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NBS TECHNICAL NOTE 873

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

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3 Located at Boulder, Colorado 80302.
6*13

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Electro-Optical Deflection

Measuring Device

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R. A. Crist, R. D. Marshall, H. I. Laursen

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

'Vau of *

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary tor Science and Technology

U,^, NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

* * \ Issued December 1975

ii iy>n i in illinium
National Bureau of Standards Technical ISote 873

Nat. Bur. Stand. (U.S.), Tech. Note 873. 24 pages (Dec. 1975) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:873). Price 75 cents. (Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

Page

Abstract v

1 . Introduction 1

2. Background 1

3. Optical Techniques 2

4. The Electro-Optical Tracking System 2

5. Calibration 3

6. Performance of the Operational System 3

7. The Test Building 4

8. Equipment Installation 4

9. Data Transmission and Recording 5

10. Experimental Results 5

11. Data Analysis 5

12. Theoretical Study of the Structure ....... 6

13. Thermal Deflection 7

14. Optical Path Interference 8

15. Summary 9

16. Future Studies 9

17. Acknowledgements 9

Figures 10

18. Bibliography 16

iii

ELECTRO-OPTICAL DEFLECTION MEASURING DEVICE

by

Robert A. Crist

Chief, Structures Section

Center for Building Technology

National Bureau of Standards

Richard D. Marshall

Structural Research Engineer

Structures Section

Center for Building Technology

National Bureau of Standards

Harold I. Laursen

Professor of Civil Engineering

Oregon State University

(Visiting Researcher, National Bureau of Standards)

ABSTRACT

The development and testing of an electro-optical device for the direct measurement of

lateral deflections of tall structures are described. The device utilizes a tracking telescope mounted on a fixed reference and a light source attached to the structure at the level

for which lateral deflections are to be measured. Operating characteristics of the system

are based on the results of tests carried out over a period of several months in one of the

elevator shafts of a 12-story building.

Key Words: Buildings; deflections; instrumentation; structural response; wind loads,

Electro-Optical Deflection Measuring Device

Introduction

With the continuing trend toward taller and mc-e flexible buildings, lateral deflections and oscillations caused by wind, ground motions and solar heating are assuming a significant role in the design process. If this trend continues, and it most likely will, new criteria must be developed to ensure satisfactory performance at acceptable levels of

risk and cost.

There is an interaction between dynamic structural deflections, the human occupants

(human response), and the building subsystems. The sensitivity of humans to building

motion depends upon a number of parameters, including acceleration, velocity, frequency,

damping, and duration of exposure. Although human response is necessarily subjective in

nature, the response of a structure and its subsystems is objective and may be quantified

through the use of appropriate instrumentation.

Analysis leading to design involves a designated forcing function and a calculated

transfer function to determine the system response. This can be expressed in block diagram

form as

F(t) G(t) El

Measurements of system response (deflection of a structure in service) allow a reverse

process. The structural characteristics can be determined if the forcing function is known

or the forcing function can be inferred if the structural characteristics are known. Thus,

proper understanding of response leads to the solution of the interaction problem and ultimately to allowable deflections as they relate to both building subsystem response and

human response.

With recent emphasis on serviceability, the direct measurement of structural response

in full scale with which to compare both theoretical predictions and model results has

become increasingly important. The lack of such data in the past has been primarily due to the fact that satisfactory instrumentation and measuring techniques have not been available.

In the following sections, the development and testing of an electro-optical system for the

direct measurement of structural deflections and oscillations are discussed. Finally,

comparisons are made between calculated and measured dynamic characteristics of a 12-story

reinforced concrete flat-plate structure.

2. Background

Early attempts to measure lateral deflections of tall structures relied on pendulums

whose natural period was much longer than that of the building (1,2). In fact, most measurements of lateral deflections reported to date have been made with inertial transducers

of one form or another (3,4).

Ideally, a complete time history of structural motion is desired. This includes both

mean and fluctuating components over periods of several months and at frequencies up to 10

Hz. Amplitudes of interest may range from 0.025 mm (0.001 in) to several millimeters. In theory, most of these requirements are within the range of existing inertial transducers,

but practical considerations severely limit their application. Deflections are usually

accompanied by rotations and it is extremely difficult to separate these effects over a reasonably wide bandwidth. That is to say, an inertial transducer will respond to both

forms of motion if the range of frequencies is sufficiently wide (5) . It is therefore

necessary to rely on more than one type of transducer to completely define time histories

of motion. Other approaches can be used which are not restricted by limitations of frequency response. The measurement of column strains is one technique. However, this is not

a direct measurement of lateral deflection and this approach requires certain assumptions
to arrive at deflections. The uncertainties involved often limit the value of these measurements. The same may be said of deflections determined from the double integration of

acceleration measurements.

