NIST NCSTAR1-2

Federal Building and Fire Safety Investigation of

the World Trade Center Disaster

Baseline Structural Performance and

Aircraft Impact Damage Analysis of

The World Trade Center Towers

(Appendices A-E)

Fahim Sadek

National Institute of Standards and Technology • Technology Administration • U.S. Department of Commerce

Appendix A

Salient Points with Regard to the Structural Design of the

World Trade Center Towers

Reproduced with permission of The Port Authority of New York and New Jersey.

NISTNCSTAR 1-2. WTC Investigation 305
Appendix A

Saliert -points with regard co the structural design of The

World Trade Cer.tcr towers: -

' -

1. The structural analysis carried out by the firm of Worthington,

Skilling, Kelle 6 Jackson is the niost complete and detailed of any

ever made for any building structure. The preliminary calculations

.-.Icuc cover 1,200 pages and involve over 100 detailed drawings.

2. The buildings have been designed for wind loads of 45 lbs. per square

foot which is l^c tines the New York City Building Code requirements

of 20 lbs. per square foot, the design load for the Empire State,

Pan American and Chrysler Buildings. In addition to static wind

looili,, a co;."rjletc c^ynaxic analysis hai btcn iuaie to take into account

extremely high velocity gusts.

3. The buildings have been investigated and found to be safe in an assumed

collision with a large jet airliner (Boeing 707 - DC 8) travelling

at 600 miles per hour. Analysis indicates that such collision would

result in only local damage which could not cause collapse or

substantial damage to the building and would not endanger the lives

and safety of occupants not in the immediate area of impact.

4. Because of its configuration, which is essentially that of a beam 209'

deep, the towers are actually far less daring structurally than a

conventional building such as the Empire State where the spine or

braced area of the building -is far smaller in relation to the height.

5. The building as designed is sixteen times stiffer than a conventional

structure. The design concept is so sound that the Structural Engineer

has been able to be ultra-conservative in his design without adversely

affecting the economics of the structure. This is not the case with

conventional buildings where a more radical approach must be used if

the building is to be constructed at reasonable cost,

.

'

. •*

306 NIST NCSTAR 1-2, WTC Investigation
Salient Points with Regard to the Structural Design of the WTC Towers

6. The structural concept is ne;? but the design principlfis, the stress analysis ar.d the theories cf ir.echcr.ics upon which the design is based

are well knc\va and are ia acccrdar.co v.-Ui ^^-^^ ^..-luoojir.- pructice.

7. The design has been reviewed by some of the most knowledgeable people

in the construction industry. In a letter to John Skilling, the Structural

Engineer for The World Trade Center, the Chief Engineer of the American

Bridge Division of U. S. Steel Corporation said:

"In reviewing this design with c-^.: Z_.^sz.zl-^ c.r. \ Cc -istr-.:c tion Ocpr.rtments,

we are very optimistic that you have tur.;::t. a now ?>-se in w..c; wo-i^.-. of

structural steel. It is high time that some new thinking be applied in

our industry. In the vjords cf our General Manager of Operating, Lester "i.

Larison, he said - 'it was the best damn thing that he has seen come

down the pike in his 45 years of experience. Imagine designing a 100-

story building for under 30 pounds per square foot.'"

8. The Engineering News-Record of January 30th carries a series of quotations

from people in the building industry with regard to The World Trade Center

design.

A. James Ruderman, one of the outstanding New York Structural Engineers

says that "The structural design of the tower buildings shows a

commendable job of rethinking, where ideas were given a lot of

thought and not just treated routinely."

B. Harold Bernhard, partner, Shreve, Lamb and Harmon Associates,

Architects, says "It's a magnificent project."

9. In an editorial in the same issue of the Record is the conment: "Thus, the PNYA vjili not build as high as permitted all over its property,

despite the high land costs in dovrntovra Manhattan. Instead, the twin towers will occupy only 12% of the site. This plan should please the numerous vociferous critics of other recent New York projects not surrounded by large open snaces. It also ne-i-w.-; ^c• *-v.o < t. . t.^t .

NISTNCSTAR 1-2, WTC Investigation 307
Appendix A

w3.t;h no setbacks without violating zoning regulations. Over-all, the

design not only sppears to be esthetically preferable to a set-back

silhouette, but also lends itself to more economical construction and

use of space. The PNYA, in addition, has engaged noted architects and

consulting engineers to design the project. From the preliminary data

released, it appears that the design of the twin towers will mark an

important advance in skyscraper construction. Tall buildings are

handicapped economically because the cost of structural framing and the

space consumed by vertical transportation rise rapidly with increasing

height. The Trade Center designers have departed from usually conventional practices to cut these costs."

10. We have been informed that the structural engineering firm of Ammann &

\'Jhitney has been approached by a leading New York architect with a

request that this structural system be reviewed for possible incox^oratioa

in a large office building which the architect is presently desigi^iog.

11. The skyscraper is one of America's contributions to World Architecture.

New York is the capital of skyscraper construction in the United States.

The design of the towers of The World Trade Center is based on the lessons

learned in constructing all the tens of millions of square feet of high

rise buildings in this great city. The towers may be said to be the

first buildings of the 21st Century and the design concepts which they

embody will be incorporated in some measure in every future high rise

building ever built.

KPLtfg

2-3-64

308 NIST NCSTAR 1-2, WTC Investigation
Appendix B

Estimation of Sectorial Extreme Wind Speeds^

Abstract

We present a procedure for estimating extreme wind speeds corresponding to a scctor-bysector approach to the estimation of extreme wind effects. We provide details of the data

sets and their treatment, as well as details of the estimates themselves, in a manner

intended to be thorough, clear, and transparent. Efforts in the direction of clarity and

transparency are in our view necessary if estimates of extreme winds and their effects are

to meet the need for effective scrutiny by users and building authorities, and if a solid

technical basis for a consensus among practitioners, standards organizations, and

professional organizations is to be created in the near future.

Introduction

The estimation of extreme wind speeds at a given site is, in principle, straightforward.

However, in practice, for any given location, differences between approaches used by

various wind engineers or other professionals can lead to widely divergent estimates. To

assess any particular extreme wind speed estimates it is necessary to scrutinize with care the procedure on which that estimate is based. This requires, in turn, that the procedure,

each of its steps, and the attendant calculations, be explained clearly, transparently, in sufficient detail, and in a manner that should be independently verifiable by users or building inspection authorities. For an example of detailed assessment of an extreme

wind speed estimation methodology and attendant calculations, see (Coles and Simiu,

2003).

At this time no sufficient guidance is available in standards for (a) the estimation of

extreme wind speeds on buildings subjected to wind tunnel testing and (b) the integration

of those wind speeds with aerodynamic data. Several procedures are used by various

practitioners, but no professional consensus appears to exist on how discrepancies

between the respective estimates can be reconciled or how the various methods should be

amended to avoid situations - which do occur in actual practice - wherein various

estimates of wind effects corresponding to the same nominal mean recurrence interval can differ by as much as 50 percent.

Some wind engineering professionals perform estimates of structural responses

corresponding to winds blowing from each of a number of sectors. The sectors we

consider here are the half-octants bisected by the NNE, NE, ENE,....,N compass

directions. Those winds are referred to as sectorial wind speeds. In this paper we describe

the estimation of sectorial wind speeds.

This appendix was co-authored by WilHam P. Fritz and Emil Simiu of NIST.

NISTNCSTAR 1-2, WTC Investigation 309
Appendix B

This paper is intended to serve as a contribution to the professional debate that, in our

opinion, is needed to create a robust basis for a consensus on extreme wind estimation.

We present here a procedure for estimating sectorial extreme wind speeds in a region

with both hurricane and non-hurricane winds, and show in some detail a numerical

example illustrating the procedure. To fix the ideas we will consider a site close to New

York City (NYC).

