01 Wind Damages Due to Typhoon Yolanda

7/25/2019 01 Wind Damages Due to Typhoon Yolanda

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1 INTRODUCTIONTyphoons in the Philippines have historically accounted for

a significant amount of damages compared to other types of

natural disasters. (Pacheco 2004; Pacheco et al 2010) Most

recently on November 8, 2013, Typhoon Yolanda

(International Name: Haiyan) made landfall in the country

and was quickly touted as being the strongest landfalling

tropical cyclone on record, at least in terms of wind speed.

(Fischetti 2013) It has claimed a record number of deaths,

and caused widespread catastrophic damage almost

comparable to that caused by Typhoon Pablo of 2012. As a

response to this event, a joint survey team from the

Philippine Institute of Civil Engineers (PICE) and Japan

Society of Civil Engineers (JSCE) conducted post-damage

surveys in several municipalities in Leyte and Samar

affected by Typhoon Yolanda. This brief report is a summary

of findings on wind damages from as far south as Dulag,Leyte to as far east as Balangiga, Easter Samar (Figure 1).

2 DETAILS OF SURVEYThe team members were on-site from December 12 to 16,

2013. The survey team had a two-pronged objective: to

survey the extent of storm surges to validate numerical

models, and to survey damages due to strong wind. In this

regard, the survey team can be divided into two sub-groups,

herein referred to as the “storm surge group” and the “wind

group.”

This report focuses on the survey of damages due to strong

wind done by the wind group. The wind group is primarilycomposed of the three co-authors of this report.

Fig. 1 Surveyed areas. Map data © 2014 Google.

The wind group was accompanied by Engr. Ferdinand

Briones, DPWH District Engineer and officer of the

Association of Structural Engineers of the Philippines

(ASEP), as well as by the survey team’s overall survey

coordinator from the PICE side, Engr. Noel Ortigas, M.PICE,

Vice-President at EDCOP. Table 1 shows the schedule of the

damage survey by the wind group.

It should be noted that given the constraints of having a

limited number of survey dates, the very large affected areas,

BRIEF REPORT ON OBSERVED WIND DAMAGES IN LEYTE AND SAMAR

DUE TO TYPHOON ‘YOLANDA’ OF 2013

Ronwaldo Emmanuel R. Aquino, Ph.D, M.ASEP, M.PICE ¹, William Mata, M.PICE ²,

Justin Joseph Valdez ³

¹ Technical Coordinator, RWDI; Senior Lecturer, University of the Philippines – Institute of Civil Engineering;

E-mail: [email protected]

² Instructor, University of the Philippines – Institute of Civil Engineering

³ Student, University of the Philippines – Institute of Civil Engineering

Abstract: Typhoons in the Philippines have historically accounted for a significant amount of damages compared to other

types of natural disasters. Most recently on November 8, 2013, Typhoon Yolanda (International Name: Haiyan) made landfall

in the country and was quickly touted as being the strongest landfalling tropical cyclone on record. It claimed a record number

of deaths, and caused widespread catastrophic damage almost comparable to that caused by Typhoon Pablo of 2012. As a

response to this event, a joint survey team from the Philippine Institute of Civil Engineers and Japan Society of Civil

Engineers conducted post-damage surveys in several municipalities in Leyte and Samar affected by Typhoon Yolanda on

December 12-16, 2013. This brief report is a summary of findings on wind damages from as far south as Dulag, Leyte to as far

east as Balangiga, Easter Samar. Damages due to wind have been clearly distinguished from damages due to storm surge. A

sampling of photos are provided to describe the types of damages that were observed. A few recommendations for post-storm

reconstruction as lessons from Yolanda’s wind effects are also offered.

Keywords: Typhoon Yolanda, structural wind damage survey, Leyte, Samar, Philippines

Leyte

Samar

Approximate Path

of Typhoon Yolanda

mailto:[email protected]






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and even just after the half-day drive-around on the 1st day

of survey, it was deemed difficult to document the wind

damages as extensively as warranted. Instead, the wind

damage survey sub-group has tried to put focus on a few key

items. First and foremost is to document and assess typical

types of damages. The second is to try to gain clues as to

wind speeds, by gathering information on simple structures

whose capacities can be calculated, which is subsequently

used to back-calculate the wind speed experienced on-site.For brevity, results of the latter objective are not yet

included. It should be additionally noted that the survey was

conducted already more than a month after Typhoon Yolanda

made landfall – a lot of clean-up operations had already been

done.