The use of a theodolite, while entirely satisfactory for long-term static observations,

does not lend itself to the measurement of fluctuating components. In evaluating various

techniques for the direct and continuous measurement of lateral deflections, it was concluded

that optical devices, coupled with suitable electronics, offered the most promising

approach.

3. Optical Techniques

A number of possible schemes to measure structural deflections by optical techniques

have been discussed by Ward (6) . Some of these schemes have been developed into working

systems, all of them utilizing a laser source projecting a beam vertically upward from a fixed datum to a position sensing device. Sensing devices that have been used successfully

include: a null-seeking array of photoelectric cells mounted on an x-y plotter (6), a matrix of photodiodes and associated scanning system (7), and a photoelectric cell in

combination with an objective lens and an optical plate having a linear variation in transmittance (8)

.

A serious disadvantage of the x-y plotter approach, in addition to requiring active

mechanical components, is that the laser beam must be "captured" by the photocell array. A momentary loss of power would generally require operator response or manual intervention.

The use of a diode matrix limits resolution (fixed by minimum diode size) and system cost

increases roughly as the square of the required dynamic range. Finally, the photocelloptical plate technique requires optics whose physical size varies directly with the dynamic

range and requires expensive accessory equipment.

In addition to the above, several other laser beam techniques were considered during

the early stages of the development program. Some of them were as follows:

1. Two overlapping, motor-driven arms with a photoelectric cell extending along the

bottom side of each arm. By mounting potentiometers or shaft encoders on the

motor shafts and by using the peak photocell output to trigger a sample-and-hold

amplifier, it would be possible to obtain the angle at which each are intercepted

the beam. Knowing the distance between centers of rotation, one could then

calculate x-y coordinates of the beam position. An advantage of this approach is

that deflections at several levels could be measured simultaneously with one

laser source.

2. A vidicon tube focused on the top surface of a translucent screen mounted at the

top of an elevator shaft. Resolution would be 600 lines in one direction and

practically infinite in the other direction. Though not developed beyond the

conceptual stage, this technique has considerable potential.

3. A fixed array of photocells in combination with servo-controlled deflecting

mirrors in the optical path. Deflection measurements of the Tagus River Bridge were obtained using a similar system (9) . A working model was constructed as part of the NBS project and successfully used, but was abandoned in favor of a purely electronic sensing device.

4. The Electro-Optical Tracking System

The approach finally adopted and developed into a working instrument is shown in Fig.

1. A light source is attached to the structure at the level for which lateral deflections

are to be measured. A collimating lens projects the light beam down to the tracking telescope where a real image is formed on the surface of a position detector by the objective

lens. The objective lens is followed by a diverging or negative lens which increases the

focal length of the telescope and facilitates focusing on the sensor. The light path in the
telescope box is folded by two mirrors at 45 degrees to keep the physical size [406 mm high

x 254 mm wide x 102 mm deep (16 x 10 x 4 in)] of the box reasonably small. A separate lens

is mounted on the side of the telescope box to simplify coarse adjustment by focusing the source image on a target engraved on the telescope base plate.

The sensor is a silicon photovoltaic cell having an output terminal at each side of

the 10 mm (0.395 in) square active surface. This is reduced to an effective area of approximately 8.5 mm (0.335 in) square because of the finite image size. The sensor can be rotated

in its mount to align the electrical axes with the geometric axes of the telescope. The

dynamic range of the system is governed by the diameter of the collimating lens, the optical

path length, the focal length of the telescope, and the surface area of the detector. A motor-mounted optical flat can be rotated into the light path just ahead of the sensor to displace the image for system calibration in amplitude.

The electronics consist of two differential amplifiers which convert the two pairs of

currents generated by the sensor into voltages. Since these voltages are a function of

both image position and light intensity, an automatic gain control is provided to compensate

for fluctuations in the source intensity and accumulation of dust on the objective lens.

The low impedance output makes the system compatible with a wide range of recorders and

display devices.

It should be pointed out that practically any light source is acceptable if the objective lens is illuminated and the image at the surface of the sensor is of sufficient intensity. A 12-volt spiral filament bulb is currently being used and is operated at 8 volts to give extended bulb life (approximately one year) . It is also important to note that the

system responds only to translations of the light source since rotations of a point are not

detected. The tracking telescope must, of course, be supported by a fixed reference.

5. Calibration

Prior to installation in an actual service situation, a prototype instrument was operated on a horizontal test range over distances of 30 and 91 m (100 and 300 feet)

The light source was mounted on a cross-slide milling table which was supported by a concrete

pier. This allowed the source to be positioned within + 0.025 mm (+ 0.001 inch) over a range of + 50 mm (+ 2 in) for each axis. The tracking telescope was also mounted on a concrete pier. The telescope was aligned with the light beam by means of three fine-thread

adjusting screws in the base plate. Once the box was properly aligned, the optical components were positioned and focused. Finally, the sensor was translated and rotated to align

the electrical zero and axes with the telescope box.

