Mobile QR Code QR CODE : Journal of the Korean Society of Civil Engineers

  1. ํ™์ต๋Œ€ํ•™๊ต ๊ฑด์„คํ™˜๊ฒฝ๊ณตํ•™๊ณผ ์„์‚ฌ๊ณผ์ •, ๊ณตํ•™์‚ฌ (Hongik University ยท alstjd131@g.hongik.ac.kr)
  2. ์ •ํšŒ์› ยท ํ™์ต๋Œ€ํ•™๊ต ๊ฑด์„คํ™˜๊ฒฝ๊ณตํ•™๊ณผ ๋ฐ•์‚ฌ๊ณผ์ •, ๊ณตํ•™์„์‚ฌ (Hongik University ยท hongju@g.hongik.ac.kr)
  3. ์ข…์‹ ํšŒ์› ยท ๊ต์‹ ์ €์ž ยท ํ™์ต๋Œ€ํ•™๊ต ๊ฑด์„คํ™˜๊ฒฝ๊ณตํ•™๊ณผ ๊ต์ˆ˜, ๊ณตํ•™๋ฐ•์‚ฌ (Corresponding Author ยท Hongik University ยท jwkang@hongik.ac.kr)



์ „์ฒดํŒŒํ˜•์—ญํ•ด์„, ํœจํŒŒ๋™, ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ, Winkler ๊ธฐ์ดˆ, ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹ ๊ตฌ์† ์ตœ์ ํ™”
Full-waveform inversion, Flexural wave propagation, Prestressed lifeline structures, Winkler foundation, Partial differential equation-constrained optimization

1. ์„œ ๋ก 

์†ก์ˆ˜๊ด€ ๋“ฑ ์ง€๋ฐ˜์— ๋งค์„ค๋œ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์€ ๋„์‹œ์™€ ์‚ฐ์—…์˜ ๊ธฐ๋Šฅ์„ ์œ ์ง€ํ•˜๋Š”๋ฐ ์ค‘์š”ํ•œ ์—ญํ• ์„ ํ•œ๋‹ค. ์ด๋Ÿฌํ•œ ๊ตฌ์กฐ๋ฌผ์€ ์ž์—ฐ์žฌํ•ด๋‚˜ ์—ดํ™”์— ๋”ฐ๋ฅธ ๋…ธํ›„ํ™”๋กœ ์ธํ•ด ์†์ƒ๋  ์ˆ˜ ์žˆ์œผ๋ฉฐ, ์ด๋Ÿฌํ•œ ๊ฒฝ์šฐ ๋„์‹œ ๊ธฐ๋Šฅ๊ณผ ์‚ฐ์—… ์ „๋ฐ˜์— ํฐ ์˜ํ–ฅ์„ ๋ฏธ์น  ์ˆ˜ ์žˆ๋‹ค. ํŠนํžˆ ์ง€๋ฐ˜์— ๋งค์„ค๋œ ์ฝ˜ํฌ๋ฆฌํŠธ ๊ด€๋กœ๋Š” ๊ฒฝ๋…„์—ดํ™”์— ๋”ฐ๋ฅธ ์ฝ˜ํฌ๋ฆฌํŠธ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ ์ €ํ•˜๋‚˜ ๋ˆ„์ˆ˜์— ์˜ํ•œ ์ง€๋ฐ˜ ๊ณต๋™์ด ๋ฐœ์ƒํ•˜๊ธฐ๋„ ํ•œ๋‹ค. ๋”ฐ๋ผ์„œ ์ง€์ค‘ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์˜ ์•ˆ์ „์„ฑ์„ ํ™•๋ณดํ•˜๊ธฐ ์œ„ํ•ด์„œ๋Š” ๊ตฌ์กฐ๋ฌผ ๋ฐ ์ง€๋ฐ˜์˜ ์ง€์†์ ์ธ ๋ชจ๋‹ˆํ„ฐ๋ง๊ณผ ์กฐ๊ธฐ ์†์ƒ ํƒ์ง€๊ฐ€ ๋งค์šฐ ์ค‘์š”ํ•˜๋‹ค. ์ง€๋‚œ ์ˆ˜์‹ญ ๋…„๊ฐ„ ๋ณด ๊ตฌ์กฐ๋ฌผ์˜ ์†์ƒ ํ‰๊ฐ€๋ฅผ ์œ„ํ•œ ๋ฐฉ๋ฒ•์œผ๋กœ์„œ ์••์ „๊ธฐ ๊ธฐ๋ฐ˜ ๊ฒ€์‚ฌ๋ฒ•, ์Œํ–ฅ๋ฐฉ์ถœ๋ฒ•, ์ดˆ์ŒํŒŒ ๊ฒ€์‚ฌ๋ฒ• ๋“ฑ ๋‹ค์–‘ํ•œ ๋น„ํŒŒ๊ดดํ‰๊ฐ€ ๊ธฐ๋ฒ•์ด ์ ์šฉ๋˜์–ด ์™”๋‹ค(Lacidogna et al., 2020; Zhang et al., 2016). ์••์ „๊ธฐ ๊ธฐ๋ฐ˜ ๊ฒ€์‚ฌ๋ฒ•์€ ๊ตญ๋ถ€ ์˜์—ญ์˜ ์†์ƒ ํƒ์ง€์— ์žˆ์–ด์„œ ๊ทธ ์„ฑ๋Šฅ์ด ๋›ฐ์–ด๋‚˜์ง€๋งŒ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์ฒ˜๋Ÿผ ๊ฒ€์‚ฌ ๋ฒ”์œ„๊ฐ€ ๋„“์œผ๋ฉด ๊ฐ์ง€ ์˜์—ญ์— ํ•œ๊ณ„๊ฐ€ ์žˆ๋Š” ๊ฒƒ์œผ๋กœ ์•Œ๋ ค์ ธ ์žˆ๋‹ค. ์Œํ–ฅ๋ฐฉ์ถœ๋ฒ•์€ ๊ท ์—ด ๋“ฑ ์†์ƒ ์˜์—ญ์„ ํ†ต๊ณผํ•˜๋Š” ์ŒํŒŒ๋‚˜ ํƒ„์„ฑํŒŒ์˜ ๋ฐ˜ํ–ฅ์„ ๋ถ„์„ํ•˜๋Š” ๊ธฐ๋ฒ•์œผ๋กœ์„œ ๋ณด ๊ตฌ์กฐ๋ฌผ์˜ ๋ฏธ์„ธ ๊ฒฐํ•จ์„ ๊ฐ์ง€ํ•˜๋Š”๋ฐ ์ ํ•ฉํ•˜์ง€๋งŒ ์ผ๋ฐ˜์ ์œผ๋กœ ๊ฒฐํ•จ์˜ ํ˜•ํƒœ๋‚˜ ์œ„์น˜๋ฅผ ์ •ํ™•ํ•˜๊ฒŒ ํƒ์ง€ํ•˜๊ธฐ๋Š” ์‰ฝ์ง€ ์•Š๋‹ค. ์ดˆ์ŒํŒŒ ๊ฒ€์‚ฌ๋ฒ•์€ ๊ณ ์ฃผํŒŒ์ˆ˜ ํŠน์„ฑ์œผ๋กœ ์ธํ•ด ๊ตฌ์กฐ๋ฌผ์˜ ๋‚ด๋ถ€ ๊ท ์—ด๊ณผ ๊ณต๊ทน์„ ํƒ์ง€ํ•˜๋Š”๋ฐ ์ž ์žฌ์  ์„ฑ๋Šฅ์ด ์šฐ์ˆ˜ํ•˜๋‹ค. ๊ทธ๋Ÿฌ๋‚˜ ๊ตฌ์กฐ๋ฌผ์˜ ๋‘๊ป˜๊ฐ€ ๋‘๊บผ์šธ์ˆ˜๋ก ์ŒํŒŒ์˜ ๊ฐ์‡ ๋กœ ์ธํ•ด ํƒ์ง€ ์„ฑ๋Šฅ์ด ์ €ํ•˜๋  ์ˆ˜ ์žˆ๋‹ค. ์ด๋Ÿฌํ•œ ๊ธฐ์กด์˜ ๋น„ํŒŒ๊ดดํ‰๊ฐ€ ๋ฐฉ๋ฒ•๋“ค์€ ๋‹ค์–‘ํ•œ ์‚ฌํšŒ๊ธฐ๋ฐ˜๊ตฌ์กฐ๋ฌผ ๋ฐ ์‚ฐ์—… ์‹œ์„ค์˜ ๊ฑด์ „์„ฑ ํ‰๊ฐ€์— ์„ฑ๊ณต์ ์œผ๋กœ ์ ์šฉ๋˜์–ด ์™”์ง€๋งŒ, ์†ก์ˆ˜๊ด€๊ณผ ๊ฐ™์€ ๋Œ€๊ทœ๋ชจ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์˜ ๊ฑด์ „์„ฑ ํ‰๊ฐ€๋ฅผ ์ œํ•œ๋œ ์‹œ๊ฐ„์— ์ˆ˜ํ–‰ํ•˜๊ธฐ ์œ„ํ•ด์„œ๋Š” ์ƒˆ๋กœ์šด ๋น„ํŒŒ๊ดดํ‰๊ฐ€ ๋ฐฉ๋ฒ•์ด ํ•„์š”ํ•˜๋‹ค.

ํŒŒ๋™ ๋ฐ์ดํ„ฐ๋ฅผ ๋ถ„์„ํ•˜์—ฌ ์‹œ์Šคํ…œ ํŠน์„ฑ์„ ์ถ”์ •ํ•˜๋Š” ์ตœ์‹  ๊ธฐ๋ฒ•์œผ๋กœ์„œ ์ „์ฒดํŒŒํ˜•์—ญํ•ด์„(full-waveform inversion, FWI)์ด ์žˆ๋‹ค(Kang and Kallivokas, 2011; Kang and Pakravan, 2013; Pakravan et al., 2016; 2017; Huh et al., 2023). ์ด ์ค‘ ํƒ„์„ฑํŒŒ FWI ๊ธฐ๋ฒ•์€ ํƒ„์„ฑํŒŒ๋ฅผ ์ด์šฉํ•ด ์‹œ์Šคํ…œ์˜ ํŠน์„ฑ์„ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ๋ฐฉ๋ฒ•์œผ๋กœ์„œ, ๊ตฌ์กฐ๋ฌผ์— ๋Œ€ํ•ด ํˆฌ๊ณผ, ๊ตด์ ˆ, ๋ฐ˜์‚ฌ๋œ ํƒ„์„ฑํŒŒ ๋ฐ์ดํ„ฐ๋ฅผ ์ตœ์ ํ™” ์•Œ๊ณ ๋ฆฌ์ฆ˜์œผ๋กœ ๋ถ„์„ํ•˜์—ฌ ๋ฏธ์ง€์˜ ์‹œ์Šคํ…œ ํŒŒ๋ผ๋ฏธํ„ฐ๋ฅผ ์ถ”์ •ํ•œ๋‹ค(Kang, 2013). ํƒ„์„ฑํŒŒ FWI๋Š” ๊ด‘๋Œ€์—ญ ์ฃผํŒŒ์ˆ˜์˜ ํŒŒํ˜•์„ ์ตœ์ ํ™”ํ•˜์—ฌ ๊ตฌ์กฐ๋ฌผ์˜ ๋ฏธ์ง€ ๋ฌผ์„ฑ ๋ถ„ํฌ๋ฅผ ์žฌ๊ตฌ์„ฑํ•  ์ˆ˜ ์žˆ์œผ๋ฉฐ, ๊ทธ ๊ฒฐ๊ณผ๋กœ ํ•ด๋‹น ๊ตฌ์กฐ๋ฌผ์˜ ์†์ƒ ์ƒํƒœ๋ฅผ ํ‰๊ฐ€ํ•  ์ˆ˜ ์žˆ๋‹ค(He et al., 2021). ์ด๋Ÿฌํ•œ ํŠน์„ฑ์œผ๋กœ ์ธํ•ด FWI ๊ธฐ๋ฒ•์€ ์ง€๋ฐ˜์กฐ์‚ฌ, ์ž์›ํƒ์‚ฌ, ์‚ฌํšŒ๊ธฐ๋ฐ˜๊ตฌ์กฐ๋ฌผ์˜ ์†์ƒ ํ‰๊ฐ€ ๋“ฑ ๋‹ค์–‘ํ•œ ์—”์ง€๋‹ˆ์–ด๋ง ๋ถ„์•ผ์— ํ™œ์šฉ๋˜๊ณ  ์žˆ๋‹ค. ์ตœ๊ทผ์—๋Š” ์›์ž๋ ฅ ๋ฐœ์ „์†Œ ๊ฒฉ๋‚ฉ๊ฑด๋ฌผ ๋‚ด๋ถ€์˜ ๊ณต๋™ ํƒ์ง€์— ์ด ๊ธฐ๋ฒ•์ด ์‚ฌ์šฉ๋œ ๋ฐ” ์žˆ๊ณ (Kim et al., 2021; Kim, 2022) ์ฒ ๋„ ์ž๊ฐˆ๊ถค๋„์˜ ๋…ธ๋ฐ˜ ํŠน์„ฑ ๋ถ„์„์— ํ™œ์šฉ๋˜๊ธฐ๋„ ํ•˜์˜€๋‹ค(Kim et al., 2020). ์ด ๋…ผ๋ฌธ์—์„œ์™€ ์œ ์‚ฌํ•œ ์—ฐ๊ตฌ ์‚ฌ๋ก€๋กœ ์ง€๋ฐ˜์— ๋งค์„ค๋œ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ๋ณด์˜ FWI๊ฐ€ ์žˆ์œผ๋‚˜(Kim, 2023), ์ด๋Š” ๊ธด์žฅ๋ ฅ์ด ์—†๋Š” ๋ณด๋ฅผ ๋Œ€์ƒ์œผ๋กœ ํ•œ ๊ฒƒ์œผ๋กœ์„œ ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์‹ฑ์ด ์ ์šฉ๋œ ์ง€๋ฐ˜ ๋งค์„ค ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์˜ FWI์™€๋Š” ๋ฐฉ๋ฒ•๊ณผ ๊ฒฐ๊ณผ์—์„œ ์ฐจ์ด์ ์ด ์žˆ๋‹ค. ์ด ์—ฐ๊ตฌ๋Š” ์•ž์„œ ์–ธ๊ธ‰ํ•œ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ๋ณด ๋Œ€์ƒ FWI๋ฅผ ๋ฐœ์ „์‹œํ‚จ ๊ฒƒ์œผ๋กœ, ํœจํŒŒ๋™์˜ FWI๋ฅผ ํ†ตํ•ด ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์‹ฑ์ด ์ ์šฉ๋œ ์ง€์ค‘ ์ฝ˜ํฌ๋ฆฌํŠธ ๊ด€๋กœ์˜ ์žฌ๋ฃŒ ๋ฌผ์„ฑ ์žฌ๊ตฌ์„ฑ ๋ฐฉ๋ฒ•์„ ์ œ์‹œํ•œ๋‹ค. ์ง€๋ฐ˜ ๋‚ด ์žฅ๋Œ€ ๊ตฌ์กฐ๋ฌผ์˜ ๋™์  ์‘๋‹ต์„ ํ•ด์„ํ•˜๊ธฐ ์œ„ํ•ด์„œ๋Š” ๊ตฌ์กฐ๋ฌผ๊ณผ ์ง€๋ฐ˜์˜ ์ƒํ˜ธ์ž‘์šฉ์„ ๊ณ ๋ คํ•ด์•ผ ํ•˜๋Š”๋ฐ, ์—ฌ๊ธฐ์„œ๋Š” Winkler ๊ธฐ์ดˆ ๋ชจ๋ธ์„ ์‚ฌ์šฉํ•˜์—ฌ ์ง€๋ฐ˜๊ณผ ๊ด€๋กœ์˜ ๋™์  ์ƒํ˜ธ์ž‘์šฉ์„ ๋ฐ˜์˜ํ•˜์˜€๋‹ค(Molina-Villegas et al., 2022). ์ด๋ ‡๊ฒŒ Winkler ๊ธฐ์ดˆ์™€ ๊ฒฐํ•ฉ๋œ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ๋ณด๋กœ ์ง€์ค‘ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์„ ๋ชจ๋ธ๋งํ•˜๊ณ  ์ถฉ๊ฒฉํ•˜์ค‘์— ๋Œ€ํ•œ ์‹œ๊ฐ„์˜์—ญ ํœจํŒŒํ˜•์„ ์—ญ์‚ฐํ•จ์œผ๋กœ์จ ๊ด€๋กœ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์™€ ์ง€๋ฐ˜๊ฐ•์„ฑ์„ ์žฌ๊ตฌ์„ฑํ•˜์˜€๋‹ค.

