklifts, cranes, etc. (2) stacking for storage

purposes and (3) short transport by forklifts, mobilizers, cranes, etc. Normal and

abnormal handling events are discussed only briefly due to the insufficient amount of

available information.

Loads occurring during field service are discussed in Chapter 6. Static design loads

are presented to account for the effects of snow, ice, wind and gravity.

Chapter 7 presents Summary and Conclusions.

7
2. RAILROAD ENVIRONMENT

The dynamic environment associated with rail transport of cargo stems from two distinctly

different operational sources: (1) over-the-rail transport and (2) coupling or humping

operations in switching yards. The cargo floor of a railcar is subjected to both continuous and transient vibrations during over-the-rail transport. The transient excitations - usually in the form of shocks - are of two types: (1) recurrent or repetitious and

2 / (2) intermittent (i.e. either single occurrence or irregular occurrence) [ 1 ]

.

Some of the causes of the shocks and vibrations comprising the railroad dynamic environment

are listed in table 1. The lists of events and sources are not exhaustive, but they include

the causes of the most severe accelerations reported to date. Also shown in table 1 are

the predominant directions and the type of dynamic excitation caused by each event. Although

shock response may occur on the cargo floor as a result of such events as starting, stopping

and slack run-ins/outs and from track irregularities, the most severe shocks are caused by

the coupling of railcars in switching yards. Dynamic loads measured during both over-therail and switching events are described in the following sections of this chapter. The

load data are intended to provide the basis for predicting design forces which are to be

resisted by field shelters.

2.1 Over-The-Rall-Transport - Although data have been obtained for both conventional

3/ (i.e. friction or rubber) and cushioned draft gear— railcars, only conventional draft gear

data are presented here. This somewhat conservative approach is based on the assumption

that shelters are usually transported by railcars with standard draft gear. Furthermore,

the continuous and transient vibrations measured aboard railcars with standard draft gear are generally more severe than those measured on cushioned draft gear cars. The draft gears

included in the standard category are 18000 ft-lbf (24404 J) , 36000 ft-lbf (48809 J) and

60000 ft-lbf (81349 J) Association of American Railroads (A.A.R.) types. The range of

predominant frequencies for typical A.A.R. 1915 truck springs are shown in figure 2.1

[2]. The range of frequencies is seen to extend'from about 4 to 8 Hz. According to Foley [1], the railcar frame will produce cargo floor excitations in the 60 to 100 Hz

range, while traversing rail joints produces accelerations in the frequency range of 10

to 30 Hz.

Foley [1] and Gens [2] have described the over-the-rail environment as generally consisting

of low-level random vibrations upon which are superimposed recurrent transient accelerations.

The dynamic characteristics differ depending upon whether the frequency range is above or

— The numbers in parenthesis indicate references which are listed in Section 8. 3/

- A draft gear is a mechanical device used to isolate shock, in the longitudinal direction

in railcars. It is installed between the coupler and the car body to reduce the shock

that is transmitted through it to the freight car.

8
TABLE 1 — Events Which Cause Railroad Dynamic Excitations

Event Source

Predominant

Directions

Type of Dynamic

Excitation

1 1 Train Leaving Switching Yard Slack Run-outs Longitudinal Shock

2 Stopping Slack Run-ins Longitudinal Shock

3

4

Crossing Roads

Intersecting Tracks

Roads, Tracks

and Switches

Lateral, Vertical

& Longitudinal

Random Vibration

and Shock

5 Switches

6 On Curves Tapered Wheels

& Wheel Flanges

Lateral Periodic

rH

*H

c

04

7 On Level Runs at Various

Speeds

Discontinuity at

Rail Joints, Flat

Spots

Vertical Random Vibration

and Shock

1<u H

1J-i 0)>o

Tapered Wheels & Wheel Flanges

Lateral Periodic

8 Climbing a Hill Slack Run-ins/

Run-outs

Longitudinal Shock

9 Going Downhill with Braking Slack Run-ins/

Run-outs

Longitudinal Shock

10 On Rough Track Track

Irregularities Vertical Random Vibration

and Shock

11 Crossing Bridges Vertical and

Lateral

Random Vibration

and Shock

12 In Tunnels

Coupling

Operations Coupling or Humping Impact Longitudinal and

Vertical

Shock

1

1

9
PEAK

VALUES

AVERAGE

CONTINUOUS

VIBRATIONS

> aanindwv noiivu31333v

10
below 10 Hz. Below 10 Hz, the accelerations are almost entirely of the repetitive shock

type, while above 10 Hz the rail environment consists of intermittent transient accelerations superimposed on broad band random excitation.

2.1.1 Random Vibration Excitation - Measurements of accelerations experienced by the

cargo floor of a flatcar with standard draft gear have been made by Sandia Laboratories [3]. The 66200 lb (30028 kg) flatcar had a wood deck and was equipped with

side and end stake pockets. It was used to transport a "radioactive fuel element

shipping cask" over a distance of approximately 500 mi (804 km) . The purpose of the

shipment was to provide an in-transit test of the cask tiedown system. The flatcar was attached to regularly scheduled trains varying in length from 65 to 120 cars. The data gathering activity consisted of obtaining acceleration time-histories in the

vertical, lateral and longitudinal directions at various locations on the deck. The

predetermined sampling events were selected to essentially encompass the over-therail sources of vibrations. Accelerometer measurements were recorded for 11 typical

events. All of the sampled events are listed in table 1. Crossing roads (item 3) was not included as an event for sampling. Although no reason was stated for omitting

this event, the effect was the elimination of some transient responses that would

have been recorded. To obtain the test results, the time-histories were passed

through a series of narrow-bandpass filters and the number of acceleration peaks

within each bandpass were counted. Thus, having obtained the distribution of acceleration peaks within each bandpass, cumulative probability distributions were computed.

The probability distributions, so obtained for each event, were combined to obtain a composite distribution for all of the sampled events. Test results were presented

by Gens [3] and Ostrem [4]. Directional composite frequency spectra obtained from

the test run are presented in figures 2.2, 2.3, and 2.4 [4]. The three figures

summarize the data obtained for the 11 over-the-rail events. The data were plotted

at the center frequency within each bandpass. The statistical-based format discussed

in section 1.3 was used in presenting these acceleration versus frequency curves.

4/

In addition to a distribution curve showing Peak— accelerations, curves of constant

probability are presented for probabilities of 90, 95, 98, 99 and 99.5 percent.

The results indicate that the most severe accelerations occurred in the low frequency

domain (i.e. < 10 Hz). Because the results are presented as composite diagrams, it

is not known which events contributed to this phenomenon. It was also observed that

in all bandpasses the peak acceleration magnitudes were highest in the vertical

direction. Generally, the lateral direction Peaks were next highest; the longitudinal

vibrations are concluded to be insignificant for the over-the-rail environment.

Further examination of figures 2. 2-2. 4 shows that above 10 Hz the Peak accelerations were below 0.72 g, irrespective of direction.

4/ When the word "peak" is capitalized in this report, it is referring to an absolute

maximum acceleration measured during the stipulated event (s).

11
(*t3d-0) *,B 'N0I1VU31333V

12

Figure

2.2

-

Railroad

composite

acceleration

versus

frequency

diagram

over-the-rail

,

vertical

direction.

(after

Ostrem)
(qart-o) $,6 N0I1VU31333V

13

Figure

2.3

-

Railroad

composite

acceleration

versus

frequency

diagram

over-the-rail^

lateral

direction.

(after

Ostrem)
!s -n

<35 E

<3 V is na +5

co 350

<3c is

<a <a 3 40 cr'K

ca «

CO • 3 K

CO O

is *3

<a 40 3

<a s

o •3

•3 rtt 40

<3 SO

is <3

<a K

SO •3

<a "C3

<3 a

<3 4^

<3

<a £

40 o

*3

COo •» E *3 o C3

<3 Sh

"<3

1 ca <3O 40

Sh i s~i is *3 <a <3 3

Dq o

C\3

<a §3

<35 *3

&4

(i|»»<-o) M N0I1VU31333V
Because the 500-mi (804-km) trip involved most of the over-the-rail-events which are sources of dynamic excitations, it is concluded that the above-mentioned test events

are representative of the over- the- rail environment experienced by standard-draftgear flatcars transporting cargo. With respect to transportation design loads for

shelters, the data shown in figures 2.2, 2.3, and 2.4 are recommended as bases for

predicting dynamic loads for over-the-rail transport. When a "worst case" design

approach is used, the Peak acceleration ordinates can be selected for the specified

frequency level. In cases where a risk-oriented design philosophy is used, lower

design accelerations can be obtained based on the level of probability specified.

Generally speaking, the accelerations caused by over-the-rail transport will not

govern the design of shelters. In fact, Foley [1] states that "the continuous

excitation experienced during rail transportation is somewhat lower in amplitude

than that experienced in road travel." The data presented above will be useful

input to a reliability-based design analysis of shelters.

2.1.2 In-Transit Shock Excitation - In the over-the-rail dynamic environment,

shocks are caused by slack run-ins/outs in the coupling system, starts, stops,

and various track conditions (see table 1). Here, shock excitation refers to the

sudden, severe, non-periodic excitation of the cargo deck. The most common format

for describing transportation-borne shock is the shock spectra^. Data presented in

this format provide an indication of the frequencies at which the most severe accelerations occur as well as giving the magnitude of the critical accelerations.