A typical plot of actual and indicated source displacements is shown in Fig. 2. This

plot covers the full range of the system for an optical path length of 30 m (100 ft)

Skewness of the axes is inherent in the sensor and can be reduced only by careful selection

of sensors. Nonlinearities at large displacements are due to both the detector and amplifier characteristics. The system has a nominal sensitivity of 200 mv per mm (5 volts per

in) of source displacement and a resolution of approximately 0.13 mm (0.005 in) for a path

Length of 30 m (100 ft), i.e., 1 part in 240,000. In the several months that the instrument was operated on the test range, long-term electrical drift was found to be negligible.

No dynamic response studies were conducted on the test range. However, the device would be

expected to exhibit satisfactory response well above 100 Hz since it is purely an electrooptical system. The current configuration uses a 50 Hz low-pass filter on the output

stage.

6. Performance of the Operational System

It was recognized early in the development program that the system, basically constituting an angle measuring device, could be adversely affected by thermal gradients producing

changes in the index of refraction. -Thermal gradients in the direction of the optical path

(thermal stratification when operated in a vertical configuration) would have no effect

while those normal to the path (convection columns for example) would have the greatest
effect at the objective lens (tracking telescope) and no effect at the collimating lens

(light source). Thus, the operating environment must be considered when using this optical

device and a detailed study of system performance under typical operating conditions was therefore undertaken. Other operating functions that were evaluated included the physical

alignment, ease of placement, effect of stray light sources, and the instrument's ability

to reliably track the motion of the structure.

7. The Test Building

Performance tests were conducted over a period of several months on two units in an elevator shaft of the Administration Building on the NBS campus at Gaithersburg, Maryland.

This building is a 12-story reinforced concrete, flat-plate structure with plan dimensions

of 15 x 67 m (49 x 220 feet). It has a reasonably clear exposure to the prevailing

NW winds and the surrounding terrain may be classified as "gently rolling" with a power law

exponent of approximately 0.25. The building and surrounding structures are shown schematically in Fig. 3.

The building has two basement levels, resulting in a clear shaft height of 51 m

(166 feet), (Fig. 4). There are two rectangular shafts, each containing two cars. The

shafts were constructed monolithically and are framed into the floor system. The south

side of each shaft forms part of the outside wall and the remaining shaft walls are sur- rounded by air-conditioned office space.

The Administration Building was selected because it was readily accessible, but more

important it was estimated that the structure would provide a severe performance test of

the electro-optical deflection measurement system. Motions of this structure are small,

frequencies are relatively high, and the elevator shafts have the potential of containing

significant thermal gradients in the shaft because of the directly exposed exterior wall.

Also, data transmission lines from the system amplifiers to the tape recorders were longer

than would normally be anticipated [approximately 820 m (2700 feet)], thus providing additional electronic problems to be evaluated.

8. Equipment Installation

Light sources were mounted in adjustable brackets which were bolted to existing hanger

slots in the shaft wall. The light sources could be rotated about two perpendicular axes, allowing the light beams to be aimed directly at the tracking telescopes on the shaft

floor. Access to the positions of the light sources was gained by climbing onto the roof

of the elevator car and running it to the top of the shaft under manual control. A variable transformer located in the hoist room directly above the shaft allowed the bulb voltage

to be adjusted for proper image intensity.

Light source locations in the east shaft are shown in Fig. 4. The resulting optical

path length was 50 m (163 feet) . The tracking telescopes were connected by cables to the system electronics located outside the shaft [approximately 6 m (20 feet)] which allowed

adjustment without interrupting elevator service.

The procedure used in the installation and adjustment of instrumentation was as follows:

The light source was focused on the approximate position to be occupied by the tracking

telescope. The telescope was then positioned and roughly aligned using the aiming lens on

the side of the box. Once the image was acquired by the position detector, the objective

lens was capped and the balance potentiometers nulled. The cap was then removed and the

three adjusting screws in the base plate were used to bring the image onto the electricalzero position of the detector. The collimating lens on the light source was next adjusted

to produce maximum intensity on the detector and the bulb voltage was regulated to produce

a detector current of approximately 5 microamperes. Typically, these adjustments can be

made in approximately 15 minutes.
In addition to the two electro-optical tracking systems, an accelerometer was installed

to measure accelerations in the N-S direction. The accelerometer was placed directly on the

concrete floor at roof level and adjacent to an E-W column line (Fig. 4) . The accelerometer

is a force-balance device with a sensitivity range of 10 2 to 10 G g's. The frequency bandwidth selected for these acceleration observations was 0.1 to 9 Hz (-3 dB point).