Extreme wind speed data

Hurricane wind speed data. We make use in this note of the NIST simulated hurricane

wind speed database which, to our knowledge, is the only non-proprietary hurricane

database currently in existence. The database is available online at the following link on

the worldwide web: tTp://ftp.nist.gov/pub,-^^^^^ This subdirectory

contains the relevant data sets of simulated hurricane wind speeds in nautical miles per

hour (nmi/hr) at 10 meters above ground in open terrain, averaged over 1-min. There are 55 files with data for locations ranging from milepost 150 (file2.dat; near Port Isabel, TX)

to milepost 2850 (file56.dat; near Portland, ME), spaced at 50 mile intervals. The

structure of each data file is as follows:

Line 1: Milepost identifier, plus other information not needed for the analysis

program.

Line 2: Blank, usually. In some files, the milepost number is repeated here.

Line 3: URATE and NSTRMS. URATE is the esfimated annual rate of

occurrence of hurricanes at and near this milepost, and NSTRMS is the

number of simulated storms used to create the data. For all data sets included in this subdirectory, NSTRMS=999.

Lines 4-1003: The wind speed data for each of the NSTRMS simulated storms. There are a total of 18 numbers on each line. The first 16 are the

maximum wind speeds in 16 specified directions, beginning with NNE

and moving clockwise to N. The 17* number is the maximum wind

speed for ANY direction (i.e., the largest of the previous speeds). The

final number ( 1 8th) number in each line is the storm identifier.

The NIST data sets are based on the "highly regarded work of Batts et al. (1980),"

(unpublished report prepared for Insurance Services Office, Inc., New York City, 1994

by Robert H. Simpson, former director of the National Hurricane Center and creator with

Herbert Saffir of the well-known Saffir-Simpson hurricane intensity scale). A variety of

other hurricane models are currently available, although the data based thereon are, to our knowledge, proprietary. Agreement between wind speeds near the coasthne based on the

NIST data sets and on data sets based on other models is very good. At milestone 2500

(one of the milestones tabulated in Simiu and Scanlan (1996, p. 117) that is closest to New York City), the estimated hurricane mean hourly speeds at 10 m above ground in

open terrain according to Batts et al. (1980), Simiu, Heckert and Whalen (1996) (both

based on the NIST database), Georgiou et al. (1983), and Vickery and Twisdale (1995)

are, respectively, about 30 m/s, 30 m/s, 30 m/s, and 29 m/s for the 50-year speeds, and

45 m/s, 43 m/s, 47 m/s, and 45 m/s for the 2000-year speeds. In evaluating these

differences it should be kept in mind that sampling errors in the estimation of hurricane

wind speeds in the New York City area have estimated coefficients of variation of

310 NISTNCSTAR 1-2, WTC Investigation
Estimation of Sectorial Extreme Wind Speeds

roughly 10% for 50-year speeds and 20% for 500-year speeds (Coles and Simiu, 2003).

Note that the sampling errors depend less on the number of simulated hurricanes in the

database (999 in our case) than on the number of historical hurricanes (about 100) used to obtain statistics of the climatological parameters on which the simulations are based (i.e., radii of maximum wind speeds, atmospheric pressure defect, hurricane translation speed

and direction, and so forth). Those statistics differ relatively little among the various

simulation packages. It is the authors' understanding that hurricane wind speeds for the

State of Florida, corresponding to various probabilities of exceedance, are currently being

estimated by the NOAA Hurricane Research Division. In our opinion it would be

desirable that this effort be expanded to cover all U.S. hurricane-prone regions.

Treatment of hurricane wind speed data. The data listed in the NIST database need to be

rank-ordered for reasons explained subsequently in this note. The rank-ordered data for

the location of interest (file 50, milestone 2550 - nearest to NYC - in the NIST database)

and for the 202.5°and 225° sectors of interest are listed in Table 1. Note that for these

sectors hurricane translation speeds and the relevant vortex speeds within the hurricanes

at and near NYC are in many instances of opposite signs, resulting in relatively small and

therefore negligible, or even vanishing, total hurricane wind speeds. It is therefore

sufficient to show in the table only the largest 55 of the total of 999 data, while keeping

in mind that all the 999 data should be accounted for in the calculations.

Table 1. Rank-ordered wind speeds (nmi/hr at 10m above ground in open terrain,

averaged over 1-min) from NIST database for 202,5°and 225° sectors at milepost 2550.

Rank."! ssw202.

5" sw

225" Rank.j?/ SSW

202.5' SW

Rank.m SSW

202.5° SW

1 88.81 86.73 21 0 29.56 41 0 22.64 74.49 61.79 0 28.96 42 0 21.59

3 73.75 52.37 23 0 28.95 43 0 21.56 4 46.59 47.91 24 0 27.89 44 0 21.25

5 39.68 42.82 25 0 27.79 45 0 20.62

6 17.46 41.97 26 0 27.74 46 0 20.09

7 14.35 41.59 27 0 27.59 47 0 20.04

8 13.81 37.13 28 0 27.35 48 0 19.07 9 13.51 36.4 29 0 27.13 49 0 18.82 10 6.8 35.85 30 0 27.01 50 0 18.55

11 4.88 34.77 31 0 26.63 51 0 16.97 12 3.49 33.64 32 0 26.59 52 0 16.67 13 0 32.41 33 0 26.45 53 0 15.49 14 0 31.79 34 0 25.82 54 0 15.14 15 0 3 1 .75 35 0 25.58 55 0 0

16 0 31.13 36 0 25.28

17 0 30.64 37 0 24.16

18 . 0 30.59 38 0 23.58

19 0 30.01 39 0 23.04 20 0 29.86 40 0 22.98

Non-hurricane extreme wind speed data. In this paper we make use of wind speeds

recorded using ASOS (Automated Surface Observing System) during the period 1983-

2002, made available to NIST by the NOAA's National Climatic Center for three airports near NYC: La Guardia (LGA), Newark International Airport (EWR), and

John F. Kennedy International Airport (JFK). The wind speed data sets include the peak

5-s gust speed multiplied by a factor of 10, for every hour within the period of record, in

NISTNCSTAR 1-2, WTC Investigation 311
Appendix B

m/s. The data were recorded at 20 ft (6.1m) above ground until May 1, 1996 at LGA and

JFK and until July 1 , 1 996 at EWR. They were recorded at 1 0 m above ground thereafter.

Treatment of non-hurricane wind speed data. The results being sought are expressed in

terms of 3-s peak gust speeds at 10 m above ground in open (airport) terrain. Therefore,

all data need to be transformed from 5-s peak gust speeds to 3-s peak gust speeds. This can be done to within a sufficient approximation through multiplication of the 5-s speeds

by a factor of 1.02 (see ASCE 7-02 Standard, Figure C6.2). The data not recorded at 10m

must also be adjusted to correspond to a 10 m elevation above ground. This involves the use of the power law

K(z,)/F(Z2)K'l/-2)' (1)

where, for 3-s peak gust speeds, the exponent d = 1/9.5 for Exposure C (see ASCE 7-02

Standard).

Note that in the data sets each wind speed is associated with the direction from which the

wind is blowing. The directions from which the wind is blowing are measured in a clockwise direction from true north, and are recorded for 36 angles in 10 degree

increments.

Data should be excluded from the analysis if (1) the record provides no direction for a recorded wind speed (this is the case for a relatively small number of speeds), and (2) if the data have a quality code other than 'good', as provided explicitly in the NOAA data

set. Only one measurement at JFK (the maximum speed in the 50° sector in 1987) and

two measurements at LGA (the maximum speeds in the 210° sector in 1983 and in the

200° sector in 1984) had a quality code other than 'good\

Maximum wind speeds are extracted from an airport data set for each of the 36 wind

directions for each year of record. For example, 20 years of maximum hourly wind

speeds produce 36 x 20 values. The dates of major hurricanes of record for NYC during

these 20 years should be checked against the dates of each tabulated maximum wind

speed. Data recorded on September 27 and 28, 1985 (hurricane Gloria) and August 19

and 20, 1991 (hurricane Bob) (Neumann et al., 1993) should not be considered and the

largest ;7o;7-hurricane wind speeds in the records should be used instead.

