STRUCTURAL DESIGN FOR VIBRATION-SENSITIVE …

STRUCTURAL DESIGN FOR VIBRATION-SENSITIVE ENVIRONMENTS

Brad Pridham, Ph.D., P.Eng.Principal, Acoustics Noise & Vibration

November 11, 2020

Learning Objectives

1. Understanding of the nature and significance of commonly encountered sources of vibration in sensitive environments;

2. Understanding criteria for the vibration design of sensitive environments;

3. Refresher on dynamics of structural systems;

4. Understanding of the significance of the vibration path and how strategic siting and layouts can reduce the cost of vibration control; and,

5. Identify some of the implications of vibration control on structural design.

Agenda

• Fundamental Concepts• Source, path, receiver• Key issues for building environments• Vibration criteria• Fundamentals of Linear Structural Dynamics

• Concepts & Control Measures• Environmental Vibration• Floor Vibration

Image obtained from: Dynamics of structures, Ray W. Clough and Joseph Penzien, McGraw-Hill, 1975.

• Mechanical equipment• Footfalls• Process tools & equipment• Stamping

• Road & Rail vehicles• Process tools & equipment

Source – Path - Receiver

http://www.alecbuck.com/mediagallery/media.php?f=0&sort=0&s=20060729222351476

• Imaging

• Microscopy

• Micro-surgery etc.

Vibration effects on image quality

Low Vibration Environments

• Work environments

• Patient care floors

Occupant Comfort Tactile Vibrations

Occupant Comfort

• Research/medical equipment

• Building services

Noise Control

1 4 8 80

0.05

0.1

0.5

RM

S A

ccel

erat

ion

(%g)

Frequency (Hz)

Vibration Criteria

1 4 8 80

0.05

0.1

0.5

RM

S A

ccel

erat

ion

(%g)

Frequency (Hz)

• 1x, ISO-OpOperating theaters

• 2x, ISO-ResResidences

• 4x, ISO-OfficeOffices

• 8x, ISO-WorkshopWorkshops

8 ×

4 ×

2 ×

1 ×

Human Comfort Criteria

1 4 8 80

0.05

0.1

0.5

RM

S A

ccel

erat

ion

(%g)

Frequency (Hz)1 4 8 80

4000

RM

S V

eloc

ity (µ

in/s

)

Frequency (Hz)

�̇�𝑦 =�̈�𝑦

2𝜋𝜋𝜋𝜋

Acceleration Velocity

Sensitive Equipment Criteriaintegration

• 1/2x, Class ALow res microscopy

• 1/4x, Class BCT scanners

• 1/8x, Class CHigh res microscopy

• 1/16x – 1/32x, Class D/EMRI, SEM, NMR 1 4 8 80

125

250

500

1000

2000

4000

RM

S V

eloc

ity (µ

in/s

)Frequency (Hz)

1 ×

12

×

14

×

18

×

116

×

132

×

A

B

C

D

E

ISO-Op

Sensitive Equipment Criteria

Criteria SummaryStructural damage concerns ~30x ISO-Workshop

Threshold of perception

Low sensitivity

Offices, residences, microscopy (<40x)

Moderately sensitive

Microscopy (100x – 400x), vivaria, surgery, CT

Ultra-sensitive

Imaging (SEM, TEM), MRI, NMR

Design may be governed by Serviceability

Vendor Criteria

MRI Electron Microscopy

Additional Comments - Criteria

• Manufacturer’s criteria should always be used when available

• Measurement data processing must be consistent with methods used to formulate criteria -> frequency resolution, integration window

• Discuss detailed requirements with end users

FUNAMENTALS OF LINEAR DYNAMICSSDOF & MDOF Linear Systems

Modal parameters Response evaluation Continuous systems

Fundamentals of Linear Dynamics

System Parameters

Frequency of Oscillation

Damping Ratio

Dynamic Amplification Factor (DAF)

Frequency Ratio


Response to Harmonic Loading


Dynamic Amplification Factor

Newton’s 1st Law Sinusoid

Response to Transient Loading


Newton’s 1st Law

Exponentially Decaying Sinusoid

Continuous Systems and Modal Analysis


Generalized Coordinates – “Modes of Vibration”


• Each mode can be examined separately to establish response contribution and evaluate control measures

