Evaluation of Creep Performance of 

Structural Insulated Panels (SIPs)

Phase 2

Dwight E. McDonald

Marshall Begel

C. Adam Senalik

Thomas Williamson

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Paper

FPL–RP–697

August

2018
Abstract

Structural insulated panels (SIPs) were creep-tested to determine 

the magnitude of long-term deflection and recovery, as well as 

changes to strength and stiffness, compared with a control group. 

This report documents the second phase of a two-phase testing

process. The first phase was reported in FPL–RN–0332. In this 

second phase, 56 SIP control specimens were tested in static 

bending to failure. Of these, 28 were 12 in. wide by 12.25 in. deep 

by 19 ft long and 28 were 12 in. wide by 6.5 in. deep by 10 ft long. 

A matching set of 56 specimens were instrumented for deflection 

and subjected to 90 days of constant (creep) load, which was set at 

approximately one-third the average short-term breaking strength 

of their control group per industry practice. They were then 

monitored unloaded for 30 days for deflection recovery. For the 

12.25-in.-deep specimens, the creep deflection was approximately 

15% of the corresponding short-term deflection at failure, with a 

total 30-day recovery of 81%. The 6.5-in.-deep specimens had a 

creep deflection of 11% of the short-term failure deflection with a 

total 30-day recovery of 91%. After creep testing, the specimens 

were static-load-tested to failure. The 12.25-in.-deep specimens 

failed at about 90% of the non-creep-tested control failure load. 

The 6.5-in.-deep specimens failed at approximately the same load 

as the non-creep-tested control specimens. This report documents 

the findings of the testing of the SIP panels as well as issues of 

concern encountered during the testing process.

Keywords: creep, static loading, structural insulated panels

August 2018

McDonald, Dwight E.; Begel, Marshall; Senalik, C. Adam; Williamson, 

Thomas. 2018. Evaluation of creep performance of structural insulated 

panels (SIPs): phase 2. Research Paper FPL–RP–697. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products Laboratory. 

12 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin.

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

Contents

Introduction............................................................................... 1

Materials.................................................................................... 1

Testing Program........................................................................ 1

Test Method .............................................................................. 2

Results....................................................................................... 3

Discussion ................................................................................. 3

References................................................................................. 5

Appendix A—Load and Deflection Graphs of SIP Creep 

Tests.......................................................................................... 6

Appendix B—Load and Deflection of SIP Recovery after 

Creep Testing ............................................................................ 7

Appendix C—Creep and Recovery Deflection of Long-Term 

Bending Tests............................................................................ 8

Appendix D—Static Bending Results of Control and 

Postcreep Tested Specimens ..................................................... 9

Appendix E—Static Bending Failures.................................... 11

Appendix F—Modeling of Creep Behavior............................ 11

English unit

Conversion 

factor SI unit

inch (in.) 25.4 millimeter (mm)

square inch (in2) 645.16 square millimeter 

(mm2)

foot (ft) 0.3048 meter (m)

pound (lb), mass 0.45359 kilogram (kg)

pound (lb), force 4.4482 newton (N)

pound per cubic 

foot (lb/ft3)

16.02 kilogram per cubic 

meter (kg/m3)

Nominal lumber 

size (in.)

Standard lumber 

size (mm)

2 by 6 38 by 140

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Introduction

Structural insulated panels (SIPs) are a composite building 

material. They are sandwiches that consist of two layers of 

structural facers with an insulating layer of foam in between. 

Typical facers are either sheet metal or oriented strandboard 

(OSB), and the foam can be expanded polystyrene (EPS), 

extruded polystyrene (XPS), or polyurethane. This study 

was limited to an evaluation of SIPs using 0.44-in. (7/16-in.)

performance category OSB facers and EPS foam cores at 

two different thicknesses.

SIPs share the same structural properties as an I-beam or Icolumn. The rigid insulation core of the SIP functions as a 

web, and the facers function as flanges. The use of SIPs 

replaces several components of conventional building such 

as studs, joists, headers, rafters, and insulation. As such,

they can be used for many different applications in wall, 

roof, and floor systems.