The 36 directions are reduced through an appropriate scheme to 16 directions that match

the NIST hurricane data. This can be accomplished by defining the wind speed data set associated with, say, the 22.5° sector as the set of maximum yearly wind speeds from the

NOAA data sets for the 10°, 20°, 30° and 40° sectors. This definition is somewhat

conservative, since the 22.5° sector is associated with the narrower sector 11.25° to 33.75°, rather than the sector 5°-45°. However, in our opinion this conservatism is warranted by the fact that the data samples at our disposal are limited to 20 years. A

longer than 20-year data set for the 1 1 .25° to 33.75° sector may contain wind speeds that,

during a 20-year interval, have actually blown within the small sectors 5° to 11.25° and

33.75° to 45°. This minor conservatism affecting wind speeds is an empirical and

312 NISTNCSTAR 1-2, WTC Investigation
Estimation of Sectorial Extreme Wind Speeds

reasonable way of accounting for possible sampling errors with respect to the direction of

extreme speeds, for which to our knowledge no applicable theory is available to date.

Estimates of extreme wind speeds

Estimation of extreme wind speeds regardless of whether they are associated with

hurricanes or non-hurricane winds. Estimates of extreme wind speeds at 10 m above

ground in open terrain at or near the site must take into account both hurricane and nonhurricane winds. We are interested in estimates of sectorial wind speeds, that is, wind

speeds that occur in a specified sector defined by the azimuth of its bisector and the total angle swept by the sector. For specificity, in this note we illustrate our estimates of

sectorial wind speeds for the 22.5° sectors defined by the bisectors with a 202.5° and a 225° azimuth (i.e., for the SSE and SE directions).

Let the probability of non-exceedance of the wind speed v be denoted by P(V<v). This

probability represents the probability that hurricane wind speeds do not exceed v and that non-hurricane wind speeds do not exceed v. Denoting the probability that hurricane wind

speeds do not exceed v by Ph{V<v) and the probability that non-hurricane speeds do not exceed v by Pnh{V<v), and noting that the occurrences of hurricane and non-hurricane

speeds are independent events, we have

The corresponding mean recurrence interval of the wind speed V is, by definition.

Estimation ofprobabilities Ph{V<v). For wind speeds blowing from any one of the 16

compass directions (corresponding to the 16 half-octants) the following procedure is used:

• Extract from the NIST database the hurricane mean rate of arrival {p =

0.305/year) and, for the wind direction of interest, the 999 hurricane wind speed

data for New York City (milestone 2550).

• Rank-order the 999 data. (This was done in Table 1 .) If the hurricane mean arrival rate URATE (henceforth denoted in this paper by /j) was 1/year, the highest speed

would have a 999-year (or approximately 1,000-year) mean recurrence interval.

However, if fj <1, then the mean recurrence interval of the highest speed in the set

is 999/ /2. (For example, if the mean arrival rate were one hurricane every two

years (/v =0.5), then the mean recurrence interval of the highest speed in the set would be 999/0.5=1998, or about 2000 years.)

• The w-th largest speed in the set of 999 speeds corresponds to a mean recurrence

interval N=999/{jU m). For example, if - as is the case for New York City area -

the estimated mean rate of arrival is 0.305, the mean recurrence intervals of the

first highest, second highest, and 65 highest speed are about

999/0.305=3275 years, 999/(0.305 x 2)=1640 years, and 999/(0.305 x 65)=50

years, respectively. Conversely, the hurricane wind speed with an A//y-year mean

NIST NCSTAR 1-2, WTC Investigation 3 1

P(V<v)^ Pm{V<v) PMV<v). (2)

A^=l/[l-P(K<v)]. (3)
Appendix B

recurrence interval corresponds to the w-th largest wind speed in the set, where

w=999/(/yA^/y).

• The probability that this wind speed does not exceed v is defined as follows:

. P„(V<v)=\-\/Nh. - (4)

Other estimation procedures are available, however to date there is no definitive consensus on which procedure is to be preferred. Some analysts believe that extreme

value distributions are inadequate owing to their validity, strictly speaking, under

asymptotic assumptions only; others believe that Weibull distributions are not appropriate

since they are distributions of the smallest values, rather than distributions of the largest

values. In spite of its theoretical non-optimality in terms of the precision of some

estimates, the non-parametric approach used in this paper appears to be relatively noncontroversial and appears to have been adopted by other analysts of hurricane wind

speeds.

Estimation of probabilities Pnh{V<v). The 7VA'//-year mean recurrence interval may be

estimated by using techniques discussed in Simiu and Scanlan (1996, Appendix A 1.7).

Although other distributional models may be adopted, the least controversial model for

extreme wind speeds of non-hurricane origin appears to date to be the Type I extreme

value distribution. The mean recurrence interval associated with the non-hurricane wind

speed V is then

^NH = exp

The mean, v , and standard deviation, 5, are calculated from the yearly maximum wind 3-

s peak gust speeds at 10 m above ground in open terrain for the sector of interest. The

probability that the wind speed, V, does not exceed v is

Pnh{V<v)=\-\/Nnh. . (6)

The requisite probability P{V<v) can be obtained from Eqs. 2, 4, and 6.

Numerical example

We seek the 50-, 500- and 720-year winds blowing from the sectors nominally associated

with the 202.5° and 225" sectors for the area around New York City. We use 20 years of

non-hurricane wind speed data measured at LGA and the NIST hurricane wind speed

data for those sectors. The choice of the LGA data set is commented upon subsequently.

Let us first consider the 3-s peak gust speed F=100 mph at 10 m above ground in open

terrain, and calculate its mean recurrence interval (Eq. 3). Recall that the estimated

hurricane arrival rate at milepost 2550 \s ju^ 0.305/year. The 100 mph, 3-sec gust wind

speed is divided by 1.525 (for conversion to mean hourly speeds), then divided by

1.15 (for conversion to nmi/hr) and finally multiplied by 1.25 (for conversion to 1-min

V -V

0.785 + 0.577 (5)

314 NISTNCSTAR 1-2, WTC Investigation
Estimation of Sectorial Extreme Wind Speeds

averaging time) (see ASCE 7-02 Standard, Figure C6.2). The 1-min speed at 10 m above

ground in open terrain corresponding to the 100 mph peak 3-s speed is therefore 71.3

nmi/hr. This value ranks in Table 1 m = 3.1 and m= ].6 for the 202.5° and 225° sectors,

respectively. The mean recurrence intervals of a 100 mph, 3-sec gust hurricane speed are

therefore:

999 N = 1057 years

rt. 202.5'

A'

0.305(3.1)

999

H.225° = 2047 years

0.305(1.6)

and the probability that the 100 mph, 3-sec wind does not exceed v is

1 ^/..202 5 (100mph,3-s<v) = l-

(100 mph,3-s<v) = l

1057

1 2047

= 0.99905

= 0.99951

Note that if a Poisson-based approach to the estimation of the mean recurrence intervals was adopted, instead of the approach used in this paper, the results would be identical for

practical purposes. The mean recurrence interval obtained by the Poisson-based approach

is 7V=l/{l-exp{-//[/77/(999+l)]}}. This yields 1058 years for 202.5° sector and 2049 years

for the 225° sector.

For non-hurricane winds, maximum hourly wind speeds at LGA airport are shown in

Table 2 for the two directions considered and for each of 20 consecutive years (1983 to 2002). The original speeds in m/s, averaged over 5-sec, and affected by a scale factor of

10 from the NOAA data set are provided in Table 2 along with their converted values in 3-s peak gusts in mph at 10 meters. Also shown are the four directions of the NOAA

data from which the maximum value is drawn for the 202.5° and 225° sectors. The mean

( V ) and standard deviation {s) of each set of 20 values are also provided.