Shock spectra envelopes for accelerations caused by slack run-ins/outs are shown in

figure 2.5 [3], The composite curves were obtained by enveloping spectra for a number of occurrences of these operations. Accelerometer measurements were made

aboard a flatcar equipped with standard draft gear. At various times during the

500-mi (804-km) trip, the test car was part of three different train lengths, varying

from 65 to 120 cars. The three curves in figure 2.5 reflect measurements taken in

three orthogonal directions. Note that the damping ratio (C/C

c

> used in computing

the shock spectra was 0.03. According to Ostrem [4], the ratio is commonly used for

cargo which are not provided with shock isolation systems. Examination of figure 2.5

shows that the highest accelerations occur in the longitudinal direction for frequencies below 20 Hz. The maximum acceleration magnitude in this frequency range is

approximately 1 g. Above 20 Hz, the highest accelerations occur in the vertical direction. The maximum acceleration, occurring at about 100 Hz, is approximately 3.5 g. Directional shock spectra envelopes for inputs resulting from road crossings, intersecting

— A shock spectra is defined as the response of a series of single-degree-of-freedom dynamic

systems to shock excitation. Thus, shock spectra differs from frequency spectra such as shown in figures 2.2 - 2.4; here the frequency and damping ratio have to be assumed prior

to determining response, and only one degree of freedom is assumed.

15
ACCELERATION,

g's

(o-peak)

Figure

FREQUENCY, Hz

2.5 - Railroad shook spectra envelopes> slack run ins/outs,

dvrectional. (after Ostrem)

16
tracks (at right angles) and switch crossings are shown in figure 2.6 [4], It is

shown that in the low frequency range (i.e. up to 10 Hz) that the maximum acceleration

is about 1 g. The most severe shocks in this range occur in the vertical direction.

It is interesting to note that Gens [2] indicates that over-the-rail shocks seldom

exceed 1 g.

Considering the data presented in figures 2.5 and 2.6 as being typical of the over- the-rail shock environment, it is concluded that the design acceleration for shelters

with natural frequencies less than 20 Hz, is 1 g in the vertical and longitudinal

direction. In the lateral direction, the design acceleration for the same natural

frequency range is 0.5 g.

As is shown in section 2.2, the shock response cause by the above-mentioned events

will not govern shelter design for the railroad dynamic environment.

2.2 Coupling Operations in Switching Yards - Railcar couplers have an automatic actuator

feature which makes it necessary for the cars to be impacted together to effect coupling.

Several factors (i.e. equipment and operators) cause a wide variation in the coupling

speeds. The literature contains several reports of studies conducted to determine the

range and frequency of occurrence of coupling speeds in switching yards. Sufficient

field data have been collected to derive a statistically-based prediction of coupling

speeds. Curve A in figure 2.7 shows a cumulative probability distribution curve that was

plotted from data presented in table 2 of Ref. 5 and table 2-1 of Ref. 6. The probability

distribution is cut off at 2 mph (0.89 m/s) because it requires a speed of this magnitude

to actuate the coupling systems. Thus, the approximate range of coupling speeds is from

2 mph (0.89 m/s) to 11 mph (4.92 m/s). It is noted that the curve is dashed for speeds

below 5 mph (2.24 m/s) because there were no specific data presented for the lower speeds.

Inspection of the histogram from which curve A in figure 2.7 was derived, shows that the

coupling speed distribution has positive skewness. The fact that the 50 percentile value

lies on the left side of the speed range also indicates positive skewness. The probability

distribution represented by curve A differs somewhat from that presented by Ostrem et al

[3, 7], For comparative purposes, Ostrem's probability distribution is shown as curve B in figure 2.7. Examination of two probability distributions shows that while 5 mph (2.24

m/s) is the 70 percentile speed on curve A, it is the 50 percentile speed on Ostrem's

curve. Likewise 8 mph (3.58 m/s) is the 90 percentile value on Ostrem's curve, while

curve A indicates 97 percentile for 8 mph (3.58 m/s). The specific source of Ostrem's

probability distribution was not indicated and therefore the differences have not been

resolved. Because curve A is based on data sources which are referenced, it will be used

in subsequent discussion of coupling speeds.

The benefit in determining the probability distribution of coupling speeds is that a basis is established for determining the likelihood that different shock levels will

17
ACCELERATION,

g's

(o-peak)

FREQUENCY, Hz

Figure 2.6 - Railroad shook envelopes, road crossingss intersecting

track and switches , directional, (after Ostrem)

18
occur for railroad cargo, and in particular for shelters. A coupling speed distribution

such as presented in figure 2.7 makes it possible to predict the probability of not

exceeding a given coupling speed as a function of the number of switching yards traversed

by a flatcar. This exercise is important in that freight cars generally traverse more

than one switching yard. Table 2 presents the probability of not exceeding specific

6 / speeds for 1-6 switching yards.—

Table 3 presents the complimentary probabilities: namely, the probabilities of exceeding

specific coupling speeds. To obtain a probabilistic design approach for the dynamic

environment caused by coupling, it is necessary to have railcar response data for the

full range of coupling speeds.

The response of railcars to coupling impact is usually described in terms of shock spectra

for each of the three principal directions. The directional spectra are obtained by

using the time-histories from accelerometers located on the cargo deck as forcing functions

for the single-degree-of-freedom systems. Then, analyses, using either analog or digital

techniques, are performed for particular natural frequencies and damping ratios. Representative shock spectra data are presented herein for coupling speeds with a relatively

high probability of occurrence and for those with a relatively low probability of occurrence.

Curve A in figure 2.7 indicates that about 70% of all observed impacts have occurred at speeds no greater than 5 mph (2.24 m/s). Envelopes of shock spectra for a coupling speed of

— The coupling speed is regarded as a random variable and the headings of the five righthand columns of table 2 indicate the event of the random variable being <_ the specified

speed (i.e. 5 mph, 6 mph, etc.). The probabilities of the five events are shown in the

first row. The left-hand column indicates the number (n) of switching yards traversed

by a particular railcar. An underlying assumption is that exactly one coupling impact

occurs in each yard. The probability entries for n = 2, ., ., were obtained by treating

the switching events as a series of independent repeated Bernoulli trials. A Bernoulli

trial is one which has only two possible outcomes, "success" or "failure." Here,

obtaining a coupling speed a specified value (e.g. 5 mph) constitutes success. Conversely, obtaining a speed greater than the statue value is called failure. The

Binomial Law can be used to express the probability, denoted by b(k; n, p) , that n independent repeated Bernoulli trials, with probabilities p of success and q(= 1 - p)

of failure, will result in exactly k successes and n - k failures (in which k = 0, 1,

., . , n) . The formulation of the Binomial Law is as follows:

The entries in table 2 were computed for the outcome, k = n, where n= 1, 2, 3, 4, 5, 6. The physical significant of k = n is that if n couplings occur all n of them will be

_< 5 mph (2.24 m/s) etc. Equation 1 reduces to:

The values of p are shown in the first row of table 2. The values for the remaining rows were obtained by using the corresponding value of n in equation 2. The values in table 3 are complements of those in table 2; they represent the probabilities of there being one or more coupling impacts—within the n trails—at greater than the specified speed.

k n-k

P q ( 1 )

b (k; n, p) = p n

( 2 )

19
BASED

ON

DATA

FROM

CO

m

o

QOin

PO

oo

CM

. OO

CD

OO

o

t/i E

a_coCD

OO

lUMMd iiiiiavscnu 3/\uvwwno

20

Figure

2.7

-

Cumulative

probability

distribution

of

railroad

coupling
TABLE 2 — Probability of Not Exceeding a Given Coupling Speed

Number

of Yards

<_ 5 mph

2.24 m/s

<_ 6 mph

2.68 m/s

<_ 7 mph

3.13 m/s

<_ 8 mph

3 . 58 m/s

< 9 mph

4.02 m/s

1 0.700 0.874 0.934 0.965 0.988

2 0.490 0.764 0.872 0.931 0.976

3 0.343 0.668 0.814 0.898 0.964

4 0.240 0.584 0.761 0.867 0.952

5 0.168 0.510 0.711 0.836 0.941

6 0.118 0.445 0 . 664 0.806 0.930

Example: If a railcar has impacts in J3 switching yards, the probability of all _3 coupling

speeds being <_ 6 mph is 0.668.

TABLE 3 — Probability of Exceeding A Given Coupling Speed

Number

of Yards

> 5 mph

2.24 m/s

> 6 mph

2.68 m/s

> 7 mph

3.13 m/s

> 8 mph

3.58 m/s

> 9 mph

4.02 m/s

1 0.300 0.126 0.066 0.035 0.012

2 0.510 0.236 0.128 0.069 0.024

3 0.657 0.332 0.186 0.102 0.036

4 0.760 0.416 0.239 0.133 0.048

5 0.832 0.490 0.289 0.164 0.059

6 0.882 0.555 0.336 0.194 0.070

Example: If a railcar has impacts in 3^ switching yards, the probability of 1 or more

coupling speeds exceeding 6 mph is 0.332.