Materials

The test specimens were sampled from a production run at a 

Structural Insulated Panel Association (SIPA) member 

manufacturer and are representative of the product under 

evaluation. For all specimens, the panel facings consisted of 

PR-N610-rated, 0.44-in. (7/16-in.), Exposure 1 OSB 

sheathing on both sides and the insulating foam was 1-lb/ft3

EPS. All test specimens were constructed and cut with the 

OSB strength axis parallel with the panel length. For this 

study, 65 12-in. by 12.25-in. by 19-ft sections, each with 

11.25-in.-thick EPS foam cores, and 65 12-in. by 6.5-in. by 

10-ft sections, each with 5.63-in.- (5-5/8-in.-) thick EPS 

foam cores, were provided for testing. Two groups were 

tested for each size selected, 28 for short-term bending tests 

and 28 for long-term creep–rupture bending tests.

Testing Program

To address the creep performance of SIPs, a two-phase

testing program was initiated. Phase 1 was a collaboration 

between the USDA Forest Service, Forest Products 

Laboratory (FPL), APA—The Engineered Wood 

Association (APA), and SIPA as a pilot test program to 

evaluate basic creep performance parameters using a limited 

number of test specimens under three load conditions.

Separate testing to evaluate shear critical loading conditions 

was conducted by APA, and the testing to evaluate bending 

critical creep performance was conducted at FPL. The 

results of this study were reported by FPL–RN–0332

(McDonald and others 2014). Based on the results of the 

pilot test program, a more in-depth test program (Phase 2) 

was undertaken using ASTM D6815 (ASTM 2015) as the 

basis to establish creep performance factors for SIPs that 

can potentially be included in the American Wood Council

National Design Specification (NDS).

A total of 112 SIP specimens of two different depths were 

tested in either static (short-term) or creep (long-term) 

bending following by static testing (Table 1). Two specimen

depths, 6.5 and 12.25 in., were selected to represent the 

range of SIPs typically used in horizontal applications. To 

determine short-term failure loads and corresponding 

deflection values, 28 control specimens of each depth were 

tested in four-point bending (Fig. 1). The spans used were 

1.5 in. less than the full length to allow stable, simple 

supports. Loads were placed symmetrically about the center 

with a load span one-third of the total span (Fig. 2).

Evaluation of Creep Performance

of Structural Insulated Panels (SIPs)

Phase 2

Dwight E. McDonald, Forest Products Technologist

Marshall Begel, General Engineer

C. Adam Senalik, Research General Engineer

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin, USA

Thomas Williamson, P.E., Technical Advisory Committee Chair

Structural Insulated Panel Association, Fort Lauderdale, Florida, USA

Table 1—Specimen and test configuration details

Test 

no.

Sample 

depth

(in.)

Sample 

widtha

(in.)

Spana

(in.)

Load 

level

No. of 

samples Duration

1 6.5 12 118.5 To failure 28 1 min

1a 6.5 12 118.5 350 lb 28 90 days

2 12.25 12 226.5 To failure 28 1 min

2a 12.25 12 226.5 350 lb 28 90 days

a

The bending creep tests used a sample width of 12 in. and a test span of 

approximately 18 times the structural insulated panel depth.
Research Paper FPL–RP–697

2

It was proposed that moment-critical testing would provide 

the required results using foam flush end-bearing conditions

(that is, no dimension lumber blocks were required at the 

ends of the specimens).

Test Method

Static Testing

Fifty-six specimens (28 of each depth) were tested in static

bending to failure. This process was consistent with “shortterm” bending tests per ASTM D6815 with the following 

deviations noted: (1) the test load used was not as defined in 

section 5.3.2, (2) the decreasing creep rate was not 

determined per section 5.4.2, and (3) the fractional 

deflection was not determined per section 5.4.3. The load 

was provided by an MTS 244.31 55-kip hydraulic actuator

(MTS Systems Corporation, Eden Prairie, Minnesota, USA)

(1 kip = 1,000 pounds, force). This was connected to a 

spreader bar and two swivel heads with 3.5-in.-wide 

dimension lumber contact points. The specimens were 

supported by roller stands with 1.5-in.-wide lumber 

contact points.

The specimens were loaded in four-point symmetric 

bending. The support span was as large as the geometry 

allowed, and the load span and shear leg were one-third of 

the support span (Fig. 2). Specimens were loaded for a 

constant load head movement of 2 in./min, which caused 

failure between 30 and 60 s.

Load and deflection data were recorded five times per 

second throughout the test. The average maximum load (that 

is, load at failure, PMax in every case) in these short-term 

tests was used to calculate an appropriate long-term creep 

load based on PMax/3. PMax/3 is the basis for establishing all 

design loads for SIPs based on Section 4.2 of ICC AC04

(ICC 2012) and is used here as the recognized standard 

industry practice.