Table 1 Schedule of wind damage survey sub-group

Day Activities

Day 1 Arrival in Tacloban;

Drive-Around Tacloban and PaloDay 2 Focus on Palo, Leyte

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Day 3 Visit different parts of Eastern and

Western Samar

Day 4 Cities and Municipalities of Tacloban,

Palo, Tolosa, Tanauan, and Dulag

Day 5 Around Tacloban Airport;

Return to Manila1On this day, the storm surge group went as far east to Guian

and Llorente, Eastern Samar

3 PRELIMINARY ESTIMATE OF WIND SPEEDSTo properly assess wind damages to structures, it is

important to gather information on the actual wind speed at

the site, as well as on surrounding terrain information. If the

National Structural Code of the Philippines 6th

ed. or NSCP

2010 (ASEP 2010) is to be used, this wind speed should be a

3-second gust speed at 10 meters height in flat, open country

terrain.

3.1 Reported Wind Speeds

There are some discrepancies in reported wind speeds,although these are usually based only on certain analysis

techniques performed on satellite imagery (presumably of

cloud formation) and not surface measurements. For

example, at 1200 UTC on November 7, 2013, the Japan

Meteorological Agency (JMA) reported that the storm’s 10-

minute maximum sustained winds reached around 230 kph,

while the Hong Kong Observatory (HKO) and the China

Meteorological Administration (CMA) placed it at around

270 kph to 275 kph. Meanwhile, at 1800 UTC on the same

day, the Joint Typhoon Warning Center (JTWC) reported

that the 1-minute sustained wind speed is around 315 kph.

Note that while these are wind speeds at 10-meters height,

they are presumably wind speeds over open ocean, not on-

land, and are not 3-second gust speeds – they are not directly

useable with the NSCP. Note too that the JMA is the official

Regional Specialized Meteorological Center for the Western

Pacific Ocean of the World Meteorological Organization.

On November 8 itself, the person in charge of taking

measurements at the meteorological station at the Tacloban

(DZR) Airport was said to be on-site just before the typhoon

hit. He allegedly observed wind speeds of 230 to 250 kph

(presumably gust wind speeds) – which mean that the actualgust wind speeds are at least 250 kph. Unfortunately, there

were no more measurements during the peak strength of the

storm at the site because there was no one anymore manning

the station, and the station itself was said to have suffered

damages.

In any case, the question here is what is the actual wind

speed due to Typhoon Yolanda that could be used to assess

structures? If, say, JTWC’s 315 kph is taken as the gust wind

speed at 10 meters height on flat, open land, Typhoon

Yolanda would have brought 7,500-year winds to the

affected areas, based on current climate models as used in

the NSCP. The NSCP, in principle, allows for no damage up

to 700-year winds. By ratio of velocities squared, the wind

forces are therefore at least 50% larger than the design wind

speed at the onset of damage. Significant damages are surely

expected, even for some buildings designed to the latest

edition of the NSCP.

3.2 Estimated Wind Speeds Using Fallen Pole Structure

The survey team found a fallen pole in Lawa-an, Easter

Samar, that is relatively covered by different residential

structures before the typhoon struck. (Figure 2)

Fig. 2 Damaged pole structure in Lawa-an, Samar

The estimated bending capacity of the steel pole at the point

of failure (near the base) is around 4 kN-m. Neglecting

combined axial and flexure effects, the lowest realistic wind



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speed estimated is around 360 kph, corresponding to a

terrain factor K d of 1.03 (applied to pressure) and a wind

force coefficient for poles of 1.4 (extrapolated for height-to-

diameter ratio h/ D of 34). This is much higher than the

earlier assumed wind speed of 315 kph. The assumed value

for K d corresponds to a coastline terrain exposure, which is

appropriate for this site. However, the direction of failure of

the pole is towards the sea – i.e. the terrain exposure for the

wind that fell it should be closer to an open terrain. It shouldalso be noted again that there was a building adjacent to this

pole, and across the street is another building still standing.

For its size, this pole is relatively buried in a typical

suburban terrain, albeit just a few meters from the shoreline.

It is also difficult to arrive at a more accurate force

coefficient, given the complex surroundings. It is possible

that the force coefficient is higher than 1.4.

Given the limitations and many assumptions made, it is

therefore recommended that another method is used to verify

this estimated wind speed. The only conclusion that can be

made is that the 3-second gust wind speed (for an equivalent

open country terrain) at 10-meters height seemingly

exceeded 315 kph. Furthermore, it can be said that this is a

very rare, extreme event, at least according to the current

climate model as being used in the NSCP.