21
approximately 3 mph (1.34 m/s) are shown in figure 2.8 [4]. The envelopes were obtained

in the same field study discussed in section 2.1.1. Acceleration time-histories were

obtained by Gens [3] during the low-speed coupling phase with the test car coupled to the

switching engine. The test car then struck a stationary, loaded gondola car at an estimated

speed of 3 mph (1.34 m/s). Figure 2.8 shows that the most severe shocks occurred in the

longitudinal direction up to 20 Hz. The maximum acceleration in the 1-20 Hz range was

about 3.5 g. The vertical acceleration magnitude was a close second over most of this

frequency range. Above 20 Hz, shocks in the vertical direction were the most severe. Lateral and longitudinal shocks were of approximately equal magnitude over the 20-150 Hz

range. The maximum acceleration—in the vertical direction—was approximately 30 g.

Probability distribution curve A in figure 2.7 indicates that approximately 98% of all

coupling impacts occur at speeds less than or equal to 11 mph (4.92 m/s). With this

statistical basis, 11 mph (4.92 m/s), is commonly regarded as the "worst case" coupling

speed in regard to potential shipping damage. Several impact test programs have been

conducted in which the maximum coupling speed is either 10 mph (4.47 m/s) or 11 mph (4.92

m/s). It is noted that in Procedure VI of MIL-STD-810C [8], a maximum coupling speed of

10 mph (4.47 m/s) is specified in the rail impact test. Thus, it is concluded that shock

spectra data for coupling speeds of 10 or 11 mph (4.47 or 4.92 m/s) would indicate the

upper limit in damage potential for shelters shipped by rail. Directional shock spectra

envelopes are presented in figure 2.9 [4] for 11 mph (4.92 m/s) coupling speed. The

results were obtained with the use of the same standard draft gear flat car mentioned in

section 2.1.1. For the high-speed coupling tests, acceleration time-histories were

obtained for such events as (1) an engine striking the standing test car and (2) an engine striking the test car in motion. The resulting shock spectra were combined to form the composite envelopes shown in figure 2.9. The response curves are presented for

C/C^ = 0.03, which according to Ostrem [3] and Foley [9] is indicative of non-shockisolated cargo. In essence, the three curves are predictions of how cargo with various

natural frequencies may respond to acceleration time-histories at the coupling speed of

11 mph (4.92 m/s). For example, a non-shock-isolated shelter with a natural frequency

(in each direction) of 10 Hz will experience a shock of approximately 2 g in both the

longitudinal and vertical direction. A shelter with a natural frequency of 20 Hz will

experience about 4.5 g longitudinally and 3.0 g vertically.

Several reports [3], [4], have shown a shock spectra envelope for a coupling speed of 11

mph (4.92 m/s) and C/C^ = 0; figure 2.10 shows the envelope as presented in Ref. 4. Ostrem [4] refers to this curve as the "upper limit of shock spectra" with regard to the

railroad coupling environment. In comparing the shock spectra curves in figs. 2.9 and

2.10 it is observed that the ordinates in the latter figure are in general, an order of

magnitude greater than the corresponding maximum values in the former figure. For example,

the maximum acceleration at 10 Hz is about 2 g for = 0.03 while the envelope curve

22
ACCELERATION,

g's

(o-peak)

FREQUENCY, Hz

Figure 2.8 - Shook spectra envelopes, railroad coupling (2-5 mph),

directional, (after Ostrem)

23
ACCELERATION,

g's

(o-peak)

Figure 2.9 - Shook spectra envelopes, railroad coupling (11 rrrph)

,

directional (after Ostrem)

24
ACCELERATION,

g's

(O-PEAK

Figure 2.10 - Maximum shook spectra envelopet railroad oouplina

(11 mph). (after Ostrem)

25
for C/C

c = 0 indicates a response of about 20 g at the same frequency. The upper limit

spectra envelope in figure 2.10 is presented for illustrative purposes, but is not recommended

for selecting design accelerations.

Structural damping has a mitigating effect on the maximum acceleration amplitudes and its

neglect results in an overly conservative design requirement. The importance of damping

emphasizes the need for additional data with which to refine the estimates of damping

used in shelter design and analysis.

26
3. ROAD ENVIRONMENT

In road vehicles, shock and vibration excitations are functions primarily of vehicle

dynamics and road irregularities. Occasionally, wind gusts are also a factor in the highway dynamic environment. Internal excitations may be caused by the engine, transmission

and drive assembly, wheel unbalance and shimmy. Such excitations are usually of low

amplitude, as a result of good engineering design and maintenance. Normal over-the-road

events which cause the more critical excitations include (1) crossing railroad tracks,

(2) traversing roads made rough by potholes and dips, and (3) crossing cattle guards. A

study of the vibration response of a 2 1/2-ton (907-kg) flatbed truck, [10, 11], identified

some "abnormal conditions" affecting the cargo's dynamic environment. The abnormal conditions

include driving with two wheels on the shoulder, driving completely on the shoulder and

driving on a dirt road. The environment, caused by the normal and abnormal over-the-road

conditions, consists principally of a low level random vibration upon which is superimposed

a large number of repetitive shock inputs [12]. Frequency spectra and shock spectra data

for typical events encountered by a 2 1/2-ton (907-kg) flatbed truck will be presented in

the following paragraphs.

Two and One-Half Ton Flatbed Truck - Typically, shelters are transported over-theroad in such military vehicles as M-35, M-55 and M-36 trucks. These are all flatbed cargo

trucks with steel cargo bodies and dual rear axles. Ideally, a description of dynamic

environment for truck transport of shelters would be based on vibration measurements on

the cargo floor of these vehicles. The vibration studies would involve both normal and

abnormal road conditions. A search of the literature on over-the-road truck studies

revealed very little applicable data for military cargo vehicles. However, it was found

that extensive vibration measurements had been taken on the deck of a commercial 2 1/2-ton

(907-kg) flatbed truck, under both normal and abnormal conditions. The truck was loaded

with cargo which consisted of a "distributed-mass dummy" mounted on an isolated pallet.

The cargo was rigidly tied down to the deck and accelerometers were located fore and aft

of the pallet/deck interface. The rear accelerometers were located over the rear wheels.

The events comprising the category of normal conditions were:

1. Backing up to a loading dock across railroad tracks,

2. Driving to a weighing station across a series of railroad tracks,

3. Going into a dip at low speed,

4. Driving from a low, level road to a raised road,

5. Driving onto a bad overpass,

6. Driving onto a heavily patched asphalt highway at 50 mph (22.4 m/s),

7. Driving on an access road parallel to an interstate highway,

8. Driving on a smooth blacktop, four-lane highway at 50, 55 and 35 mph (22.4, 24.6

and 15.6 m/s)

,

27
9. Driving in a paved construction zone at 45 and 55 mph (20.1 and 24.6 m/s), and

10. Driving on rough blacktop at 60 mph (26.8 m/s).

The events in the abnormal condition category included:

1. Driving with two wheels on the shoulder of the road at 45 mph (20.1 m/s),

2. Driving completely on the shoulder of the road at 45 mph (20.1 m/s),

3. Colliding with a loading dock,

4. Driving off the road in desert brush,

5. Driving onto the median of an interstate highway,

6. Driving on a dirt road at 45 mph (20.1 m/s),

7. Driving into a dip at 50 mph (22.4 m/s),

8. Hitting holes in a truck stop at 45 to 50 mph (20.1 to 22.4 m/s),

9. Running over a cattle guard at 45 mph (20.1 m/s), and

10. Driving over railroad tracks at 45 and 50 mph (20.1 and 22.4 m/s).

The measurements obtained from sampling the cargo deck response to these events were reduced

in one of two ways depending upon the characteristics of the vibrations.

In the first method, long-duration random vibrations were analyzed with narrow-band spectrum

analyzers . The acceleration peaks were counted within each frequency bandpass for both

the fore and aft accelerometers. The data were combined into a composite probability

distribution description for each event. Then, the results from all normal events that

caused long-duration continuous vibrations were combined to form a composite description of

the dynamic environment for normal road conditions.

Nine of the ten normal events produced long-duration vibrations. Data were reduced for

the vertical direction only because it was the observed direction in which maximum acceleration levels occurred. Six of the ten abnormal events also produced vibrations which were

analyzed in the manner described above. Composite response curves for long-duration

vibrations caused by the nine normal events and six abnormal events are presented in

figures 3.1 and 3.2 respectively [4], In addition to the Peak accelerations (i.e. the

absolute maximum accel.) curves are shown for 99.5, 99.0, 98.0, 95.0 and 90.0 percentile

accelerations. Comparison of the two sets of curves will show that the respective ordinates

are in relatively close agreement. No explanation is offered in the source documents for

the closeness of results for the two categories. It is concluded from figures 3.1 and 3.2

that the continuous excitation experienced during truck transportation has a Peak amplitude

of about 2 g in the vertical direction. It is noteworthy that the 99 percentile amplitudes

are less than 0.5 g for all frequency bands.