Creep Testing

Creep testing used the same four-point test geometry as the 

static test (Fig. 3). Load was applied by pneumatic actuators

(Marsh Bellofrom Group of Companies, Newell, West 

Virginia, USA) with a bore area of 30 in2

. One actuator was 

used per load point. Load heads had the same 3.5-in. bearing 

widths as the static tests. The specimens were supported by 

an edgewise 2 by 6, giving the same support width as the 

static tests.

Figure 1. Static bending test setup: (a) 12.25-in.-deep 

specimens; (b) 6.5-in.-deep specimens.

Figure 2. Static loading diagrams for 6.5-in.-deep specimens 

and 12.25-in.-deep specimens.

Figure 3. Creep test setup for the 12.25-in.-deep specimens.
Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2

3

Groups of pneumatic actuators were supplied with air via 

one of three main regulators. Group 1 included the actuators 

on specimens 1 through 9, group 2 included actuators on 

specimens 10 thorough 18, and group 3 included actuators 

on specimens 19 through 28. Each actuator group had a 

dedicated main regulator and shutoff. One specimen of each 

group was equipped with a load cell on each of its two 

actuators. The air supply was adjusted using the main 

regulator for each group of actuators until load cells on the 

monitored specimen within the group indicated the desired 

load.

The creep test specimens were loaded to 350 lb for both 

specimen depths, which is approximately one-third of the 

short-term failure load. The initial load rate was set such 

that the average time to attain the preselected constant stress 

level would not exceed the average time to failure of the 

short-term tests. Thereafter, the specimens were subjected to 

constant load for a minimum period of 90 days. During this 

period, midspan deflection readings were taken for each 

specimen until the 90-day time period had elapsed or until a 

failure occurred. Deflection readings were taken at regular 

intervals. No specimen experienced material failure in the 

90-day tests. The test load was removed after 90 days, and 

the midspan deflections continued to be recorded for an 

additional 30 days.

After the creep recovery period, the creep-tested specimens 

were statically tested in the same orientation to determine 

the specimens’ residual strength and flexibility after the 90-

day creep loading.

Results

Table 2 shows the load and deflection values of the control 

specimen static tests, the deflection values of the SIPs after 

the load was applied initially and that accrued after 90 days 

of constant load, and the initial deflection recovery after the 

load was removed plus the additional recovery that occurred 

after 30 days in the unloaded state. It also shows the load 

and deflection values of the postcreep specimen static tests

after the 30-day recovery period. All values are averages for 

the 28-specimen test sets. However, as discussed in the 

Testing Issues section of this report, specimens 6-10 through 

6-18 were excluded for the 6.5-in.-depth data set and 

specimens 12-11 and 12-6 were excluded for the 12.25-in.-

depth data set.

Load and deflection graphs of each test’s creep behavior are 

presented in Appendix A. Recovery graphs are presented in 

Appendix B.

Individual numeric creep and recovery results are presented 

in Appendix C. Static bending results of control and 

postcreep specimens are presented in Appendix D.

Discussion

Creep Effects

The test results are perhaps best understood when their 

values are shown in relation to each other, as in Table 3.

These results show that creep deflection was in the range of 

approximately 30% to 40% of the value of the load’s initial 

elastic deflection. There was significant recovery of total 

deflection (82%–90%) when the applied load was removed

and the specimens were allowed to rest for 30 days (with no 

load), implying that the creep behavior was at least partially 

elastic.

The postcreep static test load for the 12.25-in.-deep 

specimens was at about 90% of the 12.25-in.-deep control 

Table 2—Results summary of structural insulated panel

testing

Test

Specimen depth

12.25 in. 6.5 in.

Static failure load of control 

specimens, lb

1,014 1,032

Deflection of control specimens at 

failure, in.

1.251 1.031

Initial elastic deflection at start of 

creep test, in.

0.462 0.413

Additional deflection caused by 

creep behavior, in.

0.193 0.129

Total deflection, in. 0.655 0.543

Initial elastic recovery at removal of 

long-term load, in.

0.448 0.409

Additional recovery caused by creep 

behavior, in.

0.092 0.080

Total deflection recovered, in. 0.540 0.489

Static failure load of postcreep-tested 

specimens, lb

917 1,059

Deflection caused by static failure 

load of postcreep-tested 

specimens, in.

1.048 1.158

Table 3—Average test results as a comparison of 

deflection states

Test

Specimen depth

12.25 in. 6.5 in.