Table 2. Maximum non-hurricane wind

202.5" speeds (mph, 3-s), LaGuardia (LGA).

225'

Year

190".200°,2

0.1 m/s. 5-sec

0°.220° Mph,3-sec

210°.220°

0.1 m/s. 5-sec

230°.240° mph,3-sec 1983 319 77 267 64 1984 268 65 268 65 1985 118 28 108 26

1986 113 27 103 25 1987 170 41 118 28 1988 154 37 134 32 1989 149 36 154 37 1990 154 37 113 27

1991 113 27 149 36

1992 138 33 118 28

1993 128 31 128 31 1994 118 28 128 31 1995 118 28 113 27 1996 154 37 103 24

1997 113 26 149 34

1998 118 27 118 27 1999 144 33 118 27 2000 134 31 1 13 26

2001 123 28 123 28 2002 123 28 123 28 mean

std

35.3

13.0

32.6

11.5

NISTNCSTAR 1-2, WTC Investigation 315
Appendix B

The mean recurrence interval of the 100 mph, 3-sec gust as a non-hurricane wind is therefore:

^NH.202.5' =

100-35.3

0.78(13.0)

100-32.6

+ 0.577

+ 0.577

= 1051 years

= 3265 years

0.78(11.5)

and the probability that a 100 mph, 3-sec wind does not exceed v is

AW. 202.5' (100mph,3-s<v) = l1

NH. (100mph,3-s<v) = l1051

1 3265

= 0.99905

= 0.99969.

In our opinion it would be desirable that a concerted effort be made that would engage

NOAA on the one hand and wind and structural engineering professionals on the other,

aimed at making wind speed observations archived by NOAA available in a suitable, user

friendly format to the structural engineering community. The mean recurrence interval

for the peak 3-s gust 100 mph speed, regardless of whether it is associated with hurricane

or non-hurricane winds, is calculated using Eqs. 2, 4, and 6: = 527 years 202.5° 1-P(100<v) l-(0.99905)(0.99905)

1 1 A^„,. = = = 1250 years. 1-P(100<v) 1- (0.9995 1)(0.99969)

The procedure just described was followed for wind speeds between 60 and 105 mph.

The mean recurrence interval of the wind speeds - regardless of whether they are associated with hurricane or non-hurricane winds - is plotted in Figure 1 for the two

sectors. The mean recurrence intervals for the F=100 mph above are marked with a circle

in the respective plots.

13(B

16CB

1400

12Cn

75 60 85 90 Wind speed (mph)

/

//

/

75 80 85 Wind speed (mph)

(a) (b) Figure 1. Combined mean recurrence intervals as a function of peak 3-s gust wind

speed for the (a) 202.5° and (b) 225° sectors.

316 NISTNCSTAR 1-2, WTC Investigation
Estimation of Sectorial Extreme Wind Speeds

Estimates of the 50-, 500- and 720-year, 3-s peak gust winds are obtained from Figure 1 and are shown in Table 3

Table 3. Estimates of the NYC 50-, 500- and 720-year speeds, regardless of whether

they are associated with hurricane or non-hurricane winds, at 10m above ground in

open terrain for the 202.5° and 225° sectors.

A^-year wind (mph,3-s)

Sector 50-yr 500-yr 720-yr

202.5° 69.8 99.1 104.1

225° 63.0 86.3 91.1

Choice of LGA sectorial data versus EWR and/or JFK sectorial data

The estimated sectorial wind speeds associated with the 202.5° and 225° directions were

found to differ significantly for the LGA and EWR records, on the one hand, and the JFK

record on the other. This may be due to relatively large sampling errors associated with

wind directionality. In view of the uncertainties associated with sectorial wind speeds it appeared prudent to consider the LGA data above, whose variability for the sectors of

interest is largest. Had the EWR data been considered instead, the final results would

have been marginally lower. However, had the JFK results been used, the results would

have been significantly smaller. This is due to the absence in the JFK record of some of

the relatively high wind speeds that are present in the sectors of interest for LGA and

EWR. This is an example of the occurrence of significant sampling errors in a sectorial

wind speed record.

Rather than making use of the LGA data set alone, the analyst may be tempted to use a "super-station" comprising the data from the LGA, EWR, and JFK stations. However, in our opinion this consolidation of the three data sets into one larger data set would provide an inadequate basis for performing more precise estimates. The reason for this statement

is that the three stations are relatively close to each other. The respective wind speed

records are not necessarily independent, and gust speeds contain variabilities associated

with turbulent fluctuations that may mask the actual correlations between the three

records. In our opinion the issue of superstations constructed for stations that are geographically close needs to be researched in the future.

Comparison of extreme w ind speed estimates at the three NYC airports

It was noted in the previous section that sectorial speeds can vary fairly significantly from

station to station. It is of interest to compare extreme wind speed estimates at EWR, JFK

and LGA without regard to wind direction. To do this, maximum wind speeds,

regardless of their direction, are used in the procedure described earlier in lieu of sectorial

wind speeds. That is, we consider hurricane winds from column 17 in file 50 of the NIST

database and maximum yearly non-hurricane winds from the NOAA data set. Thus, nonhurricane data consist of 20 obser\'ations for each of the three NYC airports. Mean

NISTNCSTAR 1-2, WTC Investigation 317
Appendix B

recurrence intervals of wind speeds at each airport, regardless of whether they are associated with hurricane or non-hurricane winds, and regardless of their direction, are plotted in Figure 2. The 50-year 3-s peak gust speed at each airport, regardless of

direction, is 1 12.2 mph.

£ 50

1

ipiy _ — LGA

1

/_ /

y - /

/

60 70 80 90 100 110 120 Wind speed (mph) Figure 2. Mean recurrence intervals of wind speeds - regardless of whether they are

associated with hurricanes or non-hurricane winds, and regardless of direction - for

LGA, EWR, and JFK airports.

For any specified wind speed, the mean recurrence interval is generally shorter for winds

regardless of their direction than for winds blowing from one sector only. The remarkable

agreement between the estimates of extreme wind speeds at the three airports contrasts with the far less satisfactory agreement observed for the sectorial wind speeds. In other

words, sectorial wind speeds appear to exhibit significant sampling errors for which, as mentioned earlier, no applicable theory or research appear to be available to date. This

justifies, in our opinion, the use of the data set among the three available airport data sets

that yields the most conservative results. In light of these remarks, we believe that caution

is also warranted on the use of overly refined schemes for estimating extreme wind

speeds for any one angular sector in approaches to wind directionality effects other than

the sector-by-sector approach, e.g., the up-crossing approach.

Summary and conclusions

We presented a procedure for estimating extreme wind speeds corresponding to a sector- by-sector approach to the estimation of extreme wind effects. We provided details of the

data sets and their treatment, as well as details of the estimates themselves, in a manner

intended to be both clear and transparent. Efforts in the direction of clarity and

transparency are in our view indispensable if estimates of extreme winds and their effects are to meet the need for effective scrutiny by users and building authorities, and if a solid

technical basis for a consensus practitioners, standards organizations, and professional

organizations is to be created in the near future.

318 NISTNCSTAR 1-2, WTC Investigation
Estimation of Sectorial Extreme Wind Speeds

In the authors' opinion it would be desirable (1) that the NOAA's Hurricane Research

Division expand in the future its current efforts aimed at estimating hurricane wind

speeds, with a view to covering all U.S. hurricane-prone regions, and (2) that NOAA's

wind speed archives for non-hurricane wind speeds be made available to the wind and

structural engineering communities in a suitable, user-friendly format to be agreed upon

by NOAA and qualified representatives of those communities.

Acknowledgement

We wish to thank William Brown of the National Climatic Center (National Weather

Service) for providing valuable help on the LaGuardia, Newark International Airport, and

John F. Kennedy International Airport data sets, and information on the anemometer

height history for those sets.