28
a

*«Osa

•O

s;o

•tS +s 05 o

O 05 £ is 05 -r5 3 'O

O'

05 t-~5 ^ o

•<-5 05 +S 3

N 05 05

is

05 O -NE O •> 0) * r* S, >- c 0 +s u o

•o 3 05

*« o M +5o

+s

s- Ui fn't! <i) 3 05

t~-i 05 4S

-Ci V-, a 05O

+* O o v—

Ui

ee O

O £ • 05 u. 05 c s; +s o o

+s

05 1 +S Ofiiv

tv>?

<5 « §

I

»"S NJ

05

Os £

(iftad-o) s,6 N0I1VH 3133DV
>-o

(H«ad-o) s,6 'NOI1VU31333V

30

Figure

3.2

-

Composite

deceleration

versus

frequency

diagram,

2

1

/2-ton

flatbed

truck,

vertical

direction,

abnormal

conditions.

(after

Ostrem)
The second method of data redaction was applied to transient vibration measurements. Shock,

spectra were derived to describe the transient response caused by both normal and abnormal

events. Only one normal event, driving across a series of railroad tracks at low speed

(event No. 2), caused shock excitations. On the other hand, four abnormal events resulted

in shock excitations. Figures 3.3, 3.4, 3.6 and 3.7 [4] present the shock spectra for the

five events. It should be noted that the two curves in figure 3.4 show shock spectra

resulting from one normal event (i.e., crossing railroad tracks at a slow speed) and one abnormal event (i.e., crossing railroad tracks at 45 mph (20.1 m/s)). The acceleration

time-histories from which the two spectra curves in figure 3.4 were derived are shown in

figures 3.5.1 and 3.5.2 The time histories were used as the applied forcing functions.

Then the equivalent static acceleration (i.e. maximum dynamic response) was computed for

single-degree-of-freedom systems with particular natural frequencies and damping ratios.

Although spectra were obtained for damping ratios of 0, 0.03 and 0.10, only those obtained

for C/C

c = 0.03 are presented. As stated previously, the spectrum for 0.03 damping ratio

is regarded as an estimate of the response severity produced on non-isolated cargo systems.

Except for figure 3.7, the response curves present maximum accelerations for the vertical

direction only. This approach is due to the test observation that the severest cargo deck

excitations occurred in the vertical direction. As would be expected, colliding with a loading dock (figure 3.7) produces a higher longitudinal than vertical shock excitation

over most of the frequency range. Examination of figures 3.3 through 3.6 shows that, in

general, driving over potholes (figure 3.3) causes the severest shock excitation experienced

in the road environment. It is noted that relatively high amplitudes exists over most of

the frequency range. For frequencies less than 100 Hz, the severest acceleration, 8 g, occurs at about 3 Hz. According to Foley, et al [1], truck suspension systems provide the

relatively high intermittent transient inputs at 3 to 6 Hz.

With regard to the design of shelters for truck transport the data in figures 3.3 - 3.7 are suggested as being representative of the shock environment. Thus, vertical accelerations

can generally be selected from fig. 3.3 while horizontal accelerations are selected from

figure 3.7. The shock spectra can be used in an equivalent static force design in the

following manner. An acceleration value is selected that corresponds to the estimated

natural frequency of the shelter (or shelter component). This acceleration is used to compute an equivalent static force, which is to be applied to the shelter to account for

dynamic effects. The shelter is then analyzed for maximum stresses or deflections. The

maximum stresses and deflections are then compared to allowable values.

31
ACCELERATION,

|'s

(O-PEAK)

1.0 10 100 1000

FREQUENCY, Hz

Figure 3.3 - Maximum shock spectra3 2 1/2-ton flatbed trucks vertical

directions potholes at truck stop, (after Ostrem)
ACCELERATION,

g's

(o-peak)

FREQUENCY, Hz

Figure 3.4 - Maximum shook spectra, 2 1/2-ton flatbed, truck, vertical

direction, railroad crossings . (after Ostrem)

33
Figure 3.5.1 - Driving' Across Railroad Tracks at Stockyards

Vertical3 Aft on Truck Bed

0 510 1000 1500 2000

TIME, MILLISECONDS

Figure 3.5.2 - Driving Across Railroad Tracks at - 45 mph

Vertical3 Aft on Truck Bed.

34
ACCELERATION,

g's

(o-peak)

FREQUENCY, Hz

Figure 3.6 - Maximum shock spectra, 2 1/2-ton flatbed truck, vertical

direction, driving across cattle guards. (after Ostrem)
ACCELERATION,

g's

(o-peak)

FREQUENCY, Hz

Figure 3.7 - Maximum shock spectra3 2 1/2-ton flatbed trucks longitudinal

direction3 backing into loading docks. (after Ostrem)

36
4. AIR ENVIRONMENT

The vehicles used to transport shelters through the air are airplanes - either propeller or turbojet - and helicopters. Propeller-driven airplanes are separated into two categories:

reciprocating engine and turboprop aircraft. The dynamic environment caused by the fixedwing and rotary-wing classes of aircraft are different although they have in common several

normal service conditions. Vibrations in aircraft result from such factors as runway roughness, propulsion dynamics, propeller and rotor unbalance, aerodynamic forces and acoustical

fluctuations. Air turbulence around the aircraft will also induce vibrations. The air has

vertical components of velocity which impart vertical accelerations to the aircraft [4].

The dynamic effects of these factors on the cargo deck have been measured for all the classes

of aircraft mentioned above.

Most of the available vibration data related to air transport of cargo have been obtained

from measurements made aboard military cargo aircraft. For example, acceleration timehistories have been obtained from the cargo floor of the C-130 which is a cargo plane com- monly used for transporting shelters. While there are sufficient data with which to predict

the in-flight environment of airplanes and helicopters, there is a scarcity of information

pertaining to the dynamic environment during landing. This shortcoming is probably

inconsequential to transport by modern turbojet airplanes since recent studies of the shocks

_3 experienced during landing operations [12] indicate a relatively low probability (10 ) of

exceeding 1 g. There is no evidence given in the literature to suggest that helicopter

landing produces severe environments when the cargo is transported inside the aircraft.

The environment caused by the "landing" of cargo which is transported while suspended

beneath a helicopter is one of concern to shelter designers. However, applicable test

data are not available at this time.

4.1 Airplane Transport of Cargo - Shock and vibration measurements have been made on

the cargo floor of both turbojet and propeller airplanes [1, 4, 7, 14]. Acceleration timehistories were obtained on these aircraft during normal service operations such as: taxiing,

ground run-up, take-off, climbing, cruising, turning and descending. The accelerometers were oriented to obtain measurements in the vertical, lateral and longitudinal directions.

The dynamic environment for fixed-wing aircraft is generally characterized by broad-band

random excitations upon which are superimposed continuous single-frequency excitation [1].

The most severe accelerations are produced during the take-off and climb events and while

cruising at high altitudes. The take-off and climb events cause relatively high amplitude

inputs of short duration. On the other hand, the cruise environment is characterized by

moderate amplitudes of considerably longer duration.

4.1.1 Propeller-Driven Airplanes - Figures 4.1 through 4.4 present directional

acceleration data obtained for the C-123, 377 PG (Pregnant Guppy), C-130 and

37
o

(SI

38

Figure

4.1

-

Aircraft

acceleration

envelopes

s

C-123

propeller

plane

directional.

(after

Ostrem)
39

directional.

(after

Ostrem)
(sum) t,8 N0I1VU31333V

directional.

(

after

Ostrem)
(sujj) s,B N0IJ.VH31333V

41

Figure

4.4

-

Aircraft

acceleration

envelopes

3

C-133

turboprop

directional.

(after

Ostrem)
C-133 airplanes respectively [4, 7]. The C-123 and 377 PG are reciprocating engine,

military cargo planes. Both the C-130 and C-133 are turboprop planes with four engines

and three-b laded propellers. The original data - obtained by the Wright Air Development

Division - were presented in the form of power spectral density curves. Subsequently,

Ostrem [4, 7] computed the root-mean-square accelerations (g rms

) for the frequency

bandwidths comprising the spectra. Thus, the data were presented in the form of g versus frequency. As discussed in section 4.3, this format is not compatible with the

equivalent static force design approach. It is important to note that the four diagrams

are composites of maximum data points obtained for different accelerometer locations

and all the normal operating conditions mentioned in section 4.1. Thus, the curves are envelopes encompassing the maximum acceleration level for all flight conditions rather

than continuous response spectra.

4.1.2 Turbojet Airplanes - Turbojet airplanes are commonly used for transporting

military cargo. A search of the literature revealed that data useful in predicting the

dynamic environment of this class of fixed-wing airplane is scarce. Furthermore, the

methods of data presentation varied according to available instrumentation and intended use of the information. Dynamic load information obtained from an early version and a more recent version of one type of military turbojet is presented herein. The data are presented in the respective formats used in the source reports. They will serve as a case-in-point of the problem of having to use diverse data formats in predicting dynamic

loads generated by airplanes. This problem is addressed in section 4.3, where a common

basis is established for presenting the acceleration versus frequency data.

In 1958, acceleration measurements were made aboard a KC-135A (military version of the

Boeing 707 commercial jet) for the conditions of ground run-up, taxiing, take-off and

cruising. The accelerometer time-histories were passed through a series of narrow- bandwidth filters for the purpose of data reduction. The original Boeing report [15]

presented the data in the form of power spectral density versus frequency diagrams.