Creep deflection, as a percentage of 

initial elastic deflection

42% 31%

Creep deflection, as a percentage of 

total deflection

29% 24%

Creep deflection, as a percentage of 

failure deflection

15% 13%

30-day creep deflection recovery, as a 

percentage of 90-day creep 

deflection

48% 62%

Total deflection recovery, as a 

percentage of total creep test 

deflection

82% 90%

Static bending strength of postcreeptested specimens as a percentage of 

control specimen strength

90% 103%

Static deflection of postcreep-tested 

specimens as a percentage of control 

specimen deflection

84% 112%

Static plus creep deflection of longterm specimens as a percentage of 

control specimen deflection

91% 117%
Research Paper FPL–RP–697

4

specimen static test load. The postcreep static test load for 

the 6.5-in.-deep specimens was about the same as the 6.5-

in.-deep control specimen static test load. The postcreep 

static deflection for the 12.25-in.-deep panels was 84% of 

the control’s value, and the postcreep static deflection for 

the 6.5-in.-deep panels was 112% of the control’s value.

A comparison of average static test results before and after 

creep loading shows that the creep loading caused a loss of 

strength in the 12.25-in.-deep panels of 10% with a 16% 

lower deflection at failure. For the 6.5-in.-deep panels, creep 

loading caused a 3% increase in strength values and a 

12% increase in deflection at failure. As described in 

Appendix E, deflection is governed by flexure properties 

but strength is controlled by shear.

For the 6.5-in.-deep specimens, the 30-day recovery of 

creep deflection was 62%, whereas it was 48% for the 

12.25-in.-deep specimens.

Creep behavior for SIPs under flexural loading with respect 

to time was modeled using the power model as described in 

Taylor and others (1997). A description of the application of 

the model is given in Appendix F.

Testing Issues

As stated in the Test Method section, the long-term loads 

during the creep testing were applied using pneumatic 

actuators. Groups of several actuators were linked to 

common regulators for air supply. There were three groups

of actuators: group 1 included actuators on specimens 6-1 

through 6-9, group 2 included actuators on specimens 6-10 

through 6-18, and group 3 included actuators on specimens 

6-19 through 6-28. Each group had a separate air regulator 

that fed air to all of the actuators. As a result, similarities in 

curve shape were seen between specimens loaded by a 

particular group of actuators but not in other groups. A load 

cell was used to measure the load on one specimen in each 

group of actuators fed by a common regulator. The regulator 

pressure was adjusted until the load cell yielded the load 

desired for the long-term testing. The time-weighted load 

for each recorded load cell is given in Table 4.

During the period of loading for the 6.5-in. specimens, the 

specimen groups 1 and 3 demonstrated expected creep 

deflection. The specimen group 2 demonstrated a lack of 

noticeable creep deflection relative to specimen groups 1 

and 3. Load cell and pressure data from specimen group 2

showed multiple instances of load–pressure loss and 

readjustment that probably influenced creep displacement.

After the pressure was corrected, the creep deflection for the 

specimens loaded by specimen group 2 began to conform to 

expected creep behavior, but it was decided to exclude this 

data from the results.

There were no deflection data for the 12.25-in. creep 

specimen 12-16 because of a sensor malfunction; however, 

it was determined that the specimen was loaded correctly 

and its postcreep static test was valid. Specimen 12-11 

displayed a deflection behavior significantly different from

all other creep specimens; the deflection increased linearly 

for approximately 13 days then remained constant or 

slightly decreased for the subsequent 25 days. This behavior 

raised concerns regarding the validity of the displacement 

data and if the observed behavior represented creep or some 

other phenomenon. As a result, specimen 12-11 was 

excluded from the results.

There was an unexpected correlation between the order of 

the specimens tested and the maximum load supported

during static testing by the specimen. For the control group 

of 6.5-in. specimens, the maximum load decreased with 

increasing number of tests with an r2 value of 0.816. For the 

control group of 12.25-in. specimens, the maximum load 

increased with increasing number of tests (r2 of 0.505). For 

the 6.5-in. specimen group tested after long-term loading, 

there was no significant correlation between maximum load 

and order of testing (r2 of 0.009). For the 12.25-in.

specimens group tested after long-term loading, there was a 

slight increase in maximum load with increasing number of 

tests (r2 of 0.298). Correlation between maximum load and 

the order the specimens were tested was not expected.

Factors examined as possible causes included material 

handling after receiving the shipment of specimens to the 

laboratory, handling during testing, load cells, changes in 

test setup, and ambient conditions, but no conclusive 

explanation has yet been found for the noted unexpected 

correlation. However, the coefficient of variation for all 

static load data sets was within the expected range, and the 

PMax values were within the expected range based on Phase 

1 testing. Therefore, the creep test load of approximately 

PMax/3 or 350 lb was deemed appropriate.