9. Data Transmission and Recording

At the time this study was undertaken, research into wind pressures on Building 226

(Fig. 3) was in progress. Instrumentation associated with this program included, among

other items, a 20-m (66-ft) meteorological tower with several fast-response anemometers and

a sophisticated data acquisition system. It was therefore decided to transmit data from the

Administration Building to the recording center in Building 226, a distance of approximately

820 m (2700 ft), via telephone lines.

Instrumentation amplifiers with a gain of X10 were installed at the outputs of the two tracking telescope systems in the Administration Building. This resulted in a very satisfactory signal-to-noise ratio and capacitance of the signal lines was found to be acceptable

over the frequency range of interest. Data lines from the meteorological tower [located 60 m

(200 ft) directly north of Building 226] were already in operation.

Calibration drive motors for the tracking telescopes were activated from Building 226

by means of a battery and a momentary-on switch. This activated a high impedance relay in

the Administration Building which in turn activated a latching relay in the motor circuits.

To initiate and terminate calibration, it was necessary to temporarily use the common side

of the data lines in the relay circuit as only three line pairs to the elevator shaft were

available.

Data were recorded on digital, analog and stripchart recorders. Continuous records of

N-S motion and wind speed were recorded on paper stripcharts. A computer-controlled digital

and analog system was used to record data under strong wind conditions. The computer was programmed to initiate recording at a certain wind speed [usually 17 m/s (35 mph) ] and

record lengths were 15 minutes. Records were separated by a 30-minute hold period. For typical runs, variables recorded included N-S and E-W components of wind velocity, total

wind speed, N-S and E-W deflection measured by each tracking system, acceleration, ambient

air temperature and barometric pressure. Solar radiation will be monitored in future experiments. A detailed description of the data acquisition system can be found in Ref. (10).

10. Experimental Results

A typical 10-minute record is shown in Fig. 5. The variables are acceleration of the

building in the N-S direction, deflection of the building in the N-S direction, and the

total wind speed at 10 meters (33 feet) above ground. The analysis of these records is

considered in the following sections.

As seen in Fig. 5, the,wind speed varied from 5 to 14 m/s (11 to 32 mph), peak-to-peak

acceleration was 0.34 x 10~"Vs and the peak deflection was 0.69 mm (0.027 in). Mean wind

direction for this record was WNW.

11. Data Analysis

The records that were considered in the analysis were the N-S deflection and the N-S

acceleration of the building. In addition, mean values and spectral densities of wind

velocity were determined. These records were digitized at a rate of 20 samples per second.

The length of record considered was 7.5 minutes, resulting in 9000 data points for each

record. Autocorrelation and spectral density functions were calculated for the acceleration

and deflection records. Spectral density functions were obtained as Fourier transforms of

the autocorrelation functions (11). In calculating autocorrelation functions, 200 lags were

used.
The autocorrelation and spectral density functions for the building deflection and

acceleration due to a wind with a mean speed of 10.5 m/s (23.5 mph) in the west to east

direction and 2.3 m/s (5.1 mph) in the south to north direction, measured at the 20-m (66-

ft) height, are shown in Figs. 6 and 7. Variable DY1 represents the N-S deflection of the

building and variable AY1 represents the N-S acceleration. The rms levels for the deflection and acceleration records were 0.12 mm (0.0048 in) and 0.187 x 10 g's, respectively. As seen in Figs. 6 and 7, both analyses yield a value of about 1.0 Hz for the predominant

natural frequency of the building with some indications of the higher frequencies on the

power spectrum for acceleration. This value compares favorably with that obtained from a theoretical study of the structure which is discussed subsequently.

In view of the direction of the wind, which was predominantly along the strong axis of

the building, the results shown in Figs. 6 and 7 are due essentially to building motion

normal to the direction of the wind, i.e., cross-wind response. To examine the effects of

the change in direction of the wind relative to the weak axis of the building, records were

also analyzed for a wind with a mean speed of 6.3 m/s (14.0 mph) in the west to east direction and 1.9 m/s (4.2 mph) in the north to south direction, measured at the 20-m (66-ft)

height. The rms deflection was 0.11 mm (0.0044 in) and the resulting spectrum is shown by

the dashed line in Fig. 6. It is seen that there is negligible definition of the predominant natural frequency in the deflection record. Damping of the structure was estimated

from the rate of decay of the autocorrelation functions. The results are shown in Fig. 8. Damping was estimated to vary from 1.3 to 1.8 percent of critical damping.

12. Theoretical Study of the Structure

In order to compare measurement against theory, the natural frequency of the entire

structure was calculated using current state-of-the-art references and techniques. Two methods were used: a simple calculation using building code equations, and a frequency analysis using a general structural analysis computer program. The recommended method for

estimating the longest natural period according to UBC (12) and ANSI A58.1 (13) for building

structures is given as 0.05h

T = -—± (1)

V

D

where h is the height of the building in feet and D is the width in feet of the building

in the direction under consideration, or for moment resisting space frames which resist

100% of the lateral force,

T = 0.10N (2)

where N is the total number of stories above exterior grade. For the type of structure

being considered here, the lowest natural frequency would be estimated to lie between the

values obtained from equations (1) and (2)

.

