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Laser Scanning and 3-D Modeling of Jacobs Hall

Abhishek Salkar1, Abhinanda Dilip1 and Qudsia Wahab1

1Graduate Student, Department of Civil and Environmental Engineering, University of California, Berkeley,

Berkeley, CA 94720

ABSTRACT Laser scanning is the fastest, most accurate, and automated way to acquire 3D digital data for

reverse engineering. This technique is most effective for projects in which structural drawings of

old buildings are not available. For example, many private schools in San Francisco need to

undergo seismic evaluation. Many of these buildings were designed before 1970 and the

structural drawings are not easily attainable. In order to avoid destructive tests, laser scanning

can be used. This saves cost and also helps reduce pollution caused by destructive testing. For

this project, a complete laser scan of the Jacob’s Hall was carried out from 12 different locations.

The point cloud data obtained was stitched together using ModelSpace and Cyclone, and a 3-D

unified virtual model was developed. Using the point clouds in the model, elements were

introduced through the best fit technique and appropriate structural forms were developed. This

model was studied for imperfections in various structural members. The results could be used to

carry out structural health monitoring of Jacob’s Hall during its service life. Finally, a 3-D print

of the developed model will be obtained to visualize the amazing capabilities of the laser scan

and its future use in structural engineering.

Keywords: Laser Scanning, Point Clouds, Element Imperfections, 3-D virtual model

INTRODUCTION

Laser Scanning is often called 3-D object scanning or 3-D laser scanning. It is used to rapidly

capture shapes of objects, buildings and landscapes. This technology enables users to capture

millions of points from the subject structure which is discretized as a cloud of thousands or

millions of points in space, referred to as ‘point clouds’ as illustrated in Figure 1. Laser scanning

technology is rapidly on the rise, as it is finding applications in various fields, and the trend is

expected to continue with the development of high definition laser scanning (HDS).

Figure 1: Point Cloud of (a) St. Anne’s Church at Vilnius, Lithunia [1] and (b) Fahy’s

Bridge at Bethlehem, USA [2]

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In the past decade, laser scanning has attracted a lot of interest in academia. Research on

applications of laser scanning in structural laboratory testing and field surveys has been

summarized by K. M. Mosalam, S.M. Takhirov [3], [4], [5] and [6].

Applications

Laser Scanning has many applications in the field of structural engineering. The applications of

laser scanning can be divided into 2 parts:

Structural Engineering Laboratory Tests:

1. To capture construction errors and as-built dimensions of test specimens.

2. To measure rigid body motion and correlate with conventional measurements.

3. To assess damage and its evolution and correlate with conventional measurements.

Earthquake Engineering Structural Reconnaissance:

1. Can be applied to a large bridge structure for novel data interpretation and damage

assessment.

2. Can be applied to a group of buildings for correlation with ground surveillance

assessment.

3. Can be applied to a damaged complex structure and used for reconstruction of geometry

in 3D using point clouds.

Advantages and Disadvantages

Some of the advantages and disadvantages of laser scanning in the structural engineering field

have been summarized below:

Advantages:

1. They can capture three dimensional global deformations of large objects with

accuracy.

2. They enable graphical post-processing based on a location of the acquired points in

space and utilizing the light intensity red–green–blue (RGB) values of scanned

images.

3. They allow access to objects without spatial constraint and remote monitoring by

scanning from a distance.

4. They are relatively easy to install, and do not require much effort.

Disadvantages:

1. It can only acquire data from visible surfaces, and

2. It is relatively slow to deploy in rapid reconnaissance efforts and real-time structural

testing. However, the advances of HDS technology are expected to most likely resolve

this issue in the near future.

This project involves laser scanning of Jacob’s Hall. Opened in Fall 2015, Jacob’s Hall is a new

four story building located at 2530 Ridge Road (corner of LeRoy Avenue and Ridge Road)

adjacent to Soda Hall and Etcheverry Hall and can accommodate more than 500 students in

light-filled design studios. It covers an area of 24,000 square feet. The procedure that was

followed has been briefly summarized below in Figure 2 in the form of a flowchart.