Mode Shape - φ


input

mass

response

SDOF

𝐹𝐹𝑛𝑛 = 𝜑𝜑𝑇𝑇𝐹𝐹

𝑚𝑚𝑛𝑛 = 𝜑𝜑𝑇𝑇𝑀𝑀𝜑𝜑

�̈�𝑈𝑛𝑛 = 𝜑𝜑𝑇𝑇�̈�𝑦𝑛𝑛

• Vector of spatial distribution of motion (dynamic deflection)

for mode n

Response of Linear MDOF Systems – Mode Superposition


Linear Dynamics - Key Takeaways


1. Increasing mass:i. Reduced response by way of Newton’s 1st Lawii. Reduced system frequency – how does it affect r ?

2. Increasing stiffness increases the system frequency – how does it affect r?

3. Increasing damping is only affective at resonance

4. Continuous linear systems can be decoupled into a series of SDOFs

5. The spatial distribution of mass, stiffness, damping, and externally applied forces are important to the design for vibration control

ENVIRONMENTAL VIBRATIONConcepts & Control Measures

Equipment and procedures Slab-on-grade design Ground-borne noise Occupant comfort

Environmental Vibration Control

Sources

• Cars, trucks, buses• Railway• Activity from neighboring buildings

(MEP, heavy equipment, etc.)


Forces from Road Vehicles

• Axel hop• Body bounce


Example: Measured Road Vibrations


Forces from Rail Vehicles

Narrow-band random process

Commuter Rail (DMU), Tie-on-ballast

Light Rail, Embedded Track


Example: Measured Rail Vibrations

Freight, Tie-on-ballast


Example: Measured Stamping Vibrations

Control Measures

• Source: establish the nature and relevance of sources (spatial and temporal)

• Path: strategic layouts, structural isolation joints, wave barriers

• Receiver: foundation design, equipment/room isolation and control

Control of the path and receiver are most effective

Environmental Vibration – SOURCE Control

Source Characterization – Temporal Statistics

• 20-hours of road traffic data collected next to a highway

Environmental Vibration – SOURCE Control

Source Characterization – Site Mapping

Environmental Vibration – PATH Control

Modifying the Transmission Path: Wave Barriers

Example: Effectiveness of Wave Barriers

• Target: 25% reduction in vibration level• Concrete barrier in clay (Vs = 600 ft/s)• What size barrier is needed?

Source Source Frequency (Hz)

Barrier Dimension (ft)

Width Depth

Freight Train 4 16 131

Truck 15 7 23

LRT 40 3 7


Foundation Attenuation (Coupling Loss)

Wave Scattering


Slab “Isolation”

Perimeter isolation joint


Example: Isolated slabs

Vibration data collection



Ambient vibration

0 50 100 150 200 250 30010

-14

10-12

10-10

10-8

10-6

0 50 100 150 200 250 30010

-14

10-12

10-10

10-8

10-6

Frequency (Hz)

PS

D (g

2 /Hz)

Vertical Axis

0 10 20 30 40100

101

102

103

0 10 20 30 40100

101

102

103

Time (s)

Acce

lera

tion

(g x

10-3

)

Impact hammer on floor



Impact hammer on floor

0 50 100 150 200 250 30010

-14

10-12

10-10

10-8

10-6

0 50 100 150 200 250 30010

-14

10-12

10-10

10-8

10-6

Vertical Axis

Frequency (Hz)

Mea

n P

SD

(g2 /H

z)



Environmental Vibration – RECEIVER Control

Equipment Isolation

Ground motion

Isolator

Tool

Interaction Force

• System is designed to behave as an SDOF• “Isolator” can be passive or active• Isolator selection is based on:

– Source characteristics– Cost– Durability and robustness

• Base structure designed to be very stiff(~ 3x106 – 6x106 lb/in)


Transmissibility – Mechanical Springs and Elastomers


Passive Control Systems

Rubber mounts

Pneumatic springs

Optical tables

Negative Stiffness Isolators


Active Control Systems

SEM Base Active Table Supports Active Plinths/Platforms


Transmissibility – Precision Control


Quiet Room Design Concept

• Plinth mass 3x – 5x equipment mass• Acoustic enclosure (typically masonry) supported on

base structure not floating slab• Provide space for access to isolators• Thickened room slabs for control of local disturbances

– 8” – 12” slab in surrounding areas

FLOOR VIBRATIONConcepts & Control Measures

Occupant comfort Sensitive equipment Specialty surgical suites

Multiple sources of vibration to consider

Control Measures

• Source: strategic layouts of corridors and “source areas” – Response Mapping

• Path: optimizing mass and stiffness, partitions

• Receiver: strategic layouts, isolated structure, equipment isolation

Successful designs incorporate a combination of source, path, and receiver control

Floor Vibration – SOURCE Control

Space Layouts – Response Mapping

Walking path

Floor Vibration – SOURCE Control

Space Layouts – Response Mapping

Walking path

‘quiet’zone

Floor Vibration – PATH Control

Optimizing Design for Serviceability

• Example – footfall response of a laboratory floor

A

B

• Target criteria:

Room A VC-C, 500 µin/sRoom B VC-B, 1000 µin/s

• Response simulation for walker in corridor



• Framing – base caseBeams: W14x22

Girders: W21x44

Slab: 3.5” conc. 1.5” deck



Resp

onse

Lev

el

ISO-Workshop32,000 µin/s, 800 µm/s

ISO-Office16,000 µin/s, 400 µm/s

ISO-Residential8,000 µin/s, 200 µm/s

ISO-Operating Theatre4,000 µin/s, 100 µm/s

Class A2,000 µin/s, 50 µm/s

Class B1,000 µin/s, 25 µm/s

Class C500 µin/s, 12.5 µm/s

Class D250 µin/s, 6.25 µm/s

Class E125 µin/s, 3.125 µm/s

A

B

• Target criteria exceeded in both bays

Room A ISO-Op, 4000 µin/s

Room BISO-Res, 8000 µin/s



• Identify the problem zones & associated mode(s) of vibration

Mode 1: 7.5 Hz



• Framing revisions

Mode 1: 7.5 Hz

Beams:W14x22 → W18x60Girder:W21x44 → W24x103



Resp

onse

Lev

el

ISO-Workshop32,000 µin/s, 800 µm/s

ISO-Office16,000 µin/s, 400 µm/s

ISO-Residential8,000 µin/s, 200 µm/s

ISO-Operating Theatre4,000 µin/s, 100 µm/s

Class A2,000 µin/s, 50 µm/s

Class B1,000 µin/s, 25 µm/s

Class C500 µin/s, 12.5 µm/s

Class D250 µin/s, 6.25 µm/s

Class E125 µin/s, 3.125 µm/s

A

B

• Target criteria satisfied in both bays

Room A VC-C, 500 µin/s

Room BVC-B, 1000 µin/s



FEM

Modal Parameters

Simulation

Assessment

Mitigation

Optimization

Integration with Revit


Optimizing Design for Serviceability – Partition Effects

• Example: pre- and post-fit-out measurements

Floor Plan



• Slab-to-slab partitions run below

Floor Plan



• Free decay floor response at mid-span – no significant change in damping

0 0.2 0.4 0.6 0.8 1-0.02

-0.01

0

0.01

0.02

Time (s)

Acce

lera

tion

(g)

0 0.2 0.4 0.6 0.8 1Time (s)

Pre Fit-Out Post Fit-Out



• Change to bay frequency

0 5 10 15 200

0.05

0.1

0.15

0.2

Acce

lera

tion

(%g)

Frequency (Hz)

Pre Fit-Out: 8 Hz

Post Fit-Out: 9.5 Hz



-500

0

500

Velo

city

(µm

/s)

4 6 8 10 12 14 16 18 20

-500

0

500

Time (s)

Pre Fit-Out

Post Fit-Out

• Footfall responses


Optimizing Design for Serviceability – Modelling Partition Effects

• k = 2 kip/in/ft

Linear springs at stud wall locations

Floor Vibration – RECEIVER Control

Supplemental Damping

Tuned mass dampers, viscous dampers Active mass dampers

19” (h) x 20”m (L) x 14” (W)


Supplemental Damping – TMD vs. AMD

• AMD requires 1/10th the mass and achieves better control

• Power and maintenance issues in development

• Product release coming soon

Summary

• Generic vibration criteria are derived from the ISO Base Curve (threshold of human perception)

• Most problems encountered can be examined using the SDOF model

• Critical sources to consider for structural design of sensitive environments:– Environmental sources: road and rail traffic, nearby industrial sources– Floor vibration: occupant activity, building services, environmental sources

• Remember: Source – Path – Receiver control paths– Source: layouts and source characterization;– Path: layouts, isolation joints, barriers, optimization of structural dynamics– Receiver: layouts, tool isolation, supplemental damping(?)

HOW A VIBRATION CONSULTANT CAN HELP

Reduce uncertainty associated with vibration design elements

Experience-based guidance related to: criteria integration of vibration design with other disciplines feasibility of various solutions implementation

“Insurance check” for design team

TYPCIAL SCOPE OF WORK

Site investigation Existing installations, new sites

Design analysis/technical assessments Dynamic modelling (FEA, empirical models etc.) Isolation system design

Performance specifications for controls Coordination with vendors Monitoring and performance testing Peer reviews

THANK YOU!

Brad PridhamPrincipal, Technical Director – SLR Consulting

226 706 8080 [email protected]

Documents

STRUCTURAL DESIGN FOR VIBRATION-SENSITIVE …