The Administration Building is a reinforced concrete structure with flat-plate floors

and square, rectangular-tied columns. In rectangular plan dimensions it is 15 m (49 ft)

by 67 m (220 ft) and from subbasement to the roof of the penthouse it is 55 m (181

ft) . The building is asymmetric about both axes due to irregular spacing of columns and two elevator shafts near one end of the structure (Fig. 4). A typical structural bay through

the narrow width of the building consists of 254 mm (10 in) floors and variable column size

and spacing. The two elevator shafts are rectangular in plan with 203 mm (8 in) walls

extending approximately the full height of the structure. Interior finishings are typically

movable fiberglass metal clad partitions and the majority of floors are office occupancy

with relatively sedentary activity. The heating, ventilating and air conditioning systems

are located on the second floor. Exterior finishing on the broad faces of the structure is

glass and light metal cladding between stainless steel mullions running full height of the

structure which isolate the majority of the structural system from direct exterior exposure. The elevator shafts are directly exposed to the exterior for their full height on one narrow wall (Fig. 4). Each end of the structure has a solid brick masonry wall over the

full width and height. The walls are attached to spandrel beams at each floor level. The

structure is relatively stiff and experiences no undesirable performance.
Table I summarizes the calculation of the lowest natural frequency (f = 1/T) from

equations (1) and (2) for the direction parallel to the narrow dimension of the structure.

Equation (1), which applies more to the stiff er or shear-wall type structure, gives a frequency that is 50% higher than indicated by equation (2) , which is for moment resistant

space frames. The calculated range for the lowest natural frequency is 0.8 < f < 1.2 Hz.

The computer analysis was used to determine the first three natural frequencies in a direction parallel to the narrow dimension of the structure. The computer program SAP (14) was used on the UNIVAC 1108 EXEC 2 system at NBS . Principal assumptions used in the frequency analysis were:

1. The structure was divided into 5 typical cross-sections representing various

column arrangements. Structure heights, and elevator shafts.

2. Lateral floor displacements at a particular floor were the same for each section

(twist of the structure was not considered)

3. Axial deformation of beams and columns was neglected.

2

4. Material properties used were: E = 24 KN/m (3,500 ksi) [modulus of elasticity

estimation, ACI 318-71(15)] and u = 0.15 (Poisson's ratio).

5. Damping was neglected in the calculation of the natural frequencies.

6. The relative stiffnesses of columns and floors were based on gross concrete sections. Effective widths of the plate floors were assumed to be 9 times the slab

thickness, which was extrapolated from information in Ref. (16). The stiffness

due to block walls in the stairwells and the brick walls at the ends of the building was not considered.

7. In addition to flexural deformations, shear deformations in the elevator walls

were considered using a shape factor of 1.2.

8. The values of mass used in the analysis were based on dead load calculations and

live load values from Ref. (17). The resulting values correspond to a gross

density of 260 kg/m 3 (16 pcf) for the building.

9. Mode shapes were necessarily assumed for analysis by SAP. The first four mode

shapes were excited with a system of forces in order that reasonable numerical

accuracy could be obtained for the first three modes.

Table I gives the natural frequencies that resulted from the analysis of the structure

using SAP. The first through third frequencies are 0.7, 2.7 and 6.6 Hz respectively. The

corresponding calculated mode shapes are shown in Fig. 9. The comparison of measured frequency from analysis of response data and calculated frequency is reasonable in all cases

(Table I) . The SAP analysis indicates that the first and second natural frequencies were

detected by the deflection measurement system. Also frequencies predominant in the deflection measurements corresponded approximately to those predominant in the acceleration measurements.

13. Thermal Deflection

Along with the dynamic response to windloads, a structure responds to long term timedependent loads which are superimposed on the shorter duration loads. These long term loads

have to be accounted for in the design of a measurement system and analysis of the data.

These deflections are induced by creep and shrinkage for reinforced concrete structures and

temperature changes for structures in general. The latter is most critical in the case of

the electro-optical deflection system and will be considered here.

The Administration Building is exposed to solar heating on one broad side only the year

round. Part of this side incorporates one of the short walls of each of the elevator shafts;
the other short walls are on the interior of the structure (Fig. 4). A diurnal drift cycle

in a direction parallel to the narrow width is induced in the elevator shaft due to the

solar heating and atmospheric temperature changes. A typical record of this is shown in

Fig. 10 for a period of 48 hours during which there was range of air temperature from -2.2

to 16.1 °C (28 to 61°F) . During this period the total wind speed was less than 3.4 m/s (7.5

mph) and there were neither clouds nor overcast. Peak-to-peak displacement was 1.5 mm (0.06

in) . The record was obtained directly from the north-south component of the electro-optical

deflection system and recorded on a slow speed (1/2 in/hr) low frequency response (dc to 1Hz)

stripchart recorder. The width of the trace represents the dynamic activity of the deflection

measurement

.