Table 4—Time-weighted load by specimen group

Specimen 

ID

Thickness

(in.)

Specimen 

group

Time-weighted load 

(lb)

Load 

cell 1

Load 

cell 2 Total

6-9 6.5 1–9 184.6 182.4 367.0

6-18 6.5 10–18 — — —

6-28 6.5 19–28 185.3 191.5 376.7

12-9 12.25 1–9 197.5 200.2 397.7

12-18 12.25 10–18 192.0 179.0 371.1

12-28 12.25 19–28 178.0 180.2 358.2
Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2

5

References

ASTM. 2015. D6815-09. Standard specification for 

evaluation of duration of load and creep effects of wood and 

wood-based products. West Conshohocken, PA: ASTM 

International. www.astm.org.

ICC. 2012. ICC-ES AC04. In: Acceptance criteria for 

sandwich panels. Brea, CA: ICC Evaluation Service, Inc.

McDonald, D.; Begel, M.; Senalik, C.A.; Ross, R.J.; 

Skaggs, T.D.; Yeh, B.; Williamson, T. 2014. Creep behavior 

of structural insulated panels (SIPs): results from a pilot 

study. Research Note FPL–RN–0332, Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 12 p.

Taylor, S.B.; Manbeck, H.B.; Janowiak, J.J.; Hiltunum, 

D.R. 1997. Modeling structural insulated panel (SIP) 

flexural creep deflection. Journal of Structural Engineering.

123(12): 1658.
Research Paper FPL–RP–697

6

Appendix A—Load and Deflection 

Graphs of SIP Creep Tests

The graphs in this appendix do not show the initial elastic 

deflection that occurred when the load was first applied, 

only the deflection attributable to creep behavior. Each line 

starts at zero at the earliest time a steady state was reached

after loads were applied. For the 12.25-in.-deep specimens

(Fig. A1), this occurred at 2 h 25 min. For the 6.5-in.-deep 

specimens (Fig. A2, this was at 45 min because of the 

hyper-responsiveness of the pressure regulating instruments.

Some specimens lost pressure for a number of days during 

the planned 90-day test. To make up for this, their load time 

was extended to give them a total of 90 days under creep

loads. The time periods without load were removed from the 

graphs. The graphs only show the 90 days during which 

load was applied.

During the final 60 days of the 12.25-in. tests, data were

taken once a day, whereas for the 6.5-in. tests, data were 

taken every hour. This was done to catch discontinuities in 

the data that did not correspond to creep behavior. As a 

result, the 6.5-in. graph lines have more ‘noise’ than the 

12.25-in. lines, but they should not be considered to be any 

more or less precise.

Figure A1. Creep deflection with time, 12.25-in.-deep specimens.

 

Figure A2. Creep deflection with time, 6.5-in.-deep specimens.
Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2

7

Appendix B—Load and Deflection of 

SIP Recovery after Creep Testing

The graphs in this appendix (Figs. B1 and B2) do not show 

the initial elastic deflection that occurred when the load was 

first removed, only the long-term deflection recovery.

Figure B1. Deflection recovery after removal of load, 12.25-in.-deep specimens.

Figure B2. Deflection recovery after removal of load, 6.5-in.-deep specimens.
Research Paper FPL–RP–697

8

Appendix C—Creep and Recovery 

Deflection of Long-Term 

Bending Tests

Table C1—Creep and recovery deflection, 

12.25-in. specimens

Specimen 

ID

Initial 

deflection

(in.)

Creep 

deflection

(in.)

Initial 

recovery

(in.)

Final 

recovery

(in.)