Subsequently, Ostrem [7] integrated the spectral densities within each filter bandwidth

and computed the mean-square accelerations. The results were then presented in the

form of g rms versus frequency. The summary response diagram for the KC-135A is presented in figure 4.5 [7], The three directional-dependent curves are envelopes of the

highest values of g for all observed flight conditions. Thus, the diagram indicates

maximum g data points connected by straight lines rather than continuous functions

of frequency. The frequency range extends from approximately 10 to 2000 Hz (the accelerations corresponding to frequencies above about 500 Hz are probably superfluous for

shelter design considerations). Vertical accelerations were generally predominant.

This result is to be expected in that Foley et al [1] indicate that the most severe amplitudes are caused by take-off and climbing conditions. Examination of figure 4.5

shows that the maximum value of g rms was approximately 2 g in the vertical and lateral

42
(sun) s,6 'N0I1VH31333V

43

Figure

4.5

-

Aircraft

acceleration

envelopes

3

KC-135A

jet,

directional

(after

Ostrem)
directions. In the longitudinal (i.e. fore and aft) direction, the maximum value of

gb was about 1 g. rms

In a later test program, accelerometer time-histories were obtained aboard a NC-135 (a

military version of the commercial 707 jet) for the conditions of: take-off, full power

climb, cruising, descent and braking (after landing). Acceleration records were not

obtained for the touchdown part of the landing operation. The random vibration timehistories were processed by bandpass analysis (i.e. filtering with narrow bandpasses).

In the original report of this study, Harley [16] presented the data in the form of

cumulative probability distributions within each frequency band. This form of data

presentation was described in section 1.3. Part of the data is presented in figure 4.6

[4] in terms of the maximum values of peak accelerations and the probability of peaks

being less than indicated levels. Of the five test conditions mentioned above, takeoff produced the most severe accelerations in the frequency range of about 2-100 Hz.

Above 100 Hz, the accelerations produced by braking after landing were the most severe. Figure 4.7 [4] shows the maximum values of peak acceleration for the higher frequency

range. It is noted that data are presented for the vertical direction only since

preliminary review of the acceleration measurements indicated the predominance of the

acceleration in this direction.

4.2 Helicopter Transport of Cargo - Prediction of the dynamic environment for helicopters

is important to the design of military field shelters in that it is a convenient mode of

short- and medium-haul transportation. Movement of cargo via helicopter involves one of two dynamic environments depending upon whether the shelters are supported inside the aircraft

or are suspended from beneath the aircraft by cables.

4.2.1 Cargo Transported Inside Helicopters - The dynamic environment for cargo transported inside helicopters is primarily influenced by the same factors that affect cargo

in fixed-wing aircraft (e.g. propulsion dynamics, rotor unbalance and air turbulance)

.

However, Foley et al [1] indicate that cargo floor responses of helicopters differ from

those of airplanes. For example, there is very little difference between the vibrations

caused by take-off and those due to cruising in helicopter flight. In contrast, there

is a significant difference in accelerations caused by these two events in airplanes.

Secondly, the dynamic environment inside cargo helicopters is characterized by continuous

single-frequency vibrations, of relatively high amplitude, in the lower frequency range

(i.e. up to about 40 Hz). No such characteristic has been noted for the time-histories

obtained aboard airplanes.

Cargo floor vibrations have been measured in several military helicopters for a variety

of operating conditions. About half of the available data have been obtained inside

44
(iiead-o) s,fi N0I1VU31333V

>-o

45

Figure

4.6

-

Aircraft

acceleration

versus

frequency

diagram

,

NC-135

turbojet,

vertical

direction,

takeoff.

(after

Ostrem)

.
=

01

(HMd-o) $,B 'N0I1VU31333V

Figure

4.7

-

Aircraft

acceleration

versus

frequency

diagram

3

NC-135

turbo

jetj

vertical

direction

3

braking

after

landing,

(after

Ostrem)
(sum) s.fi ‘N0I1VU31330V

47

Figure

4.8

-

Aircraft

acceleration

envelopes

H-37

helicopter

3

vertical

longitudinal

and

lateral

directions,

(after

Ostrem)
helicopters which are carriers of cargo, including shelters. However, describing the

dynamic environment for cargo helicopters is complicated by the presentation of the

data in several different formats. Representative dynamic response data are presented below in the format used in the source document. Additional work in this area is necessary to normalize the existing data. Given a common format, the dynamic

environment on the cargo floor of helicopters can be more succinctly described. It is

important to note that the literature search did not reveal any shock and vibration

measurements incident to the landing of rotary-wing aircraft. No explanation was given

for the omission of this event from the test programs; the effect of not having test

data for landing on existing predictive models is not presently known.

Representative cargo floor vibrations were measured aboard a H-37 helicopter and reported

in Reference 7. The H-37 is a cargo helicopter with a single main rotor plus a torquecompensating tail rotor. Vibrations were measured at several locations for the following

ground and flight conditions: (1) motor starts, (2) rotor engagement, (3) take-off,

(4) hovering, (5) climbing, (6) cruising and (7) descending. A composite diagram of

the test results is presented in figure 4.8 [7]. The three curves show the envelopes

of g rms

for the vertical, lateral and longitudinal directions. Data points representing

the maximum values of g in narrow frequency bands have been connected by straight

rms lines to obtain these composite diagrams. Examination of figure 4.8 shows that the

most severe accelerations generally occurred in the vertical direction. The highest

value of g was about 7.9 and occurred at approximately 350 Hz. Between the center

rms frequencies of 5 and 80 Hz the values of grms do not exceed 1 g irrespective of

direction.

Vibration measurements were also obtained from the cargo floors of two cargo helicopters

for ten normal ground and flight conditions [17]. The CH46 and CH47 aircrafts were

used for these field tests. Both of these carrier types feature contrarotating rotors

to compensate for torque applied to the fuselage. The dimensions, weights and performance

ratings of these two helicopters are presented in Reference 18. The primary difference

between the two aircraft, for test purposes, is that the CH47 has an isolated floor in

the cargo compartment while the CH46 does not. In the latter aircraft, the airframe

structure directly supports the cargo floor. According to Reference 17, the acceleration

time-histories consisted of Gaussian random excitations; therefore, they were subjected

to narrow bandpass analyses. Mixed with the random components were decaying sinsusoidal

excitations of single frequencies. At this writing the complete data from this test program are not available. Shown in figure 4.9 is the diagram for the 99 percentile

peak amplitudes, as a function of frequency. It is noted that for the dual rotor types

of helicopter the highest levels (about 3 g) were obtained in the longitudinal and

transverse directions between 350 and 500 Hz. This is the only frequency band in which

the accelerations in the vertical direction were not dominant. The highest level in

the vertical direction was about 2.5 g. Although no supporting diagrams were presented

in Reference 17, it was stated that the absolute peaks were in the 350 - 500 Hz band

(i.e. 425 Hz center frequency) at 5.2 g for all axes.

48
49

Figure

4.9

-

Aircraft

acceleration

diagram

,

99%

level

CH46

and

CH47

helicopters,

directional,

(after

M.

Gens)
Another acceleration measurement program involved the use of the HH43B helicopter [4,

18]. Familiarly known as the "Huskie," this aircraft is principally a personnel

carrier. However, it is constructed similarly to the cargo type carriers with con- trarotating rotors. Two sets of accelerometers were placed in the passenger-cargo area

and three-directional vibration measurements were obtained for 18 ground and flight

conditions. Landing was included as one of the test conditions, but equipment malfunction

prevented data collection during the landing event. Figure 4.10 presents a summary

diagram for all the data acquired inside the HH43B. In constructing the constant

probability curves, weighting factors were assigned to the 17 events to account for the

relative frequency of their occurrence. Examination of figure 4.10 shows that the

highest Peak occurred in the 350 - 500 Hz band (i.e. 425 Hz center frequency). The

absolute maximum magnitude was approximately 2 g. Between 0 and 180 Hz, the maximum

zero-to-peak acceleration was 1 g.

4.2.2 Cargo Transported Outside Helicopters - In addition to the factors which influence

the dynamic environment of cargo transported inside a helicopter, cargo suspended

beneath a helicopter's fuselage is subjected to aerodynamic pressures. In the case of

field shelters, such pressures are applied to all exposed surfaces. Qualitatively, the

aerodynamic pressures would seem to cause a more severe load application than would be

experienced by a shelter supported inside the aircraft. In addition, an externally

supported shelter will receive shock excitations when it is "landed" in the field.

Shelter design criteria for the air transport mode must account for these sources of

design loading. However, no quantitative information has been found to provide a basis

for predicting dynamic loads on externally-transported helicopter cargo. The state-ofthe-art consists of directional load factors which are presented in American National

Standard MH5 . 1 [19] for the design of containers. The load factors are applicable to containers supported either inside or outside rotary-wing aircraft. In addition, MH5.1

requires that for externally-supported containers, their end structures and top structure

2 be designed for a uniformly-applied aerodynamic pressure of 80 psf (391 kg/m ) and 35

2 psf (171 kg/m ) respectively.

4.3 Summary of Air Transport Environment - The air transport dynamic environment is

produced by three classes of aircraft: propeller-driven airplanes, jet airplanes and

helicopters. Test programs involving the measurement of vibrations aboard military cargo

and personnel aircraft in each of these categories have been reported in the literature.

