別紙41 (課程博士(英文))Date ofsubmission (month day, year)
Depa血nentof
Architecture and civil

StudentlD Number

D189502

SupeTvisors

Yukihiro Matsumoto

ShojiNakazawa

En部neerin
Pplica11t's nameJuly 06小 2021

Fen宮ky satriaYoresta

Tomoya Matsui

Abstract (Doctor)
Title ofThesis

Improvin宮 Bucklin宮 Behavior ofsteelMembers using unbonded carbon Fiber
Reinforced polymer (CFRP) Laminates

Approx.80o words
A caTbon Fiber Reinforced polymer (CFRP) 1aminates has been increasin牙ly favored as amaterial for stren曾thenin曾 Steel structures replacin牙 Conventional methods of bolting or weldin宮add辻ional steel plates. The cFRp is chosen as it 0任ers several advanta曾es such as highStren曾th"to、wei牙ht ratio, exceⅡent fatigue behavior, resistance to corrosion (exce11ent durability),
Ii宮ht wei宮ht, and ease ofhandlin今 and insta11ation.1n recent decades, many research pr0牙rams on
Stren曾thenin牙 Steel structures with cFRp have been conducted. However, the main focus ofthePrevious researches has been limited to stren曾thenin曾 With an adhesive'bondin曾 technique. Thisresearchpr0宮ram investigates a new and promisin曾 method ofstren套thening steelusin牙 Unbonded
CFRp laminates which is manufactured throU牙h a process of vacuum'assisted Resin TI.ansfer

Moldin旦(vaRTM). The main advanta牙e of this stren宮thenin牙 method is that steel surface
treatments are completely n0 10n宮er needed. This method of stren号thenin曾 does not rely on thebond stren套th bet工刃een steeland cFRp because these two materials are separated by an unbonded
Iayer. The cFRp is expected to contribute only through itS 臼.exuralri宮idity because elastic modulusOfcarbon flber is much hi宮her than steel. The proposed unbonded cFRp stren宮thenin曾 is intended
to be used for improving bucklin曾 Performance ofaxialcompression steel members
The 6rstexperiment wasconducted to axialcompression steelbars with a diameter of32 mm.
Atotalofei牙ht specimens were prepared consistin牙 ofsix stren曾thened specimens and two contr01Specimens. The cFRp requirements for stren曾thenin牙 Were derived throU曾h analytical models.
Three conditions considered included stiffness, sttength, and circumferential stren宮th. TheUnbonded cFRp was applied to only a part of length of the steel bar and positioned at center.
Variation ofspecilnens was made for bucklin牙 length, CFRp len曾th, and number of cFRp layers'
However,the numberofspecimens prepared waslimited asitwas apreliminary investi牙ation. The
test resuHs showed that the P王'oposed unbonded cFRp stren号thenin牙 Succeeded to increase thebucklin牙 Capacity of the steel bars. The stren曾thenin宮 effect can even reach almost 50%. Besides
that,the cFRP SU丘ered no dama曾e at aⅡ. Bucklin宮 Curvature ofthe specimen chan宮ed fromplastic
hin牙e at middle bei曾ht ofspecimen to plastic hin曾e at ω'ound the end ofcFRP.
A method of equivalent slenderness ratio was then proposed to determine a desi曾n(recommendation) stren今th ofthe unbonded cFRp stren曾thened specimens. The effectiveness ofthis method was confronted against the performed experimental results and the resuHs ofnumerical simulations developed for 30 strengthened models. R was clear that the proposed
method ofequivalent slenderness ratio provides very 晉ood results and canbe used to determine therecommended stren今th of axial conlpression steel bars strengthened with unbonded cFRP
Iaminates

The next experiment was carried out to an曾le steel. This pr0牙ram aimed to explore thePotential use ofthe pToposed unbonded cFRp to stren曾then a real or lar曾e、scale stee11nember
An窒le steelis chosen because 北 is very popular for lateral resistin牙 element in steel buildin号SThus, it is very prone to buckHng failure. The stren今thenin宮 Scheme was stiⅡ Same with thatapplied to the steel bars where cFRp was applied to cover the entire steel surface alon牙 theStren宮thenin曾 area (an宮le stee1 6.111y・jacketed cross section). Besides that, the cFRp was also
insta11ed at middle span ofthe specimen through the vaRTM process. A total oftwelve specimens,including two control un、stren牙thened an宮le steels, were prepared and tested. They were aⅡOwed
to buckle in their weak axis only. The specimens were divided into two different 牙roups based on
the specimen length, i.e.,1618 and 1218 mm.1n the first 宮roup, in addition to bare steel, threeSpecimens were stren宮thened with looo mm length cFRR and the other three were S仇'engthened
With only 50o mm lengu) CFRP' However,in GrouP 2, with also one controlun、stren牙thened an牙leSteel, the other four specimens were stl'engthened W北h the same cFRp len容th, namely,50o mm.1t
Could be con丘rn].ed that the cFRp created from the vaRTM process in this study has higher flberContent, at approximately 60%. TI〕is deS仇'ibed the advanta曾e ofvaRTM over hand layup process

Which is commonly used in adl〕esive bonding stren曾thening techniques. The test results showedthatthe buckHn曾 Performance ofthe angle steelcan be we11improved. Load、bearin宮 Capacity ofthe
an曾le steel can be increased by 8.5%"54.3%. The increase in load・bearin牙 Capacity occurred asincreasing number of cFRp layers and cFRp len今th. However, for stren号thenin牙 With the same
number of layers and cFRp len宮th, a greater capacity increase was attained in specimens withSlnaⅡer an牙le steel slenderness ratios. validation ofthe effectiveness ofthe equivalent slenderness
ratio method in determining stren宮th Tecommendation of the an宮le steel stren宮thened withUnbonded cFRp showed that this method 宮ives better results. The safety factor fe11 Within therange of l.14'1.34 With coefflcient variations of 2.2%,0.6%, and 5.2% dependin牙 on strengthenin曾Varlatlons

The 6nal investi牙ation also involved an曾le steel with the same properties but its
Stren牙thenin号 Scheme is sli容htly different 丘om the previous l'esearch. The unbonded cFRp wasOnly applied to both legs of the angle steel (an号le steel partia11y・jacketed cross section). This
experimental prograln was conducted to explore another alternative stren牙tbenin号 Scheme thata110ws reduction in amount of cFRp usa牙e. The test results also showed that the proposedUnbonded cFRp stren曾thenin宮 Can improve the buckling performance ofangle steel. The buckling
Capacity ofan牙le steelcould be increased by up to about 69%.
In conclusion, the investigation results presented in this dissertation clearly prove that theProposed unbonded cFRp stren曾thenin宮 method can be used for improvin套 bucklin宮 behavior ofaxial compression steel men)bers. However, there are some topics that have not been explored
Within this study and the author has identified them as recommendations for future research.