12-1 0.597 0.243 0.534 0.107

12-2 0.640 0.186 0.535 0.116

12-3 0.592 0.192 0.462 0.098

12-4 0.598 0.220 0.512 0.109

12-5 0.580 0.229 0.524 0.120

12-6 0.588 0.127 0.644 0.071

12-7 0.611 0.147 0.654 0.075

12-8 0.538 0.160 0.470 0.094

12-9 0.564 0.217 0.499 0.105

12-10 0.522 0.157 0.462 0.086

12-11a — — — —

12-12 0.325 0.132 0.275 0.022

12-13 0.353 0.154 0.323 0.074

12-14 0.309 0.103 0.274 0.067

12-15 0.344 0.154 0.333 0.062

12-16b — — — —

12-17 0.338 0.146 0.332 0.072

12-18 0.329 0.178 0.327 0.067

12-19 0.380 0.206 0.378 0.117

12-20 0.383 0.143 0.469 0.079

12-21 0.384 0.241 0.414 0.098

12-22 0.444 0.256 0.485 0.096

12-23 0.434 0.251 0.444 0.101

12-24 0.441 0.245 0.479 0.100

12-25 0.432 0.230 0.428 0.129

12-26 0.433 0.220 0.448 0.104

12-27 0.405 0.193 0.423 0.069

12-28 0.450 0.276 0.518 0.152

Average 0.462 0.193 0.448 0.092

CoVc, % 23 24 22 29

a

Specimen 12-11 displayed a behavior significantly different from

all other specimens and was excluded.

b

As discussed in the Testing Issues section, there were no 

deflection data for the 12.25-in. creep specimen 12-16 because of

a sensor malfunction; however, it was determined that the 

specimen was loaded correctly and its postcreep static test was 

valid.

c

CoV, coefficient of variation.

Table C2—Creep and recovery deflection, 

6.5-in. specimens

Specimen 

IDa

Initial 

deflection

(in.)

Creep 

deflection

(in.)

Initial 

recovery

(in.)

Final 

recovery

(in.)

6-1 0.473 0.115 0.397 0.074

6-2 0.471 0.109 0.393 0.083

6-3 0.473 0.098 0.390 0.081

6-4 0.454 0.115 0.384 0.074

6-5 0.417 0.111 0.366 0.071

6-6 0.435 0.096 0.375 0.066

6-7 0.426 0.097 0.369 0.070

6-8 0.402 0.080 0.339 0.065

6-9 0.417 0.083 0.359 0.075

6-19 0.344 0.147 0.391 0.083

6-20 0.394 0.160 0.459 0.083

6-21 0.390 0.162 0.439 0.076

6-22 0.371 0.150 0.427 0.077

6-23 0.384 0.158 0.445 0.081

6-24 0.410 0.182 0.475 0.086

6-25 0.457 0.163 0.506 0.123

6-26 0.425 0.152 0.465 0.101

6-27 0.368 0.142 0.412 0.082

6-28 0.344 0.142 0.380 0.075

Average 0.413 0.129 0.409 0.080

CoVb, % 10 24 11 16

a

As discussed in the Testing Issues section, specimens 6-10 through 

6-18 were excluded for the 6.5-in. analysis. b

CoV, coefficient of variation.
Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2

9

Appendix D—Static Bending Results 

of Control and Postcreep-Tested 

Specimens

Table D1—Static bending results for 12.25-in. specimens

Control tests Postcreep tests

Control 

specimen ID

Maximum

load 

(lb)

Deflection at

max. load 

(in.)

Specimen 

ID

Maximum

load 

(lb)

Deflection at

max. load 

(in.)

12-1 ctrl 1,017 1.235 12-1 911 1.032

12-2 ctrl 907 1.158 12-2 854 0.913

12-3 ctrl 1,003 1.173 12-3 896 0.975

12-4 ctrl 873 1.030 12-4 920 1.045

12-5 ctrl 883 1.025 12-5 883 0.931

12-6 ctrl 902 1.103 12-6 802 0.794

12-7 ctrl 1,022 1.228 12-7 912 1.076

12-8 ctrl 967 1.154 12-8 909 1.043

12-9 ctrl 941 1.180 12-9 928 1.034

12-10 ctrl 966 1.176 12-10 821 0.876

12-11 ctrl 918 1.111 12-11 829 0.940

12-12 ctrl 994 1.251 12-12 813 0.970

12-13 ctrl 1,061 1.384 12-13 858 0.968

12-14 ctrl 1,082 1.360 12-14 805 0.899

12-15 ctrl 1,039 1.272 12-15 882 1.023

12-16 ctrl 1,062 1.298 12-16 999 1.188

12-17 ctrl 1,079 1.358 12-17 957 1.152

12-18 ctrl 1,086 1.452 12-18 971 1.129

12-19 ctrl 1,068 1.269 12-19 1,063 1.299

12-20 ctrl 1,069 1.305 12-20 997 1.167

12-21 ctrl 1,000 1.249 12-21 999 1.157

12-22 ctrl 1,081 1.286 12-22 920 1.088

12-23 ctrl 1,045 1.246 12-23 984 1.189

12-24 ctrl 1,093 1.368 12-24 951 1.092

12-25 ctrl 1,034 1.263 12-25 973 1.063

12-26 ctrl 1,065 1.352 12-26 954 1.074

12-27 ctrl 1,054 1.319 12-27 954 1.140

12-28 ctrl 1,069 1.410 12-28 927 1.088

Average 1,014 1.251 Average 917 1.048

CoVa, % 7 9 CoVa, % 7 11

a

CoV, coefficient of variation.
Research Paper FPL–RP–697

10

Table D2—Static bending results for 6.5-in. specimens

Control tests Postcreep tests

Control 

specimen ID

Maximum

load 

(lb)

Deflection at

max. load 

(in.)