Data obtained aboard 4 propeller-driven planes (C-123, C-130, C-133 and 377-PG), 2 turbojets

(KC-135A and NC-135) and 4 helicopters (H-37, CH46, CH45 and HH43B) were presented in

sections 4.1 and 4.2. Examination of figures 4.1 through 4.9 shows that the air transport

data have been presented in several different formats. The use of various forms of

acceleration versus frequency diagrams makes it difficult to establish a single predictive

model for the entire environment. However, it was noted in the literature that broad-band

50
>-o

()|B8d-o) s,fi 'N0I1VU31333V

51

Figure

4.10

-

Composite

acceleration

versus

frequency

diagram

,

HH43B

helicopter,

all

directions

,

all

flight

conditions

except

landing.

(after

Ostrem)
random excitation with peaks distributed approximately in a Gaussian manner, was a component

common to the three classes of aircraft. Based on this observation, a common format was established for presenting the test data. The 99 percent level of zero-to-peak accelerations

was selected for summarizing the various data.

The data presented in figure 4.6 for the NC-135 turbojet were used to establish a scaling

or multiplying factor for the root-mean-square data in figures 4.1 through 4.5 and

figure 4.8. Properties of the Standard Normal curve were used to calculate g rms

(i.e. the

sample standard deviation for a zero mean transformation process) values for the frequencies shown in figure 4.6. At each selected frequency, a value of the ratio g^ _„/g was ° 0.99 rms computed. Then an average value was computed from the set of ratios. The average value

of the ratio, which was 3.5, was applied as the multiplying factor to the g values given

in figures 4.1 - 4.5 and figure 4.8. It is acknowledged that the scaling factor derived

in this manner is dependent on the period over which the observations are made. Nevertheless,

it is felt that the use of 3.5 as a multiplier provides a reasonable estimate of the 99

percentile accelerations. The transformation formula is as follows:

®0. 99 (o-peak)

= 3.5 x (3)

The directional summaries are presented in figures 4.11, 4.12 and 4.13. Thus, the ordinate

of each diagram is 99% level ac< of the various frequency bands.of each diagram is 99% level acceleration (g

Q peaj<.) and the abscissa is the center frequency

Figure 4.11 presents the summary diagram for the C-123, C-130, C-133 and 377-PG aircrafts.

The directional curves are composites obtained by enveloping the maximum 99% value of

go p ea k at the indicated frequencies. Typical of the aircraft environment, the vertical

direction predominates over most of the frequency range.

Figure 4.12 presents the summary diagram for the KC-135A and NC-135 turbojets. Composite

curves have been plotted for both the vertical and lateral directions. It is recommended

that the values given for the lateral direction be used for the longitudinal direction, due

to the close agreement between the two directions. Comparison of figures 4.11 and 4.12

reveals that the acceleration magnitudes obtained from propeller-driven airplanes are higher at all frequencies than those produced by turbojets.

A summary of helicopter data obtained from cargo aircraft is presented in figure 4.13. It should be noted that the data from the HH43B were not included in the directional composites

because this aircraft is not a likely carrier of shelters. Examination of figure 4.13

shows that there is no clear predominance of any one direction. By comparing figure 4.13

with figures 4.11 and 4.12 it is seen that the acceleration amplitudes are highest for the

helicopter cargo area, over the entire comparable frequency range.

52
ACCELERATION,

g

0

9g

(o-peak)

100

10

1.0

0.5

0.1

1 5 10 50 100

FREQUENCY, Hz

Figure 4.11 - Composite aircraft acceleration versus frequency diagram

99% level , C-12S, C-130, C-133 and 377 PG propeller

planes, directional.

500 1000
ACCELERATION,

g

0

. 99

(o-peak)

10

5

0.10

.05

.01

1 5 10 50 10 © 500

FREQUENCY, Hz

Figure 4.12 - Composite aircraft acceleration versus frequency

diagram3 99% level , NC-135 and KC-13SA turbojets } vertical and lateral directions .

1000

54
ACCELERATION,

g

0

.99(o-peak)

100

Figure 4.1Z - Composite aircraft acceleration versus frequency

diagram, 99% level, H-37, CH46, CH47 helicopters,

directional.

55
The summary data presented above can be used independently or collectively in the prediction

of air transport dynamic forces. An equivalent static force can be established for either

of the three classes of carrier, when the design/analysis operation is narrow in scope.

The critical design frequencies would be based on the known or estimated natural frequency

of the shelter or shelter component. When the design/analysis considers the entire air

transport mode, the absolute maximum 99% level can be obtained by comparing the respective

ordinates at the designated frequencies. It should be noted that the predictive models

presently do not include excitations resulting from landing any of the three classes of

aircraft. The composite diagrams could change as a result of incorporating data obtained

from the landing event. Also, no information is included for the case of cargo suspended

beneath the fuselage of a helicopter. ANSI MH5.1[19] should be referred to when this mode

of transportation is a likelihood.

It is possible to predict acceleration amplitudes for other designated percentiles by

using the following formula:

g

P 8

0 . 99

(Z 0.99Z ) 80. 99

P (4)

where

g^

= The acceleration amplitude at percent level P,

a0.99 = The 99% acceleration amplitude obtained from figures 4.11-4.13,

Z 0.99 = The standard normal variable for P = 0.99

and Zp = The standard normal variable for percent level P.

For example, the determination of the 90% level vertical acceleration for a frequency of

30 Hz, for helicopter transport is as follows:

gQ>90 = 1. 75 - (2.326 - 1.282)

80.90

= X - 75 “ °’ 52 = 1 ‘ 23

(5)

The value of 1.75 for g^ was obtained from figure 4.13, while the values of Z^

99

and

Z

Q 9 p were obtained from table A-2 of Reference 21.

56
5. HANDLING ENVIRONMENT

A mobile shelter will generally experience handling loads at different stages of its

useful life. For example, there is a sequence of handling events which may occur between

the point of manufacture and a place of temporary storage. The dynamic environment incident

to handling shelters results from two different classes of excitations. First, normal

handling excitations are those produced by such terminal activities as stacking, crane

hoisting, forklift transfer, jacking, towing and pushing. These excitations are deterministic

in that they are primarily dependent upon the dynamic characteristics of the handling device.

However, the normal handling environment cannot be adequately predicted because the available

data pertaining to normal handling operations are scarce. As will be discussed below, the

only documented handling vibrations found were obtained in a test involving forklifts. The

second class of excitations is produced by rough handling activities such as accidental

dropping, collisions and expedient handling practices. Because these handling excitations

are largely dependent on human elements, the dynamic environment is non-deterministic . Predictive models in this handling mode are non-existent. However, judiciously observed handling

instructions and procedures can significantly limit the severity of this source of loading.

5.1 Normal Handling Excitations - Three operations which comprise a large portion of the

normal handling activities for shelters are: (1) lifting at various terminal points,

(2) placement during stacking and (3) short transport by handling devices such as forklifts.

5.1.1 Lifting Inputs - Depending upon the type and location of the terminal point,

shelters may be lifted by a range of machines, including forklifts, cranes, mobilizers,

straddle carriers and side loaders. The dynamic input from these types of handling

devices are generally characterized by intermittent—^ transient inputs of relatively

low amplitudes, for normal conditions. Furthermore, although the dynamic forces

resulting from lifting are repeated a number of times during the life of a shelter,

their cumulative effect on a shelter's fatigue life is considered to be insignificant

by comparison with the effect of transportation and field service loads. The basis

for this conclusion is the fact that normal lifting events caused relatively low

dynamic load magnitude. Thus, the absence of shock and vibration data incident to lifting is not critical to establishing design criteria. Furthermore, the acquisition

of data from lifting experiments should receive relatively low priority in a transportation and handling test program.

5.1.2 Placement During Stacking - Shelters are often stacked two-high or three-high

for storage purposes. The bottommost shelter should be capable of withstanding the

inertial forces generated by the second and third tier shelters. No data are available

7/ Intermittent infers a single occurrence or irregularity in occurrence. For example:

the response caused by railcar coupling or a truck crossing railroad tracks.

57
to provide a basis for predicting the dynamic environment experienced by containers

and shelters during stacking. However, ANSI MH-5.1 [19] requires that a downward

"Design Load Factor" of 2.0 be applied in an equivalent static force analysis to account for the impact loads. The use of a load factor of 2.0 may be somewhat

conservative in that the normal practice for accounting for suddenly applied loads

requires a factor of 2.0 only for elevator support design.

5.1.3 Short Transport by Handling Devices - Short distance transportation of shelters

may be accomplished by a number of handlers including forklifts, cranes amd mobilizers.

Trucks are also used as short transport carriers. In addition, shelters may be towed

and pushed at prior to being stored, transported over longer distances or erected.

The dynamic environment caused by short-distance moving activities has not been

adequately described. The only handling device for which data were found was the

forklift. Gens [20] reported on the results of a study conducted to determine the

dynamic loads imparted to cargo during transfer by four different industrial forklift

trucks. The forklifts ranged in capacity from 2000 lb (907 kg) to 7000 lb (3175 kg).