References

American Society of Civil Engineers. ASCE Standard ASCE 7-02, Minimum Design

Loads for Buildings and Other Structures, American Society of Civil Engineers,

Reston, Virginia, 2002.

Batts, M.E., Russell, L.R., and Simiu, E. (1980), "Hurricane Wind Speeds in the United

States," Journal ofthe Structural Division, ASCE 100 200 1-2015.

Coles, S., and Simiu, E. (2003), "Estimating Uncertainty in the Extreme Value Analysis

of Data Generated by a Hurricane Simulation Model," Journal of Engineering

Mechanics 129 1288-1294.

Georgiou, P.N., Davenport, A.G., and Vickery, B.J. (1983), "Design Wind Loads in

Regions Dominated by Tropical Cyclones," Journal of Wind Engineering and

Industrial Aerodynamics 13 139-152.

Neumann, C.J., Jarvinen, B.R., McAdie, C.J., and Elms, J.D. (1993), "Tropical Cyclones

of the North Atlantic Ocean, 1871-1992," Historical Cliniatology Series 6-2,

National Climatic Data Center, Ashville, NC in cooperation with the National

Hurricane Center, Coral Gables, PL.

Simiu, E., Heckert, N.A., and Whalen, T.M. (1996), "Estimates of Hurricane Wind

Speeds by the 'Peaks over Threshold' Method," NJST Technical Note 1416,

National Institute of Standards and Technology, Gaithersburg, MD.

Simiu, E. and Scanlan, R.H. (1996), Wind Effects on Structures, New York: Wiley.

Vickery, P.J. and Twisdale, L.A. (1995), "Prediction of Hurricane Windspeeds in the

U.S.,'''' Journal ofStructural Engineering 121 1691-1699.

NISTNCSTAR 1-2, WTC Investigation 319
Appendix B

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320 NISTNCSTAR 1-2, WTC Investigation
Appendix C

Wind Tunnel Testing and the Sector-By-Sector

Approach to Wind Directionality Effects^

abstract

We examine the sector-by-sector approach used by some wind tunnel operators to specify

extreme wind effects. According to this criterion the design of a structural member

subjected to wind loads is adequate if the stresses induced by the largest sectorial wind

speed with a 50-yr mean recurrence interval does not exceed the maximum allowable

wind-induced stress for that member, sectorial wind speeds with a 50-yr mean recurrence

interval being estimated separately for each of the eight 45° (or the sixteen 22.5°)

azimuthal sectors. We show that this approach leads to estimates of wind effects that are

unconservative (i.e., on the unsafe side), owing to their failure to consider the overall

effects of winds blowing from all sectors. introduction

The sector-by-sector approach to the estimation of wind directionality effects consists of

estimating, separately, the wind speeds with a 50-yr mean recurrence interval (MRI) for

winds blowing from each of the eight 45° sectors of the horizontal plane. Those wind

speeds are referred to as the 50-yr sectorial speeds. For defmiteness we consider the case

of eight 45° sectors and of a 50-yr MRI, but the same definition can be extended for

sixteen 22.5° sectors and any desired MRI.

' To appear in the Journal ofStructural Engineering, ASCE, July, 2005. This appendix was coauthored by

Emil Siiniu, ASCE, NIST Fellow, Structures Group, National Institute of Standards and Technology, Gaithersburg,

MD 20899-861 1, and James J. Filliben, Leader, Statistical Engineering Group, National Institute of Standards and

Technology, Gaithersburg, MD 20899-8980.

NISTNCSTAR 1-2, WTC Investigation 321
Appendix C

Some wind tunnel operators specify wind effects based on the following criterion,

henceforth referred to as the sectorial design criterion: for any given member, the

maximum allowable wind-induced effect, R, (e.g., the maximum allowable wind-induced

stress) must not be exceeded by the largest of the wind effects Qi,so (/'==1,2,..,8) induced by

the eight 50-yr sectorial speeds vj^so- We denote by k the sector where this largest wind

effect, denoted by Qk.so, occurs. The purpose of this work is to show that the sectorial

design criterion is unconservative (i.e., on the unsafe side) relative to the physicallybased criterion, henceforth referred to as the regular design criterion, which states that

the maximum allowable wind-induced effect R should not be exceeded by the 50-year

effect induced by wind blowing from any direction (rather than just from the sector k).

It would be desirable to address this question by making use of the joint extreme

value probability distributions (including correlations) of the wind speeds at the location

of interest. Unfortunately, to our knowledge, expressions for such distributions do not

exist. Bounds for the joint probabilities of interest may be estimated (Simiu et al., 1985;

Simiu, Leigh, and Nolan, 1986), but such an approach can be unwieldy owing to

combinatorial explosion problems. For the purposes of this work, which is addressed to

structural engineers, it also has the drawback of not being sufficiently intuitive.

ASSESSMENT OF THE SECTORIAL DESIGN CRITERION

Intuitive Approach. Let v/ a/, denote the sectorial wind speeds that blow from the sector j

(1<7<8) and cause the allowable wind effect R (the subscript N/ denotes the mean

recurrence interval of the wind speed v/./vy). For J=k we have Nt = 50 years. For ji^k the

mean recurrence intervals A'; exceed 50 years. (If Nj were 50 years or less for any ji^ k.

322 NISTNCSTAR 1-2, WTC Investigation
Wind Tunnel Testing and the Sector-By-Sector Approach to Wind Dir Effts

then R would be attained under sectorial wind speeds vy 50, rather than under the sectorial

wind speed v^.so, which would be contrary to the sectorial design criterion.)

Let Fo{Q<R) denote the probability that the largest yearly wind effect regardless of

the direction from which the wind blows, does not exceed R. If the number of sectors

were limited to one, then we would have, with notations similar to those used earlier,

where V] denotes the wind speed inducing the effect O. In this particular case the sectorial

design criterion would be adequate.

For multi-directionally defined wind speeds and responses the following relation

is consistent with the use of the sectorial design criterion:

in which one of the indexes 7=1, 2, .., 8 has the value A', to which there corresponds the

sectorial speed v^^aa- with Nk=50 years, all other N/s being larger than 50 years. Let us

consider the following three cases: positively correlated speeds, independent speeds, and

negatively correlated speeds. For each of these cases we will examine the probability

F(Q<R). If it were true that F{Q<R)=0.9S, the sectorial design criterion design would be

adequate. If F(Q<R)<0.9S, the design performed in accordance with the sectorial design

criterion would be unconservative. If F{Q<R)>0.9S the opposite would be the case.

Case 1. The speeds vi, V2,..., vg are perfectly, positively correlated. This means that

for all we have vj = a, va-, where «} are constants. Therefore,

Fo{Q<R)=^roh (vi<v5o) = 1 - 1/50=0.98,

Fo{Q<R) = Prob(Vi<Vi.A/i, V2<V2.a'2,..., V8<V8.a'8) (1)

Fo{Q<R) = Prob(vA<VA,5o) (2)

=0.98.

NISTNCSTAR 1-2, WTC Investigation 323
Appendix C

Equation 2 is valid because, by the definition of the sectorial design criterion, the

occurrence of the event va-<va 50 implies the occurrence of the events v/<vy,A// for all j. It

follows that in Case 1 the sectorial design criterion is adequate.