4 TYPICAL WIND DAMAGES

4.1 Damages to schools, hospitals, and other public

buildings

Photos of some surveyed schools, hospitals, and other public

buildings are shown in Appendix A. The surveyed schools

were the UP Manila School of Health Sciences (Palo

Campus), Maslog Elementary School, Bolusao Elementary

School, Osmena Elementary School, Telegrafo ElementarySchool and Tanauan Elementary School. The surveyed

hospitals were the Schistosomiasis Research and Training

Center in Palo, Leyte, and Rural Health Unit of Tolosa,

Leyte. The surveyed public buildings were the Daniel Z.

Romualdez Airport Terminal Building, Philippine Ports

Authority (PPA), TESDA Provincial Office, Leyte,

Cathedral of the Transfiguration of Our Lord (Palo), Palo

Municipal Building and the OSCA Office and Day Center

for Senior Citizens.

Note that schools, hospitals, and public buildings were not a

primary objective of the survey; hence a number of other

schools, hospitals, and public buildings are not reported here.

It can be observed from the photos that most of the school

buildings are one-storey structures with large windows used

primarily for ventilation but without any protection from

wind or wind-borne debris, and light metal roof sheets

typically on wooden trusses supported on RC columns. A

roof diaphragm is also typically not present, but this should

not be the primary issue for wind-resistant design of the roof.

Damages to hospitals are similar to the damages to schools,

except it can be seen here that steel roof framing (trusses)

have been used on one relatively newer building and it has

been damaged itself, albeit only partly. It should be noted

that the contents of the floor directly exposed to the

damaged roofs likewise are typically completely damaged.

4.2 Damages to residential structures

Most residential structures have blown away steel roof

sheets, which is the most common roof construction material.

These could either be screwed to steel roof trusses, or most

likely nailed to timber roof trusses. Figure 3 shows two

examples roof failures on residential buildings, whereas the

rest of the (concrete) main structure is still intact. This is

also generally applies to non-residential structures, although

as shown later and also in the example of the OSCA Officeand Day Care for Senior Citizens building in Appendix A,

concrete walls could also be damaged together with the roof

if they were not properly designed for wind actions.

Related to the above, timber structures are the most

devastated. Figure 4 shows one of many residential

buildings with a concrete lower floor and a timber upper

floor, but here shown with a missing upper floor after

suffering heavy wind damage.

Damages to residential structures can also be in the form of

flying or falling hazards such as the example of a fallen tree

adjacent to a residential building in Figure 5.

Another observation is that people have seemingly

reconstructed especially slightly damaged roofs the same

way as pre-storm. There are improvements that can be made

in reconstruction, but the rarity of this event should justify

the economic savings – assuming that for the design (700-

year) events the common types of construction can survive.

Fig. 3 Damaged roof sheets supported on timber trusses

(above) – note the supporting frames are also damaged; and

damaged roof sheets on relatively newer building with steel

roof trusses (below) which are still generally intact.



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Fig. 4 Completely destroyed timber upper floor of

residential building. The lower floor structure survived, but

the contents of the lower floor were likewise affected by the

damaged upper floor.

Fig. 5 Nearby trees as an example of a falling hazard on

adjacent residential and other structures. It is not clear here

but the roof sheet may have been the one that caused the tree

to fall in the first place.

Fig. 6 Large group of fallen trees in Palo, Leyte, all pointing

south (to the left)

4.3 Damages to other structures

As alluded to above, trees and other materials (e.g. roof

sheets) can become falling or flying hazards that could cause

damages to structures. However, trees themselves are

“structures” that can provide livelihood to certain people.

One of the people we interviewed said that her coconut

plantation was wiped out after the storm. Figure 6 shows a

number of coconut trees in Palo, Leyte that have fallen down

due to strong winds. It is also interesting to note that these

trees fell towards the south, which means that the winds that

affected them are winds coming from the north – i.e. even

before Yolanda made landfall, the winds were already strong

enough to make these coconut trees fall down. The easterly

winds north of the eye are typically the strongest winds

within a tropical cyclone system.

Lattice towers such as those used for cellular and other radio

communications were not all spared. Figure 7 shows one

that collapsed – although it should be noted that the

connection design were sufficient; it’s just the strength of the

members that were not sufficient to carry the very extremewind forces. Figure 7 also shows a second photo of a lattice

tower on top of a hill/ridge that appears to have remained

standing, whereas it was just a few hundred meters away

from the fallen lattice tower in the first photo.

As with many residential buildings, many windows, curtain

walls and other cladding elements that were unprotected and

easily damaged during the storm. Figure 8 shows a photo of

the BIR District Office in Leyte, as an example of one with

large, unprotected glass windows that have been damaged

due to strong wind pressures. It is important to protect

windows as sometimes it is through their breakage that

internal pressures rise and increase the uplift pressures on

roofs.