Another test variable was the type of tires mounted on the forklifts. Both pneumatic

and solid rubber tires were used. Three-directional vibration measurements were made

at the base of the handling cradle. The dynamic environment was characterized as consisting of high-amplitude discrete (i.e. transient) excitations superimposed on a background of low-amplitude pulses. Thus, the three-direction data were reduced in

the form of shock spectra. Envelopes of the response spectra for each type of forklift

truck are shown in figure 5.1 [20], Note that the diagrams are presented for a damping ratio of 0.03. Examination of figure 5.1 sho^s that accelerations in the

vertical direction are, in general, the highest. Due to the relatively small amount

of vibration data obtained for forklift operations, no prediction of the dynamic

environment should presently be undertaken. Because the response amplitudes shown in

figure 5.1 are relatively high, future transportation and handling research programs

should assign high priority to measuring shelter responses during forklift transport.

5.2 Abnormal Handling Excitations - Dynamic inputs from abnormal handling events are inherently non-deterministic. These events are caused by human error, accident or expedient

procedures and, therefore, they have not been extensively documented. A statistical basis

for predicting abnormal handling loads does not presently exist. The state-of-the-art

consists of simulating accidental and rough handling events in standardized test methods.

For example, shelters are commonly subjected to a drop test in which one end of the unit

is dropped 51 in (1.3 m) onto a well compacted surface. This test simulates the accidental

drop from the cargo bed of a M-35 truck. After conducting the test the specified number

of times, acceptance is dependent upon compliance with damage criteria. Several other

test procedures are available for simulating other abnormal handling events.

58
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tu

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to

CD £

CO CD C ^3 O

CX • to S;

03

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(9) 3SNQdS3U (9) 3SNOdS3U

59
6. FIELD SERVICE LOADS

Both expandable and rigid shelters are required to resist loads caused by equipment,

furnishings, occupants, snow, ice, wind and thermal gradient during field service. Snow,

ice, wind, thermal and gravity loads imposed during storage must also be considered in

shelter design. In the case of expandable shelters, their conformation during storage may

be considerably different from their expanded outline while in service. The designation

of design loads to account for human occupancy, furnishings and equipment is relatively

uncomplicated because design load tables are readily available and the loads are invariant

with the locale of shelter deployment. On the other hand, snow, ice and wind loads are complicated by being geography dependent. Also, these environmental loads are dependent

upon the geometry of the structure, the nature of the terrain surrounding the structure

and the degree of exposure to wind. The above-mentioned sources of service loads will be

discussed in the following paragraphs.

6.1 Gravity Loads - The weights of materials, constructions and fixed equipment used in

shelters are generally known quantities. Therefore, the design dead loads will be defined

for both the storage and erected conditions. The floor live loads should be predicted on

the basis of the intended use or occupancy of the shelter. The maximum load which will

probably occur at least once during the shelter's useful life should be used in the design.

Furthermore, the minimum uniformly distributed floor live loads should conform to Table I of AFM 88-3, Chapter 1 [22] and Table 1 of ANSI A58. 1-1972 [23]. Where applicable, a concentrated load of 2000 lbf (8896 N) should be applied on any area, 2 1/2 ft (0.76 m)

square, to produce the maximum stress condition.

6.2 Snow and Ice Loads - Shelter roofs should be designed to support the snow and ice

loads listed in Table II and shown on the isoline map (Figure 2) of AFM 88-3, Chapter 1 [22]. Reference should be made to paragraph 4.d of AFM 88-3 for specific design requirements.

It should be noted that the uniform load values presented in Reference 22 are based on statistical analyses of extremes of ground snowpack for a 25-Year Mean Recurrence Interval.

For structures built solely for use in the continental United States, Figure A7 of ANSI

A58. 1-1972 [23] should be consulted. The snow load values given in Reference 23 can be

used as a guide for establishing values for other parts of the world. To account for

loads which may be imposed during maintenance and repair, the roof design load should in

2 no case be less than 20 psf (958 N/m )

.

6.3 Wind Loads - While transportation and handling loads are likely to govern the design

of field shelters, the structures should be analyzed for wind pressure resistance. Structural

elements such as the walls and roof must be designed to resist localized wind pressures.

Also, the shelter structure and its tie-down system must resist uplift, sliding and overturning. The state-of-the-art in wind load design of lowrise buildings is to compute

60
external and internal pressures which are the products of the effective velocity pressure

and pressure coefficients. The general formulation for wind pressure is as follows:

P q C

e p qX

Pi

where q = Effective velocity pressure for external pressure,

e Cp = External pressure coefficient,

q^

= Effective velocity pressure for internal pressure,

and C . = Internal pressure coefficient.

Pi

(6)

The effective velocity pressure is a function of the basic wind speed for a given site.

The appropriate basic wind speed needed for computing the effective velocity pressure can be obtained from tables and maps which have been constructed for various mean recurrence

intervals. Wind load provisions in AFM 88-3, Chap. 1 [22] should be used to obtain the

design wind loads. The basic wind speeds can be obtained from Table II and Figure 1 of

AFM 88-3, Chapter for locations in the United States and overseas.

6.4 Thermal Loads - The design of field shelters must consider the effects of temperature

changes in addition to the effects of the service loads mentioned above. Temperature

gradients through the thickness of structural elements may cause significant forces and

deformations when the elements are restrained from expansion and contraction. Although

the thermal forces are not classified as dead or live loads, they are additive to other

field service loads. To establish the design loads caused by temperature changes, it is

first necessary to quantify temperature differentials which may result from various

environmental conditions.

The design temperature differential is taken to be the difference between the temperatures

of the exterior and interior faces of the structural elements. The problem of defining

the temperature differential for field shelters is caused by a lack of data regarding

surface temperatures on the exterior faces of the shelters and a lack of information as to the profile of the temperature gradient for typical shelter constructions such as foam and

beam and honeycomb-core sandwich panels. During the useful life of most shelters the

inside ambient temperature will remain relatively constant, having only small deviations

from normal room temperature. Hence, the interior ambient temperature can be considered a reliable estimate of the interior surface temperature. On the other hand, the exterior

ambient conditions are almost continuously changing. Furthermore, exterior conditions are dependent on seasonal characteristics and on the variation in environmental conditions

with locale. The factor of air temperature variation notwithstanding, statistical data

on the maximum and minimum daily temperatures can be obtained from local weather bureaus

and the Environmental Data Service (of the National Oceanic and Atmospheric Administration)

in establishing exterior ambient conditions. The problem lies in the fact that the surface

temperature may differ substantially from the ambient temperature. The surface temperature

61
depends of wind and solar radiation effects and on the thermal properties of the sandwich

panels. No data were found regarding the heat conduction characteristics of sandwich

panels typically used in field shelters.

Considerable study is warranted in the area of heat conduction through wall and roof

elements of shelters to establish models of thermal gradients for various environmental

conditions. Then, upon defining the boundary restraint conditions for the structural

elements, the induced thermal loads can be specified. In this way, the shelter analysis

can consider thermal loads in combination with the other service loads.

6.5 Combination of Field Service Loads - The above-mentioned field service loads should

be combined in accordance with section 4, ("Combination of Loads") of ANSI A58.1 [23] to establish the most unfavorable design conditions.

62
7. SUMMARY AND CONCLUSIONS

It is recognized that a cycle of shelter use includes a number of possible transportation

and handling activities which may affect the shelter's design. The concept of a cycle of

shelter use has been illustrated in figure 1.3. Toward the objective of developing a comprehensive design criteria, a study of the dynamic environment produced by transportation

and handling events was conducted. A literature search was performed to obtain test data

resulting from measuring the dynamic response of cargo decks on trucks, trains, airplanes

and helicopters in their respective modes of transport. The dynamic environment produced

by typical military cargo carriers was of particular interest in the search because it is

expected that military carriers will generally be used to transport shelters. The underlying premise of this activity was that the various loadbed responses could in turn be

used as forcing function inputs in shelter design analysis. Also considered in the study were the forces incident to normal and abnormal handling activities and those produced

while the erected shelters are in service. In view of the fact that shelters may be

deployed in many parts of the world existing military and nonmilitary design load standards

have been referenced for guidance in the selection of applicable snow and wind load values.

It was found that representative shock and vibration data for some transportation events were well documented and directly applicable to the stated objective. However, for several

other transportation events and most handling operations the dynamic load data were either

non-existent or insufficient. Summaries of the areas of sufficient and insufficient data

are presented in the following paragraphs.

7.1 Areas of Sufficient Data - Sufficient shock and vibration data have been reported to describe the dynamic environment for railroad, road and fixed-wing aircraft transportation

modes. Measurements obtained from the cargo area of several military helicopters have

also been well documented. The data have usually been presented in the literature in the

form of acceleration versus frequency diagrams. For continuous vibration excitations the

results have been presented in several forms: cumulative probability distribution diagrams,

root-mean-square acceleration versus frequency curves and zero-to-peak accelerations

versus frequency curves. Cargo bed responses to shock excitations have been presented in

the form of shock spectra and composite diagrams derived from enveloping spectra for

different events.

Representative acceleration versus frequency diagrams have been presented in this report

for commercial flatbed trucks, cargo trains, propeller-driven and turbojet airplanes and

helicopters which transport the cargo inside the aircraft. Some data are also presented

for four different industrial forklifts. The various transportation modes and handling

operations are discussed independently to permit flexibility in the process of combining

various loads.