Case 2. The speeds V], V2,..., vg are mutually independent. The mutual correlations

of pairs of sectorial speeds then vanish. This implies

Fo(Q</?) = Prob(vi<vi.A/i, V2<V2,A/2,..., V8<V8./v8) -(3a)

= Prob(vi<vi.A'i) Prob(v2<V2^'2)--- Prob(v8<V8jv8) (3b)

<0.98, (3c)

i.e., the mean recurrence interval of the event Q<R is equal to or less than 50 years. The

inequality (3c) holds because in Eqs. 3, as in Eq. 1, one of the indexes j=\, 2, .., 8 has the

value k, to which there corresponds the sectorial speed VkM with 7Va=50 years, and all

other A'^'s are equal to or larger than 50 years. Consider, for example, the case in which

the effects from one of the sectors were dominant, that is, the mean recurrence interval of

the event that winds from that sector would cause R to be exceeded would be 50 years,

while for the other sectors the corresponding mean recurrence intervals would be much

longer, say 250 years. Then, Fo{Q<R)={i - 1/50) x (1 - 1/250)^ = 0.98 x 0.996^ « 0.95,

corresponding to a mean recurrence interval of the event 0>R equal to 1/(1 - 0.95)=20

years. In other words, the sectorial design criterion would lead to an underestimation of

the wind effect. It is reasonable to expect that this statement remains true even if the

correlations do not vanish but are relatively small.

Case 3. The speeds V], V2,..., V8 have negative correlations. To illustrate the

significance of this case from the point of view of the problem considered in this note, we

consider the model consisting of one die with two sets of numbers, one in blue and one in

324 , NISTNCSTAR 1-2. WTC Investigation
Wind Tunnel Testing and the Sector-By-Sector Approach to Wind Dir Effts

red, as follows. For faces 1, 2, 3, 4, 5, 6, the blue numbers are 1, 2, 3, 4, 5, 6, and the red

numbers are 6, 5, 4, 3, 2, 1, respectively. The correlation coefficient between the red and

blue outcomes is -1. The probability of the event of throwing a 4 or larger number,

regardless of color, is 1 ~ to which there corresponds a mean recurrence inten'al of one

throw. (Blue and red numbers would correspond in our analogy to north and south winds,

say.)

Instead the model just described, we now consider a model consisting of one die with

two sets of numbers, one in blue and one in red, but with the following sets of numbers

for faces 1, 2, 3, 4, 5, 6. Blue: 1, 2, 3, 4, 5, 6, and red: 1, 2, 3, 4, 5, 6, respectively. In this

case the correlation coefficient between the red and blue outcomes is 1 {perfect positive

correlation). The probability of throwing a 4 or larger number, regardless of color, is 1/2,

to which there corresponds a mean recurrence inten'al of two throws, rather than one

throw, as in the case of the die with negative correlation. If exceeding the critical value 4

is undesirable, it is seen that the case of negative correlation is more unfavorable than the

case of positive correlation (the undesirable outcome occurs more frequently in the

former than in the latter case).

It is of interest to also consider the case of throwing two ordinary dice, one with the

blue numbers 1, 2, 3, 4, 5, and 6, and the other with red numbers 1, 2, 3, 4, 5, and 6. In

this case the correlation vanishes, and the probability of getting in a throw of the two

dice an outcome of 4 or larger is 27/36^0.75, i.e., the mean recurrence inten'al of this

outcome is 1.33 throws. Again, this outcome occurs more frequently than in the case of

positive perfect correlation, which is consistent with our earlier comparison between Case

1 and Case 2.

NISTNCSTAR 1-2, WTC Investigation 325
Appendix C

The preceding arguments suggest that considering the case of strongly positive

correlation when the correlation is in fact low or negative would overestimate the mean

recurrence inten'al of the critical event. This statement is valid not only for the cases of

perfect positive correlation and negative or zero correlation. This can be checked by

considering, for example: (a) Instead of a die with perfectly negatively correlated red and

blue outcomes, one in which the blue and the red numbers are 1, 2, 3, 4, 5, 6, and 4, 3, 2,

2, 1, 1, respectively; for this die the correlation coefficient is -0.75, and the mean

recurrence interval of an outcome of 4 or larger, regardless of color, is 1.5 throws, (b)

Instead of the two dice considered earlier, two dice with blue and red numbers 1, 2, 3, 4,

5, 6, and 1, 1, 2, 2, 3, 4; in this case the correlation coefficient is again zero, and the mean

recurrence interval of a blue or red outcome of at least four is 1.7 throws, (c) Instead of

the die with perfectly positive correlation, one in which the blue and red numbers are 1

,

2, 3, 4, 5, 6, and 1, 1, 2, 2, 3, 4, respectively; in this case the correlation coefficient is 0.86

and the mean recurrence interval of an outcome of 4 or larger, regardless of color, is 2

throws. Thus, the mean recurrence interval of this outcome is, again, shorter for both the

uncorrelated case (1.7 throws) and the negatively correlated (1.5 throws) case that it is for

the positively correlated case (2 throws).

Our choice of an intuitive argument is deliberate - it is intended to render our finding

as clear as possible to practicing structural engineers, who may or may not have a

theoretical probabilistic background. More basic probabilistic arguments are now

adduced that strengthen and generalize our finding, without injecting unduly elaborate

probabilistic manipulations.

326 NISTNCSTAR 1-2, WTC Investigation
Wind Tunnel Testing and the Sector-By-Sector Approach to Wind Dir Effts

Probabilistic approach. The advantage of a probabilistic argument is that is it more

general. We invoke the definition of conditional probability .

from which it follows:

P{E,,E^ = P{E,\E^)P{E,)

(5a,b)

^P{E,\E,)P{E,).

In Eqs. 4 and 5 P{E],E2) is the probability of occurrence of both events E] and E2,

P{E]\E2) is the conditional probability of occurrence of event E] given that event E2 has

occurred, PiEz) is the probability of event E2, and similar definitions hold for the second

the above equalities. It follows from Eqs. 5 that

P(EuE2)<mm{P{E,), PiE.)} (6a)

For three events E], E2, and £3, it can be shown that

P{EuE2E,) <min{P(£,),P(£'2),P(£3)}, (7)

By induction, Eq. 7 may be extended for any number of events £„, ( w=l ,2,. . .).

Let the event vy<v/,5o be denoted by Ej. The application of the extension of Eq. 7 for 8

events Ej (i.e., to Eq. 1 ) shows that FoiQ^) ^ 0.98.

Another, more intuitive way of conveying this result is the following. If the structure

was strengthened so that it could fail only in direction k, the return period of the

NISTNCSTAR 1-2, WTC Investigation 327
Appendix C

exceedances of R would be 50 years. Hence for the unstrengthened structure the return

period must be shorter.

CONCLUSION

We conclude that, except for the case of strong positive correlations between sectorial

wind speed - a case that is rarely if ever encountered in nature, - designs based on the

sectorial design criterion underestimate the 50-year wind-induced effects, and are

therefore unconservative (on the unsafe side). Results of calculations based on Bonferroni

bounds (Simiu et al., 1985, and Simiu, Leigh, and Nolan, 1986) are consistent with this

conclusion. However, owing to combinatorial explosion issues those calculations could

not be conducted to the degree of usefulness rendered possible by current computational

capabilities. We believe similar calculations should be performed in the future by using

such capabilities. Pending such calculations, the assumption of independence among

sectorial wind speeds provides a lower bound of the actual mean return period of interest. A rigorous estimation of probabilities Fq{Q<R) by reducing the multidirectional

problem to a one-dimensional problem was described by Rigato, Chang, and Simiu

(2001) for structures with no dynamic amplification effects. A similar solution applicable

for structures exhibiting dynamic effects is in progress.

References

Rigato, A., Chang, P., and Simiu, E., "Database-assisted Design, Standardization, and Wind

Direction Effects," J. 5/ri/c7. £/7g., 127 855-860 (2001).

Simiu, E., Hendrickson, E., Nolan, W., Olkin, I., and Spiegelman, C, "Multivariate Distributions

of Directional Wind Speeds," J. Struct. Eng., Ill 939-943 (1985).

Simiu, E., Leigh, S., and Nolan, W., "Environmental Load Direction and Reliability Bounds," J.

£/7g., 112, 1199-1203 (1986).