Methods have been recommended (references 18, 19 and 20) for calculating temperatureinduced deflection of high-rise structures. These methods consider temperature gradients in

structural members, geographical temporal changes, and the structural characteristics among

the items for consideration. The complexity of temperature deflection calculations for the

Administration Building extends the study beyond the scope of this report. However, the

small magnitude of measured deflection as previously shown would be anticipated for this

structure. The columns at the exterior faces of the structure are shielded from direct

exposure. The only directly exposed portions of the structure are the exterior walls of the

elevator shafts. Thus the majority of temperature induced deflections of the entire structure would be introduced only at the elevator walls.

The thermal deflection is mentioned here only to demonstrate the ability of the electrooptical device to record these deformations even though they are in the order of 0.25 mm

(0.01 in).

An important consideration is the ratio of peak-to-peak lateral thermal deflection in

typical diurnal cycles to the possible peak-to-peak dynamic response activity during the same period of time. For example, the ratio of thermal deflection to peak-to-peak dynamic

deflection was determined from the windstorm and thermal deflection perviously presented,

figures 5 and 10 repsectively. This ratio is 1.5 which shows that each deflection is of the same order of magnitude. Linear ranges and deflection capacity of the instrumentation must

be within acceptable boundaries when these deflections are superimposed. In this case, measurements obtained by the electro-optical deflection system are well within acceptable

boundaries.

14. Optical Path Interference

As previously described, refraction of the light beam from the light source to the

tracking telescope causes an apparent deflection to be recorded. Temperature gradients

perpendicular to the light path will cause such refraction. This was studied in the performance evaluation of the electro-optical deflection system by the comparison of the N-S

deflection components of two deflection systems placed in the same elevator shaft. One

system from which the data has been previously described was placed at the interior wall and

the other system at the exterior wall (Fig. 4). It was assumed that torsional displacements

were insignificant. The exterior wall is exposed directly to the atmosphere. Outputs of

the two deflection systems were analyzed by comparing the ratio of calibration signal [approximately 3.8 mm (0.15 in)] to background signal activity (noise due to thermal activity, electronics and dynamic structural response). The ratio of signal activity was obtained when

the wind was negligible; thus most of the signal activity was attributed to the optical

path interference, electronics and mechanically induced structural vibration of which the

latter two were estimated to be small by separate studies. The ratio of calibration output

to background signal activity magnitude varied from 10 to 20 for the interior wall and was con- sistently 3 at the exterior wall. This provides conclusive information that the location of

deflection measurement near the exterior wall has approximately 5 times as much noise attributable to optical path inference. It also demonstrates the importance of consideration of positio

for instrumentation when installing an optical deflection measurement system. The ratio of

calibration to signal activity level of 10-20 was found to be acceptable for adequate structural response measurements; however, the ratio of 3 was found to be marginal for resolution

of short-term dynamic response measurements.
In order to establish consistency of readings between the two units, mean deflections

measured by the two tracking telescopes are compared in Fig. 11. The readings were obtained

under a variety of conditions, ranging from clear to overcast and from calm to windy. They

are based upon visual averages of 30-second records and are estimated to be within + 0.13

and + 0.3 mm (+ 0.005 and + 0.01 in) of the true mean for Units No. 1 and No. 2, respectively.

The sensitivities of the two units are obviously different. Problems were encountered

with the calibrating mechanism of Unit No. 2 shortly after installation in the shaft. This,

coupled with optical path interference, made it difficult to establish identical sensitivities for the two units. Even though the sensitivities were not the same, the measured

amplitudes are consistent.

15. Summary

Measurements made with the electro-optical deflection system have been shown to be in

agreement with trends of observed structural response and with theoretical models.

The Administration Building provided a critical performance environment for the measurement system due to low magnitude and high frequency dynamic response. It is believed

that this instrument performed adequately in this structure and that the system could be

expected to perform as well or better in a more flexible and taller structure.

16. Future Studies

An operational deflection measurement system has been developed and tested. However,

certain details of the system's behavior remain to be studied. Deflections associated with

thermal environment will be more closely correlated by the simultaneous measurement of solar

radiation and atmospheric temperature. Optical path interference will be studied in more

detail by the comparison of the dynamic response of the two deflection systems and by con- trolled variation of the optical path environment. This will qualify the use of the optical

system in various environments which might be encountered. Long term stability of the

electronics (on the order of months to a year) , associated mechanical components and light

source will be studied in a controlled environment calibration tunnel to quantify possible

measurement errors.

Various types of incandescent and monochromatic laser light sources are being investigated. The present system is compatible with these types of light sources. The light source selection has to be made compatible with each configuration in which the system is

required to operate.