63
It is recommended that the acceleration magnitudes obtained from the diagrams be used in an equivalent static force analysis. The frequencies at which the design accelerations

are selected are dependent upon the fundamental frequencies of the various elements (i.e.

walls, floor and roof) of the shelter. The acceleration versus frequency curves have

generally been presented for the three orthogonal axes of the carriers. Insufficient

research has been performed to resolve the questions of how to combine the loads acting in

three directions. Therefore, the load combinations assumed in section 6.2 of ANSI MH5.1 [19]

provide the bases for the following recommendations. It is recommended that the equivalent

static vertical load (accounting for inertial and gravity loads) be uncoupled from the

horizontal loads and be considered as acting singly in all transportation modes. The

equivalent static lateral and longitudinal design loads should be applied simultaneously

with the dead load (i.e. gravity load) of the shelter.

7.2 Areas of Insufficient Data - It is recognized that shelters may be transported for

relatively short distances while being suspended beneath cargo helicopters. At this

writing, the frequency of occurrence of cable-supported shelter transport is unknown.

But, it is felt that a comprehensive design criteria should account for the shocks and

vibrations produced by this mode of cargo transport. However, sufficient vibration measurement data does not exist to quantify the dynamic loads applicable to shelters supported

outside helicopters. Since helicopter transport provides the most severe environment for

air transport, design load data should be developed for this case.

It was found that there is a scarcity of information regarding shocks and vibrations

produced in the cargo areas of airplanes and helicopters during landing. Some data obtained

_3

for turbojet airplanes indicate that there is a low probability (10 ) of exceeding 1 g acceleration during landing. When compared with acceleration amplitudes produced by such

events as take-off and climbing, landing accelerations appear not to govern the design

load for shelters transported via turbojets. However, there is insufficient evidence to conclude that landing is inconsequential to the dynamic environment of other classes of

cargo carriers. Therefore, future vibration measuring programs involving airplanes and

helicopters should include landing as a test event.

Although it was observed in the literature search that various transportation studies had

included acceleration measurements on various parts of military trucks (e.g. transmission

housing and rear axle) , insufficient data exist on cargo bed accelerations for military

cargo trucks. Trucks are likely carriers for short and medium transport of shelters. As

noted in Chapter 3, normal road conditions range from stretches of smooth blacktop to surfaces containing a number of dips and bumps. Abnormal conditions such as that created

when a truck drives entirely on a rough shoulder should also be considered. While well

documented data exist for commerical trucks traversing various normal and abnormal road

conditions, future programs should include typical military cargo trucks such as the M-35

and M-55.

64
As indicated in figure 1.3, handling is the most frequently recurring event in the use cycle of a shelter. This fact notwithstanding, handling operations are the least documented

events covered in this study. No dynamic load data were found for rough handling activities

such as accidental dropping and collisions. However, this insufficiency is not considered

critical to shelter design provided safe handling practices are prescribed and observed.

There is also a scarcity of applicable data pertaining to normal handling activities such

as lifting, placement during stacking and short transport by handling devices. Since

lifting operations generally produce low amplitude accelerations for relatively short time

periods, the absence of quantitative data is not critical in shelter design criteria. The

absence of dynamic load data for placement during stacking can be compenstated for by

using the design load factor of 2.0 that is recommended in ANSI MH-5.1[19]. With regard

to short transport by handling devices, further study is needed to establish design load

values. A study involving four types of industrial forklifts provides the only documented

description of dynamic environment resulting from this handling event.

Finally, as the fundamental frequencies of the shelter's structural elements will determine

the accelerations used in the equivalent static force analysis, it is essential to know

the frequency magnitudes for various types of shelter construction. This information is

needed before any of the data presented above can be used. At present such information is

not available. This deficiency should be addressed by experimentally determining representative fundamental frequencies for both honeycomb core and foam and beam core sandwich panel

construction.

65
8. LIST OF REFERENCES

1. Foley, J. T., Gens, M. B. and Magnuson, C. J., "Current Predictive Models of the

Dynamic Environment of Transportation," The Journal of Environmental Sciences , Jan. /Feb., 1973, pp. 18-28.

2. Gens, S. G., "Rail Vehicles," Chapter 45, Shock and Vibration Handbook , Vol. 3,

McGraw-Hill, pp 45-21 to 45-39.

3. Gens, M. B., "The Rail Transport Environment," The Journal of Environmental Sciences , July/August, 1970, pp 14-20.

4. Ostrem, F. E., "A Survey of Environmental Conditions Incident to the Transportation

of Materials," Report PB-204-442 , General American Transportation Corporation, Niles,

Illinois, October, 1971.

5. Simmons, L. C., and Shackson, R. H., "Shock and Vibration on Rail Movement of Freight,

Shock, Vibration and Associated Environments , Bulletin No. 31, Part III, U.S. Naval

Research Lab., Washington, D.C., April, 1973, pp. 94-101.

6. Anon., "The Railroad Environment - A Guide for Shippers and Railroad Personnel,"

Technical Research Department, New York Central Railroad, Co., 1966.

7. Ostrem, F. E. and Rumerman, M. L., "Shock and Vibration Transportation Environmental

Criteria," Report MR 1262-1 , General American Research Division, Niles, Illinois,

Sept. 1965.

8. MIL-STD-810C , Military Standard Environmental Test Methods," Wright Patterson Air

Force Base, Ohio, March, 1975.

9. Foley, J. T. and Gens, M. B., "Environment Experienced By Cargo During Normal Rail

and Truck Transport-Complete Data," Report SC-M-71-0241 , Sandia Laboratories,

Albuquerque, New Mexico, August, 1971, 318 pp

.

10. Foley, J. T., "Normal and Abnormal Environments Experienced by Cargo on a Flatbed

Truck," Report SC-DR-67-3003 , Sandia Laboratories, Abuquerque, New Mexico, Feb. 1968.

11. Foley, J. T., "Normal and Abnormal Dynamic Environments Encountered in Truck

Transportation," The Shock and Vibration Bulletin , Bulletin 39, Part 6, U.S. Naval

Research Lab., Washington, D. C., March, 1969, pp. 31-45.

66
12. Foley, J. T., "The Environment Experienced by Cargo on a Flatbed Tractor-Trailer

Combination," Report SC-RR-66-0677 , Sandia Laboratories, Albuquerque, New Mexico,

Dec. 1966, 155 pp

.

13. Hunter, P. A. and Fetner, M. W., "An Analysis of VGH Data Collected from One Type of

Four-Engine Turbojet Transport Airplane," NASA TN D-5601 , National Aeronautics and

Space Administration, Jan. 1970.

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15. Webb E. L. and Kann, A. J., "Vibration Measurements on the KC-135A Airplane During

Ground, Take-Off and Flight Conditions with J57 P-43W Engines, " Report on Test

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Washington, January 1958.

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1967 Proceedings , 13th Annual Technical Meeting, Vol. I, Institute of Environmental

Sciences, April, 1967, pp . 83-93.

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D. C., January 1972, pp . 153-161.

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March 8, 1971, p. 76.

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67
N BS-1 1 4A IREV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 77-1254

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. title: and subtitle: 5. Publication Date Transportation, Handling and Field Service Loads for Air July 1977

Mobility Shelter Systems 6. Performing Organization Code

461.01

7. A UTHOR(S)

C.W.C. Yancey

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 461-5414

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Civil & Environmental Engineering Development Agency

Tyndall Air Force Base, Florida 32401

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) To fulfill one of the prerequisites for establishing design criteria fcr military field

shelters, transportation, handling and field service load data are presented. An

extensive literature search was conducted to determine the magnitude and frequencies of

shock and vibration responses produced by railroad, road and air cargo vehicles and by

devices commonly used in handling field shelters. Summary data, extracted from field

study reports, are presented in the form of acceleration versus frequency diagrams. Thi

acceleration values can be used to compute an Equivalent Static Force input for the

design and analysis of shelter models. The probabilities of occurrence of the acceleration amplitudes generated by the three transportation modes are indicated in cases where data reduction included statistical analyses. Sources of the dynamic load data

included flatbed trucks, propeller and jet airplanes, helicopters, railroad flatcars

and forklift trucks. Where possible, the shock and vibration data used in presenting

the summary diagrams were restricted to that obtained from tests involving military

vehicles commonly used to transport shelters. Recommendations are presented for the

selection of static design loads to account for gravity, snow, ice and wind effects.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Acceleration; airplanes; cargo; dynamic loads; equivalent

static force; gravity load; handling devices; handling loads; military field shelters;

probability distributions; shocks, snow load; static load; trains; transportation;

trucks; vibrations; wind load.

18. AVAILABILITY Unlimited 19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

For Official Distribution. Do Not Release to NTIS 73 UNCL ASSIFIED

* Order From Sup. of Doc., U.S. Government Printing Office 20. SECURITY CLASS 22. Price Washington. D.C. 20402. SD Cat. No. 03 (THIS PAGE)

IXl Order From National Technical Information Service (NTIS) . | izr\ Springfield, Virginia 22151 UNCLASSIFIED

USCOMM-DC 29042- P 74