328 NISTNCSTAR 1-2, WTC Investigation
Appendix D

SOM Project 2, Progress Report No. 3, WTC Wind

Load Estimates

NISTNCSTAR 1-2, WTC Investigation 329
SOM

NIST - World Trade Center Investigation

PROJECT 2: Baseline Structural Performance and Aircraft Impact Damage Analysis

Progress Report No. 3

WTC Wind Load Estimates

Outside Experts for Baseline Structural Performance

13 April 2004

Skidmore, Owings & Merrill LLP

Suite 1000, 224 South Michigan Avenue, Chicago, Illinois 60604

312 554-9090, Fax 312 360-4545, www.som.com
SOM

1.0 Table of Contents

1 .0 Table of Contents 1 2.0 Overview 2

2.1 Project Overview 2

2.2 Report Ovei-view 2

3.0 NIST-Supplied Documents 2

3 . 1 RWDl Wind Tunnel Reports 2

3.2 Cennak Peterka Petersen, Inc. Wind Tunnel Reports 2

3.3 Conespondence 3 3.4 NIST Report 3

4.0 Discussion and Comments 3

4.1 General 3 4.2 Wind Tunnel Reports and Wind Engineering 3

4.2.1 CPP Wind Tunnel Report 4

4.2.2 RWDl Wind Tunnel Report 4

4.2.3 Building Period used in Wind Tunnel Reports 5

4.2.4 NYCBC Wind Speed 5

4.2.5 Incorporating Wind Tunnel Results in Structural Evaluations 5

4.2.6 Summary 6

4.3 NIST Recommended Wind Loads 6

5.0 References 6

Progress Report No. 3

WTC Wind Load Estimates 1
2.0 Overview

2.1 Project Ovei'\'iew

The objectives for Project 2 of the WTC Investigation include the development of reference

structural models and design loads for the WTC Towers. These will be used to establish the baseline perfonnance of each of the towers under design gravity and wind loading conditions.

The work includes expert review of databases and baseline structural analysis models developed

by others as well as the review and critique of the wind loading criteria developed by NIST.

2.2 Report Overview

This report covers work on the development of wind loadings associated with Project 2. This

task involves the review of wind loading recommendations developed by NIST for use in stmctural analysis computer models. The NIST recommendations are derived from wind tunnel

testing/wind engineering reports developed by independent wind engineering consultants in support of insurance litigation concerning the WTC towers. The reports were provided

voluntarily to NIST by the parties to the insurance litigation.

As the third party outside experts assigned to this Project, SOM's role during this task was to review and critique the NIST developed wind loading criteria for use in computer analysis

models. This critique was based on a review of documents provided by NIST, specifically the wind tunnel/wind engineering reports and associated correspondence from independent wind

engineering consultants and the resulting interpretation and recommendations developed by

NIST.

3.0 NIST-Supplied Documents

3.1 Rowan Williams Davies Irwin (RWDI) Wind Tunnel Reports

Final Report

Wind-Induced Structural Responses

World Trade Center - Tower 1 New York, New York

Project Number: 02-1310A

October 4, 2002

Final Report

Wind-Induced Structural Responses

World Trade Center - Tower 2 New York, New York

Project Number:02-1 31 OB

October 4, 2002

3.2 CeiTnak Peterka Petersen, Inc. (CPP) Wind Tunnel Report

Data Report

Wind-Tunnel Tests - World Trade Center

New York, NY

CPP Project 02-2420

August 2002
SOM 3.3 Correspondence

Letter dated October 2, 2002

From: Peter Irwin/RWDl

To: Matthys Levy/Weidlinger Associates Re: Peer Review of Wind Tunnel Tests World Trade Center

RWDI Reference #02-1310

Weidlinger Associates Memorandum dated March 1 9, 2003

From: Andrew Cheung

To: Najib Abboud

Re: ERRATA to WAI Rebuttal Report

Letter dated September 12, 2003

From: Najib N. Abboud/Hart-Weidlinger

To: S. Shyam Sunder and Fahim Sadek (sic)/NIST Re: Responses to NIST's Questions on: ''Wind-Induced Structural Responses, World Trade

Center, Project Number 02-1 31 OA and 02-1 31 OB

October 2002, By RWDI, Prepared for Hart- Weidlinger

Letter dated April 6, 2004

From: Najib N. Abboud /Weidlinger Associates To: Fahim Sadek and Emil Simiu

Re: Response to NIST's question dated March 30, 2004 regarding "Final Report, WindInduced Structural Responses, World Trade Center - Tower 2, RWDI, Oct 4, 2002"

3.4 NIST Report

Estimates of Wind Loads on the WTC Towers

Emil Simiu and Fahim Sadek

April 7, 2004

4.0 Discussion and Comments

4.1 General

This report covers a review and critique of the NIST recommended wind loads derived from wind

load estimates provided by two independent private sector wind engineering groups, RWDI and

CPP. These wind engineering groups performed wind tunnel testing and wind engineering

calculations for various private sector parties involved in insurance litigation concerning the destroyed WTC Towers in New York. There are substantial disparities (greater than 40%) in the predictions of base shears and base overturning moments between the RWDI and CPP wind

reports. NIST has attempted to reconcile these differences and provide wind loads to be used for

the baseline structural analysis.

Progress Report No. 3

WTC Wind Load Estimates 3
Wind Tunnel Reports and Wind Engineering

The CPP estimated wind base moments far exceed the RWDl estimates. These differences far exceed SOM's experience in wind force estimates for a particular building by independent wind

tunnel groups.

In an attempt to understand the basis of the discrepancies, NIST perfonned a critique of the

reports. Because the wind tunnel reports only summarize the wind tunnel test data and wind

engineering calculations, precise evaluations are not possible with the provided information. For

this reason, NIST was only able to approximately evaluate the differences. NIST was able to numerically estimate some corrections to the CPP report but was only able to make some

qualitative assessments of the RWDI report. It is important to note that wind engineering is an emerging technology and there is not consensus on certain aspects of current practice.

Such aspects include the correlation of wind tunnel tests to full-scale (building) behavior,

methods and computational details of treating local statistical (histoi"ical) wind data in overall predictions of structural response, and types of suitable aeroelastic models for extremely tall and

slender structures. It is unlikely that the two wind engineering groups involved with the WTC

assessment would agree with NIST in all aspects of its critique. This presumptive disagreement

should not be seen as a negative, but reflects the state of wind tunnel practice. It is to be expected

that well-qualified experts will respectfully disagree with each other in a field as complex as wind

engineering.

SOM's review of the NIST report and the referenced wind tunnel reports and correspondence has

only involved discussions with NIST; it did not involve direct communication with either CPP or RWDI. SOM has called upon its experience with wind tunnel testing on numerous tall building

projects in developing the following comments.

4.2.1 CPP Wind Tunnel Report

The NIST critique of the CPP report is focused on two issues: a potential overestimation

of the wind speed and an underestimation of load resulting from the method used for integrating the wind tunnel data with climatic data. NIST made an independent estimate

of the wind speeds for a 720-year return period. These more rare wind events are dominated by hurricanes that are reported by rather broad directional sectors (22.5

degree). The critical direction for the towers is from the azimuth direction of 205 to 210

degrees. This wind direction is directly against the nominal "south" face of the towers

(the plan north of the site is rotated approximately 30 degrees from the true north) and

generates dominant cross-wind excitation from vortex shedding. The nearest sector data

are centered on azimuth 202.5 (SSW) and 225 (SW). There is a substantial drop (12%)

in the NIST wind velocity from the SSW sector to the SW sector. The change in velocity

with direction is less dramatic in the CCP 720-year velocities or in the ARA hurricane

wind roses included in the RWDI report. This sensitivity to directionality is a cause for concern in trying to estimate a wind speed for a particular direction. However, it should

be noted that the magnitude of the NIST interpolated estimated velocity for the 210

azimuth direction is similar to the ARA wind rose. The reduction of forces has been

estimated by NIST based on a square of the velocity, however, a power of 2.3 may be

appropriate based on a comparison of the CPP 50-year (nominal) and 720-year base

moments and velocities.