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Figure 2: Procedure followed for this project

A picture of Jacobs Hall has been shown below in Figure 3 so that its laser scans shown later in

the paper can be compared to it. The experimental setup has been described in detail later in the

experimental setup section.

Figure 3: Jacobs Hall, University of California, Berkeley

Jacobs Hall was laser scanned from 12 different locations

Point cloud data was grouped and stitched together to obtain a 3-D unified virtual model

Unnecessary moving objects (noise) like people and cars were cleaned in order to get a clearer model

A short movie providing views of Jacobs Hall from different angles, and also a walkthrough around the building was created

Sections were sliced at various locations in order to obtain section and elevation views.

Various structural elements were modeled and checked for imperfections, which could have risen from construction errors.

A 3-D model of Jacobs Hall will be obtained in the near future.

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WORKING PRINCIPLE OF LASER SCANNER

The laser scanner used was introduced by Leica Geosystems HDS, Inc. for high definition

surveying. The laser consists of a transmitter that emits a short laser pulse and a receiver that

captures the reflected rays from surrounding objects. Based on the travel duration of the laser

pulse, it estimates the distances of the surfaces that obstruct the pulse. The measured distances

are then converted to a coordinate corresponding to each point, relative to the scanner location.

The scanner also captures the intensity of the reflected pulse which can be correlated to the color

of the object. The Leica ScanStation C10 scanner shown in Figure 4 was used in this study. It is

an all-in-one platform with the scanner, battery, controller, data storage, and video camera in a

compact off-line form. In addition, ScanStation C10 also features major advances in

productivity, versatility, and ease-of-use for as-built and topographic HDS. It has a maximum

360ᵒ ( horizontal) × 270ᵒ (vertical) field-of-view with a parallax-free aiming/sighting and angular

accuracies of ±4mm (±0.16in) and ±60micro-radians, respectively, and a beam spot size of only

4.5mm (0.18in) from 0 to 50m (0–164ft) range. Its scanning rate of 50,000 points/sec and

vertically rotating mirror on horizontally rotating base, enable deployment in virtually any

orientation. Each scanner location generates a database of points called ‘point cloud’. This

collection of scanned point sets is oriented with respect to the current relative position of the

scanner. The scans of different densities from all stations can be clubbed together through the

process of ‘registration’ by matching common target points between the point clouds. This

process reduces the layers to a single layer of data which is based on a common coordinate

system. The registered point clouds can be referenced from any coordinate system for

convenience of analysis. Data point acquisition and data reduction is carried out in Cyclone, a

software application developed by Leica Geosystems HDS, Inc. In this study, the software

application ModelSpace was used for post processing of data.

Figure 4: Leica ScanStation C10 scanner

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EXPERIMENTAL SETUP AND PROCEDURE

An extendable tripod was used to install the laser scanner in position. Once the tripod was fixed

firmly to the ground, it was leveled and the scanner was mounted on top as shown in Figure 5

(a). To ensure perfect orientation, the in-built spirit level in the scanner was adjusted as shown in

Figure 5 (b). The process of leveling is the most time consuming as it has to be done accurately

so that the scans have the correct alignment and coordinate system relative to the scanner. The

settings in the scanner were modified before starting the scan. The resolution of the scan was set

to medium in order to achieve quick scans. High resolution scans would take about 4-5 hours.

Figure 5: (a) Laser Scanner mounted on the tripod and (b) Leveling the Scanner

Jacobs Hall was scanned from 12 different locations. Among the 12 locations, 3 were on the roof

of Etcheverry Hall, 2 were on the roof of Soda Hall, and the remaining 7 were taken on ground

level to capture each corner of Jacobs building. The locations from where Jacobs Hall was

scanned have been shown in Figure 6.

Figure 6: Locations used for Laser Scanning of Jacobs Hall

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The point clouds obtained from the 12 locations were input in Cyclone software. On giving the

auto-meshing command, Cyclone divided the 12 point clouds into 3 groups depending on the

areas covered by each scan. However, the auto-meshing carried out by Cyclone did not give

desirable results due to lack of common reference points among the 12 point clouds. Hence, the

point clouds were grouped together manually by taking common reference points or common

reference lines in the form of stationary objects like buildings, trees, etc. This process was

carried out until we were able to obtain a unified 3-D virtual model which included data points

from all 12 point clouds, overlapped perfectly as shown in Figure 7.