Specimen 

ID

Maximum

load 

(lb)

Deflection at

max. load 

(in.)

6-1 ctrl 1,154 1.204 6-1 1,093 1.179

6-2 ctrl 1,179 1.250 6-2 1,062 1.143

6-3 ctrl 1,127 1.199 6-3 1,074 1.141

6-4 ctrl 1,127 1.171 6-4 1,053 1.151

6-5 ctrl 1,029 1.080 6-5 942 1.017

6-6 ctrl 1,121 1.153 6-6 913 0.956

6-7 ctrl 1,137 1.173 6-7 1,085 1.232

6-8 ctrl 1,072 1.035 6-8 1,198 1.410

6-9 ctrl 1,117 1.121 6-9 1,076 1.223

6-10 ctrl 1,016 0.949 6-10 —a —

6-11 ctrl 1,179 1.254 6-11 — —

6-12 ctrl 1,054 1.003 6-12 — —

6-13 ctrl 1,047 1.061 6-13 — —

6-14 ctrl 1,079 1.036 6-14 — —

6-15 ctrl 1,033 0.986 6-15 — —

6-16 ctrl 997 0.931 6-16 — —

6-17 ctrl 1,000 0.962 6-17 — —

6-18 ctrl 1,001 1.017 6-18 — —

6-19 ctrl 953 0.904 6-19 1,036 1.141

6-20 ctrl 955 0.901 6-20 1,124 1.308

6-21 ctrl 996 0.883 6-21 1,021 1.040

6-22 ctrl 981 0.997 6-22 1,059 1.156

6-23 ctrl 934 0.945 6-23 1,105 1.218

6-24 ctrl 909 0.877 6-24 1,134 1.331

6-25 ctrl 931 0.908 6-25 1,104 1.160

6-26 ctrlb 911 0.911 6-26 1,121 —

6-27 ctrl 942 0.991 6-27 1,067 1.162

6-28 ctrl 909 0.959 6-28 862 0.882

Average 1,032 1.031 Average 1,059 1.158

CoVc, % 8 11 CoVc, % 8 11

a

As discussed in the Testing Issues section, specimens 6-10 through 6-18 were excluded for the 

6.5-in. analysis. b

There were no deflection data for specimen 6-26 because of a sensor malfunction. Maximum 

load is valid.

c

CoV, coefficient of variation.
Evaluation of Creep Performance of Structural Insulated Panels (SIPs): Phase 2

11

Appendix E—Static Bending 

Failures

The static bending tests typically failed in shear at the 

manufactured discontinuities in the EPS web (Fig. E1).

These discontinuities are points of dramatically decreased

shear strength. Also, the industry-published design values 

for both depths and spans are controlled by shear, and this

finding supports the validity of the test data.

Appendix F—Modeling of Creep 

Behavior

Creep behavior for SIPs under flexural loading with respect 

to time was modeled by Taylor and others (1997). Taylor 

and others examined four distinct models for creep 

behavior: a three-, four-, and five-element viscoelastic 

model and a power model. Based on a comparison with the 

raw data, the five-element viscoelastic model and the power 

model were recommended for modeling creep behavior of 

SIPs for a 3-month load duration. In this study, SIPs were 

placed under constant load for a period of 90 days. The 

power model was selected to represent the creep behavior.

The general form of the power model is given in the 

following equation:

2 0 1 ( ) A ∆ =∆ + t At

Where Δ(t) is the total time-dependent deflection; Δ0 is the 

initial deflection, and A1 and A2 are creep parameters 

(Taylor and others 1997). The initial deflection, Δ0, is equal 

to zero, simplifying the power model to the form given in 

the following equation:

2 1 ( ) A ∆ = t At

There were 28 SIP specimens measuring 12.25 in. thick. Of 

the 28 specimens, 26 conformed to expected creep behavior 

regarding deflection with respect to time. As stated in 

Appendix C, there were no usable deflection data for 

specimen 12-16. Specimen 12-11 displayed a deflection 

behavior significantly different from all other creep 

specimens; the deflection increased linearly for 

approximately 13 days then remained constant or slightly 

decreased for the subsequent 25 days. The behavior raised 

concerns regarding the validity of the displacement data and

if the observed behavior represented creep or some other 

Figure E1. Typical static bending failure: (a) 12.25-in.-

deep specimens; (b) 6.5-in.-deep specimens.