More surprisingly though are the concrete (usually CHB)

walls that have been damaged, such as the one shown in

Figure 9 and also in Appendix A for the OSCA building. It is

possible that these failed because of strong wind pressure

directly applied to their faces and meanwhile they are not

typically designed to withstand such wind pressures. Or it is

possible that the whole structural system (roof and frames)

have been designed properly and deformed, putting stress on

these connected walls. It is also possible for the structure in

Figure 9 that both direct pressures and load redistribution

lead to the collapse of the overall structure.

Fig. 7 Collapsed (above) and still-standing (below) lattice

communication towers within a kilometer of each other



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Fig. 8 Windows are all broken (as well as the roof) on this

BIR District Office building

Fig. 9 Completely collapsed large building with steel roof

trusses supported on RC frames with concrete (CHB) walls

Fig. 10 Open steel roof structure (painted red) originally

providing shade on top of the “open (basketball) court”,

lifted up and tossed to the side

Fig. 11 Seemingly inadequate roof-to-frame connection

Another typically damaged type of structure are “open

court” roofs, such as the one shown in Figure 10, which was

the worst seen by the survey team. The whole steel roof

system was torn from its base, lifted up and tossed to the

side. In other cases (e.g. Figure 11), the roof-to-frame

connection appears to be inadequate. Typically, the steel

rebars are extended out of the concrete column to provide a

hook for the steel roof.

4.4 More photos of damaged structures

There are plenty other damaged structures, including steel

and concrete fences, electrical pole structures, and so on, but

on some occasion there were also a few undamaged

structures which are interesting to study. More photos of all

of these can be found on http://sdrv.ms/JUn8MM. More

photos may be added here in the future, once they become

available. Updates will be disseminated via the Philippine

Institute of Civil Engineers and/or Association of Structural

Engineers of the Philippines.

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CONCLUDING REMARKSMany of the wind damages found are typical of any strong

windstorm, although some of them can be remedied. For

example, broken windows can be prevented by boarding

them up right before the storm, or using shutters to protect

them. It is possible that roof sheets cannot be prevent from

being blown away when an extreme wind event occurs, but

they could become flying hazards that could damage other

structures. Concrete roof deck/slabs could be used as an

alternative which can surely protect from wind, although

their supporting structures would consequently need to be

stronger than when light roof sheets and trusses are used.

The use of timber is strongly discouraged unless testing andnew systems can demonstrate that they are capable of

withstanding winds. Finally, it appears that the “weakest

link ” is almost always the point where structures fail. It

could be the roof sheet screws or nails, or the connection

detailing, or in some cases, it is another element altogether

that is not at all typically designed for wind such as concrete

(CHB) wall systems. The only solution to this is to check all

exposed elements against wind loads, not just roofs and

other lightweight structures. However, it should be noted

that the adequacy of the current wind-resistant design

standard (i.e. the NSCP 2010) has not yet exactly been tested.

If it can be ascertained that all structures conform to the

NSCP 2010 (or at least its predecessor), and yet these

damages still occur, then there is a need for a major revision

of the code. However, such is not yet the case.

ACKNOWLEDGMENTS

The authors express sincere gratitude to the PICE-JSCE

Joint Survey Team members and organizers. The authors

also extend their thanks to Prof. Yukio Tamura, Tokyo

Polytechnic University, for sharing anecdotal information on

wind speed measurements in Tacloban before Typhoon

Yolanda made landfall, as well as to Dr. Gerry Bagtasa, UP

Institute of Environmental Science and Meteorology, for

sharing his knowledge on the reported Yolanda wind speeds.

http://sdrv.ms/JUn8MM





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REFERENCES

Association of Structural Engineers of the Philippines

(2010), National Structural Code of the Philippines, 6th

ed.,

NSCP 2010.

Fischetti, M. (2013). “Was Typhoon Haiyan a Record

Storm?,” Scientific American, November 27, 2013.

Pacheco, B.M. (2004). “Introduction to Disaster Mitigation

and Preparedness Strategies: the DMAPS Program of thePICE,” Proc., PICE 2004 National Midyear Convention,

Davao City, Philippines.

Pacheco, B.M., Aquino, R.E.R. and Tanzo, W.T. (2010).

“Typhoon Engineering Efforts in the Philippines,” Proc.,

Workshop on Wind-Related Disaster Risk Reduction,

Incheon, South Korea.

Wikipedia, “Typhoon Haiyan,” retrieved 20 January 2014,

http://en.wikipedia.org/wiki/Typhoon_Haiyan

APPENDIX A

Photos 1 to 40: Damages to schools

Photos 41 to 53: Damages to hospitals

Photos 54 to 86: Damages to other public buildings

The above are also available on http://sdrv.ms/JUn8MM.








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