Figure 7: 3-D unified virtual model obtained after stitching of point cloud data

The single layer of data was imported into ModelSpace and post-processing was initiated. The

data had to be reduced to include only necessary point clouds required for analyzing Jacobs Hall.

The moving objects which were captured during this scan created a haze to the image and hence

had to be removed from the point cloud data. The group of features that needed to be removed

was isolated and deleted from the project one by one. This process of reduction of data to remove

unwanted noise is called ‘cleaning’. The moving vehicles, pedestrians, etc were carefully picked

out and cleaned in such a way that no important data was lost. In order to get an idea of the

surroundings of the building, the neighboring streets, electrical wirings were retained.

Observations made from point cloud data

A few important observations were made while laser scanning Jacobs Hall:

1. Shadows: Due to obstructions like people, cars and trees, it was not possible to obtain the

point cloud data at few locations. The scanner works on the principle of reflection of light

and hence the laser beam cannot penetrate opaque objects. As a result, there were

shadows created in certain locations of the point cloud data as illustrated in Figure 8.

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Figure 8: Shadows created due to trees on (a) eastern side and (b) western side of campus

This problem can be resolved by copying point cloud data from similar locations, and pasting it

in the shadows. For example, the shadows caused in Figures 9 (a) and (b) can be replaced by

point clouds from other parts of Jacobs Hall. However, this method cannot be used if the

shadows are being created at unique locations of the point cloud data. Since the shadows created

in the point clouds for this project were not critical to our objective, they were not replaced by

similar point clouds.

2. Blind Spot: The laser scanner used for the experiment was able to scan its surroundings

efficiently except for the area exactly beneath it. This gives rise to a circular shadow right

below the scanner with a low density of point clouds visible as illustrated in Figure 9.

Figure 9: Blind Spots at (a) southern side of campus and (b) northern side of campus

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Blind Spots can also be resolved in the same manner as shadows. Point clouds from similar

locations can be copied and replaced in the blind spots to get rid of the circular shadows. As

mentioned earlier, this method cannot be used if the blind spot lies at a unique location.

However, it is very unlikely for blind spots to lie in unique locations relevant to the respective

projects, and hence are not difficult to resolve. Since all our laser scans were taken from outside

Jacobs Hall, the blind spots did were not critical and were not resolved for this project.

Elevations and Plans of sections of Jacobs Hall

Once a desirable 3-D view of the building was obtained, 3-D models were developed to

document building conditions. Then 2-D drawings were created from point cloud models to

represent as-built conditions. For example, a horizontal cut was taken from the building to obtain

the plan view of the building in Figure 10.

Figure 10: Floor Plan of Jacobs Hall

It is noticeable that the lines are not continuous in the plan view. The reason for this is that the

stations that were chosen did not allow penetration of the rays. If a higher number of stations

were chosen around the building, more desirable results would have been obtained.

An elevation of Jacobs Hall from the southern side of the campus was also obtained as shown in

Figure 11. There is only a narrow corridor which separates Jacobs Hall and Soda Hall. The two

buildings are connected by a bridge on the second floor. The obstruction caused by this bridge

makes it impossible to obtain an elevation view from the southern side of Jacobs Hall by

conventional methods. Hence, this elevation view is an excellent example of how laser scanning

technology is able to obtain drawings of structures which would not have been possible by any

other means. The elevations of different points were also marked with reference to the ground.

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Figure 11: Elevation View of Jacobs Hall from the southern side of the campus

Slicing to obtain cross sections

Once the floor plan and elevation views were obtained, the building was sliced at various

locations to obtain cross sections. For example, the building was sliced at section A-A from the

elevation view from the southern side of campus as shown in Figure 12.

Figure 12: Section A-A sliced in the elevation view from southern side of campus

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The cross section obtained after slicing the building at section A-A has is shown in Figure 13.