Figure F1. Representative deflection versus time data set 

with fitted power model curve. The data shown are from 

the 12.25-in. specimen 12-28. The solid gray line is the 

recorded data; the dashed black line is the fitted power 

model.
Research Paper FPL–RP–697

12

phenomenon. As a result, specimens 12-11 and 12-16 were 

excluded from the modeling process. A typical SIP 

deflection curve for the 12.25-in. specimens with respect to 

time is shown in Figure F1. There are two distinct regions, 

an early time region in which the change in rate of 

deflection changes rapidly, and a late time region in which

the change in rate of deflection is steady. For the 12.25-in.-

thick panels used in this study, the transition between those 

two deflection rates was at approximately 30 days. If the 

power model fit included the early time region of the 

deflection curve (0 to 30 days), then the modeled deflection 

overestimated the actual deflection during the late time 

region and became increasingly divergent with increasing 

time. If the power model excluded the early time region, 

then the modeled deflection estimated the late time region 

deflection to within ±5%, but the early time region was 

overestimated. It was decided to fit the power model only to 

the late time region (30 to 90 days) because the model 

converges with the actual data at time 0 and 30 days. The 

same modeling process was used on the 6.5-in. SIP 

specimens. A typical SIP deflection curve for the 6.5-in.

specimens with respect to time is shown in Figure F2.

The creep parameters, A1 and A2, and the coefficient of 

determination (r2

) values for each of the 26 12.25-in.

specimens and the 19 6.5-in. specimens are given in 

Table F1. As stated in the Testing Issues section, a reduction 

in load to cylinder group 2 during the long-term loading 

caused a lack of noticeable creep for a significant portion of 

the test. As a result, it was decided to exclude the specimens 

in specimen group 2, which included specimens 6-10 

through 6-18, from the modeling process.

Figure F2. Representative deflection versus time data set 

with fitted power model curve. The data shown are from the 

6.5-in. specimen 6-24. The solid gray line is the recorded 

data; the dashed black line is the fitted power model.

Table F1—Structural insulated panel creep parameters (A1

and A2) and coefficient of determination (r2) for 12.25-in. and 

6.5-in. specimens

Specimen 

ID

12.25-in. specimens 6.5-in. specimens

A1 A2 r2 A1 A2 r2

1 0.0322 0.459 0.9844 0.0096 0.585 0.8938

2 0.0188 0.517 0.9747 0.0063 0.658 0.8903

3 0.0270 0.440 0.9759 0.0068 0.633 0.8954

4 0.0289 0.459 0.9810 0.0059 0.693 0.9145

5 0.0362 0.421 0.9815 0.0084 0.606 0.8846

6 0.0191 0.441 0.9875 0.0070 0.618 0.8728

7 0.0326 0.350 0.9655 0.0076 0.598 0.8565

8 0.0241 0.431 0.9716 0.0044 0.669 0.8538

9 0.0319 0.439 0.9831 0.0042 0.700 0.9153

10 0.0139 0.548 0.9635 — — —

12 0.0236 0.383 0.9828 — — —

13 0.0368 0.316 0.9864 — — —

14 0.0154 0.421 0.8989 — — —

15 0.0301 0.363 0.9874 — — —

17 0.0329 0.337 0.9871 — — —

18 0.0439 0.310 0.9596 — — —

19 0.0178 0.546 0.9843 0.0138 0.539 0.9474

20 0.0143 0.518 0.9916 0.0122 0.581 0.9815

21 0.0286 0.475 0.9872 0.0208 0.465 0.9341

22 0.0331 0.458 0.9867 0.0148 0.525 0.9500

23 0.0359 0.432 0.9841 0.0147 0.537 0.9589

24 0.0313 0.460 0.9869 0.0191 0.503 0.9635

25 0.0322 0.440 0.9893 0.0145 0.544 0.9700

26 0.0218 0.514 0.9809 0.0142 0.536 0.9573

27 0.0206 0.504 0.9807 0.0151 0.502 0.9477

28 0.0303 0.494 0.9851 0.0136 0.538 0.9560