"A compact electronics package is being considered which would ruggedize the system for

transportation to the site, reduce setup time and extend unattended operational periods.

Such items as a variable range of signal amplification and variable filter bandwidths are

also being considered.

Uses of the system other than in high-rise structures should be investigated. The

system is not restricted to direction and is operational in lateral as well as vertical

orientations. Remote measurement of deformations on an inaccessible structure is a possible

application. The placement of a simple target on the structure will facilitate this type of

measurement.

17. Acknowledgements

The cooperation and assistance of Mr. H. D. Cook (Electronic Optical Development Section,

National Bureau of Standards) in the design and assembly of the first operational model and

initial calibration are gratefully acknowledged. The assistance of Mr. Randy Williams and

members of the electronics group of the Structures Section is also acknowledged.
FIG. 1 TRACKING TELESCOPE IN OPERATING POSITION (COVER REMOVED)

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FIG. 2 INDICATED AND ACTUAL SOURCE DISPLACEMENTS - UNIT NO. 2.

11
TRANSMISSION OF DATA

(TELEPHONE CABLE, 2700 FT) DEFLECTION

WIND VELOCITY MEASUREMENTS

TWO-DIMENSIONAL LATERAL

DEFLECTION MEASUREMENT

^UNIAXIAL ACCELERATION MEASUREMENT

NBS

ADMINISTRATION

BUILDING

DATA RECORDING AND PROCESSING

FIG. 3

BUILDING 226

SCHEMATIC OF NBS CAMPUS

Note: 100 feet = 30.48 m

LIGHT SOURCES- ACCELEROMETER

C>N

49 FT 30 FT

c\j _ O 6

z z

o o

UJ UJ ff

20.67 FT

N

220 FT

MASONRY WALL

L22FJI22TD 7|?2FT| 71.58 FT t ACCELEROMETER

i JT

8.33 FT

TYPICAL COLUMN

BAYS

DEFLECTION UNITS

PLAN

-TRACKING TELESCOPES Note: 100 feet = 30.48 m

SECTION ELEVATION AT ELEVATOR SHAFT

FIG. 4 PLAN VIEW AND SECTION ELEVATION OF ADMINISTRATION BUILDING

12
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PL. Q

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13
SITE CODE - 101 TIHE CODE - 351 1950 VRRIRBLE - Ori

^-00 2.00 4.00 6.00 8.00 10.00 LfiG (SECSI

SITE CODE - 101 TIME CODE - 351 1950 VRRIBBLE - DTI

Up 5 3 « iiiiiio" 5 5 ' 5 6 7 a 9 'i o° *~ FREOUENCT eTTPio1

FIG. 6 AUTOCORRELATION AND SPECTRAL DENSITY FUNCTIONS FOR N-S

DEFLECTION

SITE COOE - 101 TinE COOE - 351 1950 VRRlReLE - ATISITE CODE - 101 TIHE CODE - 351 195(5 VHRIHBLE - HT1

] 0" 2 3 * 5 6 7 6 9 '] 0" 2 3 * 5 6 789^(f 2 3 * 5 6 7 6 9 \

0' FREOUENCT

FIG. 7 AUTOCORRELATION AND SPECTRAL DENSITY FUNCTIONS FOR N-S

ACCELERATION

14
010

0.08

006

0.04

0.02

- DYI ^^^^^ • • ^~"-~-^^

• — •

.

• "

• -^*

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100 0.071 b "(9)(2n)

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i i

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0? i i 1 i i 2 4 6 8 10 NUMBER OF CYCLES NUMBER OF CYCLES

FIG. 8 DAMPING ESTIMATES OBTAINED FROM DEFLECTION AND ACCELERATION

AUTOCORRELATION FUNCTIONS

r I

FIRST

/

\

SECOND THIRD

FIG. 9 CALCULATED MODE SHAPES FOR ADMINISTRATION BUILDING

15
en

UJx

o

o

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0.06 h

0.05

0.04

x 0.03

cro

z 0.02

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x 0.01

i-8 0.02

1.5

1.0

0.5

6 10 2AM 6 10 2PM 6 10 2AM 6 10 2PM 6

TIME, HOURS

o

UJ

UJQ

0.5

FIG. 10 DIURNAL THERMAL DRIFT CYCLES FOR ADMINISTRATION BUILDING,

N-S DIRECTION

Source

Natural Frequencies, Hz

f accel.