Figure 13: Cross section obtained after slicing at section A-A

DEVELOPING A MODEL FOR ANALYSIS

From the point cloud data taken during the construction phase, the structural skeleton was

obtained. As it can be seen in Figure 14, the lateral system comprises of Buckling Restrained

Braces (BRB) and the gravity framing is made of steel framing with slab on metal deck. The

elements of the framing along with the connections can be observed in detail from the model.

Figure 14: Jacobs Hall Skeleton

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Modeling Procedure:

1) The point cloud information is isolated for a particular element as circled in Figure 15,

and imported into a new layer in ModelSpace.

Figure 15: Isolating an element

2) As can be seen from Figure 16, alongwith the point cloud information for the element,

there are data points from nearby objects too. These are removed from the data points and

a clean point cloud comprising of the element data points are obtained as shown in Figure

17.

Figure 16: Isolated Point Cloud Data

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Figure 17: Reduced Element Data

3) An object is created from the cleaned data points by choosing a wide flanged shape to fit

the data. The best fit is obtained through trial and error and an element that matches well

with the virtual cloud is produced as shown in Figure 18.

Figure 18: Modeled Element

4) The element created is then assigned into a new layer and added a distinct color to

distinguish it from the rest of the elements as shown in Figure 19.

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Figure 19: Assignment to a new layer

5) The element is then imported into the model as shown in figure 20.

Figure 20: Importing Modeled Element

6) This process is repeated for a portion of the structure and a partial framing is obtained as

shown in Figure 21.

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Figure 21: Partial Framing

7) The element sizes were obtained from the modeled objects. The depth of web, width of

flanges, thickness of webs and thickness of flanges were compared to the standard

sections in AISC manual and the closest sizes were picked up. Figure 22 shows a typical

connection with sizes of members and thickness of connecting plates.

Figure 22: Model of the Connection with Sizes

Gusset Plate

thickness: 1” 12x12x5/8 W 18x71 W 14x90 W 14x90 W 14x90 W 14x90 Weld Size: 1/2”

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CONCLUSION

The structural skeleton of Jacobs Hall was successfully extracted from the scanned data.

Modeling of the elements was easily achieved using ModelSpace software. While modeling,

minor imperfections like warping of beam webs at welded connections, were observed. Since

Jacobs Hall was recently constructed, very few modeling issues were encountered. The elements

were fairly aligned and local distortions were not too prominent in the elements. The sizing of

the elements and the detailing of the connections could also be easily approximated. The model

obtained can be imported to any structural analysis software and analyzed for required forces.

Thus, it can be concluded that laser scanning is an excellent alternative for older buildings when

drawings are not available. Also, for new buildings, deflections of all forms can be easily

visualized and can be compared with measurements from conventional instruments. Due to its

accuracy and flexibility, this is a better technique to monitor structural health during the service

life of the structure.

From the modeled structural framing of Jacobs Hall, a small scale 3-D replica will be printed in

the near future. A walkthrough video of Jacobs Hall was also created, and will be uploaded on

YouTube in a few weeks.

REFERENCES

1. http://www.cyark.org/news/laser-scanning-success-at-st-annes-church

2. http://www.pobonline.com/articles/97132-firm-employs-3d-laser-scanning-to-fix-bridge

3. S.M. Takhirov, K.M. Mosalam (2014) “Application of Laser Scanning to Structural

Testing in Earthquake Engineering, Field Survey, and Structural Assessment of an

Earthquake Aftermath”, Tenth U.S. National Conference on Earthquake Engineering,

Anchorage, Alaska

4. K.M. Mosalam, S.M. Takhirov, S. Park (2013) “Application of laser scanning to

structures in laboratory tests and field surveys”, Wiley Online Library, DOI:

10.1002/stc.1565

5. K.M. Mucmullin, S.M. Takhirov, S. Gunay, S. Yarra, E. Tai (2014) “Scanner and Digital

Displacement Technology for Documentation of Experimental Building Façade Test

Specimens”, Tenth U.S. National Conference on Earthquake Engineering, Anchorage,

Alaska

6. S.M. Takhirov (2010) “Laser Scanners in Structural Assessment and Finite Element

Modeling”, Structures Congress ASCE, pp 2226-2237.