The NIST critique of the CPP use of sector by sector approach of integrating wind tunnel

and climatic data is fairly compelling. The likelihood of some degree of underestimation
SOM is high but SOM is not able to verify the magnitude of error (15%) which is estimated by

NIST. This estimate would need to be verified by future research, as noted by NIST.

4.2.2 RWDI Wind Tunnel Report

The NIST critique of RWDI has raised some issues but has not directly estimated the

effects. These concerns are related to the wind velocity profiles with height used for hurricanes and the method used for up-crossing.

NIST questioned the profile used for hurricanes and had an exchange of correspondence

with RWDI. While RWDFs written response is not sufficiently quantified to permit a precise evaluation of NlST's concerns, significant numerical corroboration on this issue

may be found in the April 6 letter (Question 2) from N. Abboud (Weidlinger Associates)

to F. Sadek and E. Simiu (NIST).

NIST is also concerned about RWDTs up-crossing method used for integrating wind

tunnel test data and climatic data. This method is computationally complex and

verification is not possible because sufficient details of the method used to estimate the return period of extreme events are not provided.

4.2.3 Building Period used in Wind Tunnel Reports

SOM noted that both wind tunnel reports use fundamental periods of vibrations that exceed those measured in the actual (north tower) buildings. The calculation of building

periods are at best approximate and generally underestimate the stiffness of a building

thus overestimating the building period. The wind load estimates for the WTC towers are

sensitive to the periods of vibration and often increase with increased period as demonstrated by a comparison of the RWDI base moments with and without P-Delta

effects. Although SOM generally recommends tall building design and analysis be based on P-Deha effects, in this case even the first order period analysis (without P-Delta)

exceeds the actual measurements. It would have been desirable for both RWDI and CPP

to have used the measured building periods.

4.2.4 NYCBC Wind Speed

SOM recommends that the wind velocity based on a climatic study or ASCE 7-02 wind

velocity be used in lieu of the New York City Building Code (NYCBC) wind velocity.

The NYCBC wind velocity testing approach does not permit hurricanes to be

accommodated by wind tunnel testing as intended by earlier ASCE 7 fastest mile

versions because it is based on a method that used an importance factor to coiTCCt 50-year

wind speeds for hurricanes. Because the estimated wind forces are not multiplied by an importance factor, this hurricane correction is incorporated in analytical methods of

determining wind forces but is lost in the wind tunnel testing approach of detennining

wind forces.

4.2.5 Incorporating Wind Tunnel Results in Structural Evaluations

It is expected that ASCE 7 load factors will also be used for member forces for evaluating

the WTC towers. Unfortunately, the use of ASCE 7 with wind tunnel-produced loadings

is not straight forward. Neither wind tunnel report gives guidance on how to use the provided forces with ASCE 7 load factors.

Progress Report No. 3

WTC Wind Load Estimates 5
The ASCE 7 load factors are applied to the nominal wind forces and, according to the ASCE 7 commentary, are intended to scale these lower forces up to wind forces associated with long return period wind speeds. The approach of taking 500-year return period wind speeds and dividing the speeds by the square root of 1 .5 to create a nominal

design wind speed; deteraiining the building forces from these reduced nominal design

wind speeds; and then magnifying these forces by a load factor (often 1.6) is, at best,

convoluted. For a building that is as aerodynamically active as the WTC, an approach of

directly detennining the forces at the higher long return period wind speeds would be

prefeiTed. The CPP data did provide the building forces for their estimates of both 720-

years (a load factor of 1 .6) and the reduced nominal design wind speeds. A comparison

of the wind forces demonstrates the potential eiTor in using nominal wind speeds in lieu of directly using the underlying long period wind speeds.

It should also be noted that the analytical method of calculating wind forces in ASCE 7 provides an importance factor of 1 . 1 5 for buildings such as the WTC in order to provide more conservative designs for buildings with high occupancies. Unfortunately, no

similar clear guidance is provided for high occupancy buildings where the wind loads are detennined by wind tunnel testing. Utilizing methods provided in the ASCE 7 Commentary would suggest that a retuin period of 1800 years with wind tunnel-derived

loads would be comparable to the ASCE 7 analytical approach to determining wind loads

for a high occupancy building.

It would be appropriate for the wind tunnel private sector laboratories or NIST, as future research beyond the scope of this project, to address how to incorporate wind tunnel

loadings into an ASCE 7-based design.

4.2.6 Summary

The NIST review is critical of both the CPP and RWDI wind tunnel reports. It finds

substantive eiTors in the CPP approach and questions some of the methodology used by

RWDI. It should be noted that boundary layer wind tunnel testing and wind engineering

is still a developing branch of engineering and there is not industry-wide consensus on all aspects of the practice. For this reason, some level of disagreement is to be expected.

Determining the design wind loads is only a portion of the difficulty. As a topic of future research beyond the scope of this project, NIST or wind tunnel private sector laboratories

should investigate how to incoiporate these wind tunnel-derived results with the ASCE 7 Load Factors.

NIST Recommended Wind Loads

NIST recommends a wind load that is between the RWDI and CPP estimates. The NIST

recommended values are approximately 83% of the CPP estimates and 1 15% of the RWDI

estimates. SOM appreciates the need for NIST to reconcile the disparate wind tunnel results. It

is often that engineering estimates must be done with less than the desired level of infomiation.

In the absence of a wind tunnel testing and wind engineering done to NIST specifications, NIST

has taken a reasonable approach to estimate appropriate values to be used in the WTC study.

However, SOM is not able to independently confirm the precise values developed by NIST.

The wind loads are to be used in the evaluation of the WTC structure. It is therefore recommended that NIST provide clear guidelines on what standards are used in the evaluations

and how they are to incorporate the provided wind loads.
SOM

5.0 References

[1] American Society of Civil Engineers, Minimum Design Loadsfor Buildings and Other

Structures, ANSl/ASCE 7-02, 2002.

[2] American Society of Civil Engineers, Minimum Design Loads for Buildings and Other

Structures, ANSI/ASCE 7-93, 1 993.

Progress Report No. 3

WTC Wind Load Estimates 7

Appendix E

Still Images of the Video Records Used in Chapter 6

This appendix provides still images of the video records (Figures E-1 through E-9) used to estimate the

initial impact conditions of the aircraft that impacted World Trade Center (WTC) 1 and WTC 2

(see Chapter 6). A short description of each of these videos is provided in Table 6-1.

Figure E-1. Still image from Video V1 (WTC 1 impact).

NISTNCSTAR 1-2, WTC Investigation 339
Appendix E

"*3¥al Hlavel 2001 . All rights reserved I

§1 200

46 ^^-TFigure E-2. Still image from Video V2 (WTC 1 impact).

340 NISTNCSTAR 1-2, WTC Investigation
still Images of the Video Records Used in Chapter 7

NISTNCSTAR 1-2, WTC Investigation 341
Appendix E

Figure E-4. Still image from Video V4 (WTC 2 impact).

342 NISTNCSTAR 1-2, WTC Investigation
still Images of the Video Records Used in Chapter 7

Figure E-5. Still image from Video V5 (WTC 2 impact).

NISTNCSTAR 1-2, WTC Investigation 343
Appendix E

Figure E-6. Still image from Video V6 (WTC 2 impact).

344 NISTNCSTAR 1-2, WTC Investigation
Still Images of the Video Records Used in Chapter 7

NISTNCSTAR 1-2, WTC hvestigation 345
Appendix E

Figure E-8. Still image from Video V8 (WTC 2 impact).

346 NISTNCSTAR 1-2, WTC Investigation
still Images of the Video Records Used in Chapter 7

Figure E-9. Still image from Video V9 (WTC 2 impact).

NISTNCSTAR 1-2, WTC Investigation 347
Appendix E

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348 NISTNCSTAR 1-2, WTC Investigation