f

l

Fundamental Second Third

UBC/ANSI A5 8 Eq. 1 1.2 - - 0.83

UBC/ANSI A5 8 Eq. 2 0.8 - - 1.25

SAP Computer

Analysis 0.7 2.7 6.6 1.43

Measured from

Defl. Spectrum 1.0 2.9* - 1.0

Measured from

Accel. Spectrum 1.0 3.0 5.8 1.0

* Estimated

TABLE I. COMPARISON OF CALCULATED AND MEASURED FREQUENCIES OF TEST

STRUCTURE

16
FIG. 11 COMPARISON OF N-S DEFLECTIONS MEASURED BY THE TWO TRACKING

TELESCOPES

17
18. Bibliography

1. Engineering News, Vol. XXXI, March 1, 1894, p. 169.

2. Rathbun, J.C., "Wind Forces on a Tall Building," Transactions, ASCE, Vol. 105, Paper No. 2056, 1940, pp. 1-41.

3. Crawford, R. and Ward, H.S., "Wind-Induced Vibrations and Building Modes," Bulletin of

the Seismological Society of America, Vol. 56, No. 4, August 1966, pp. 793-813.

4. Wiss, John F. , and Curth, Otto E. , "Wind Deflections of Tall Concrete Frame Buildings,"

Journal of the Structural Division, ASCE, Vol. 96, No. ST7, July 1970, pp. 1461-1480.

5. Dalgliesh, W.A. , and Ward, H.S., Discussion of "Wind Deflections of Tall Concrete Frame

Buildings," Journal of the Structural Division, ASCE, Vol. 97, No. ST4, April 1971,

pp. 1370-1374.

6. Ward, H.S., "Some Reasons and Techniques for Measuring Large Structural Displacements,"

Engineering Journal, Vol. 54/6, June 1971, pp. 14-21.

7. Melbourne Research Laboratories, "Instrumentation, BHP House," December 1971, pp. 13-19.

8. Tanizaki, H. , et al. , "Laser Displacement Follower for Vibration and Displacement

Measurement of High-Rise Buildings," Nippon Electric Company, Ltd., Catalog No. 456-6-J,

July 1971.

9. Morecos, J., Castanheta, M. and Trigo, J.T., "Observation of Tagus River Bridge in

Lisbon," Ministerio Das Obras Publicas, Laboratorio National de Engenhama Civil, LNEC - Proc. 37/1/2203, March 1968.

10. Marshall, R.D. and Hsi, George, "Techniques for Measuring Wind Loads on Full-Scale

Buildings," Proceedings of Seminar - Wind Loads on Structures, University of Hawaii,

October 1970, pp. 133-148.

11. Bendat, Julius S. and Piersla, Allan G. , "Random Data: Analysis and Measurement

Procedures," John Wiley and Sons, Inc., New York, 1971.

12. Uniform Building Code, 1970 Edition, Vol. 1.

13. ANSI Standard A58. 1-1972, Building Code Requirements for Minimum Design Loads in

Buildings and Other Structures.

14. Wilson, Edward L., "SAP - A General Structural Analysis Program," University of

California, Berkeley, California, September 1970.

15. American Concrete Institute, Building Code Requirements for Reinforced Concrete, 1971.

16. Qadeer, Aslam and Smith, Bryan S., "The Bending Stiffness of Slabs Connecting Shear

Walls," Journal of the American Concrete Institute, No. 6, V. 66, June 1969.

17. Bryson, J.O. and Gross, D. , "Techniques for the Survey and Evaluation of Live Floor

Loads and Fire Loads in Modern Office Buildings," NBS Building Science Series 16,

December 1967.

18. Fintel, Mark and Kahn, Fazlur R. , "Effect of Column Exposure in Tall Structures - Temperature Variations and Their Effects," ACI Journal, Proceedings , V. 62, No. 12,

December 1965, pp. 1533-1556.

19. Kahn, Fazlur R. and Fintel, Mark, "Effect of Column Exposure in Tall Structures - Analysis for Length Changes of Exposed Columns," ACI Journal, Proceedings , V. 63, No. 8, August 1966, pp. 843-864.

18
20. Kahn, Fazlur R. and Fintel, Mark, "Effects of Column Exposure in Tall Structures - Design Considerations and Field Observations of Buildings," ACI Journal, Proceedings , V. 65, No. 2, February 1968, pp. 99-110.

19
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLK ATION OR REPORT NO. NBS TN-873

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Electro-Optical Deflection Measuring Device

5. Publication Date December 1975

6. Performing Organization Code

7. AUTHOR(S)

R.A. Crist, R.D. Marshall, and H.I. Laursen

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. 4615132

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as 9.

13. Type of Report & Period Covered Final, 1972-1973

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.)

The development and testing of an electro-optical device for the direct

measurement of lateral deflections of tall structures are described.

The device utilizes a tracking telescope mounted on a fixed reference

and a light source attached to the structure of the level for which

lateral deflections are to be measured. Operating characteristics of

the system are based on the results of tests carried out over a period

of several months in one of the elevator shafts of a 12 -story building.

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)

18. AVAILABILITY Fj£ Unlimited

~j_ For Official Distribution. Do Not Release to NTIS

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