์ด ๋…ผ๋ฌธ์˜ ๊ตฌ์„ฑ์€ ๋‹ค์Œ๊ณผ ๊ฐ™๋‹ค. 2์žฅ์—์„œ๋Š” Winkler ๊ธฐ์ดˆ ๋ชจ๋ธ์„ ๊ธฐ๋ฐ˜์œผ๋กœ ์ง€๋ฐ˜์— ๋งค์„ค๋œ ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ๋ณด์˜ ํœจํŒŒ๋™ ์ „ํŒŒ๋ฅผ ๊ณ„์‚ฐํ•˜๋Š” ์ •ํ•ด์„ ๋ฌธ์ œ๋ฅผ ์ •์˜ํ•˜๊ณ  ๊ทธ ๋ณ€๋ถ„์‹์„ ์ œ์‹œํ•œ๋‹ค. 3์žฅ์—์„œ๋Š” ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹ ๊ตฌ์† ์ตœ์ ํ™” ๋ฌธ์ œ๋ฅผ ์ •์‹ํ™”ํ•˜๊ณ  ๋ถ€์ •ํ™• ๋ผ์ธ ํƒ์ƒ‰๋ฒ• ๋ฐ conjugate gradient method (์ผค๋ ˆ๊ธฐ์šธ๊ธฐ๋ฒ•)๋ฅผ ์‚ฌ์šฉํ•ด ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์™€ ์ง€๋ฐ˜๊ฐ•์„ฑ์„ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ๊ณผ์ •์„ ๊ธฐ์ˆ ํ•œ๋‹ค. 4์žฅ๊ณผ 5์žฅ์€ ์ˆ˜์น˜์˜ˆ์ œ๋ฅผ ํ†ตํ•ด ๋ณด์™€ ์ง€๋ฐ˜์˜ ์žฌ๋ฃŒ ๋ฌผ์„ฑ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ๋ฅผ ์ œ์‹œํ•˜๊ณ  ํœจํŒŒ๋™ FWI ๊ธฐ๋ฒ•์˜ ์„ฑ๋Šฅ์„ ํ‰๊ฐ€ํ•œ๋‹ค.

2. ํœจํŒŒ๋™์˜ ์ •ํ•ด์„

์ง€๋ฐ˜ ๋งค์„ค ์†ก์ˆ˜๊ด€ ๋“ฑ ๋‹ค์–‘ํ•œ ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ์ฝ˜ํฌ๋ฆฌํŠธ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์€ Fig. 1๊ณผ ๊ฐ™์ด ์ง€๋ฐ˜๊ฐ•์„ฑ $k(x)$๋กœ ํ‘œํ˜„๋˜๋Š” Winkler ๊ธฐ์ดˆ๋กœ ์ง€์ง€๋˜๊ณ  ๊ธด์žฅ๋ ฅ $P$๊ฐ€ ์ž‘์šฉํ•˜๋Š” ๋‹จ์ˆœ์ง€์ง€ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ๋ณด๋กœ ๋ชจ๋ธ๋งํ•  ์ˆ˜ ์žˆ๋‹ค. ์—ฌ๊ธฐ์„œ $EI$, $A$, $\rho$, $L$์€ ๊ฐ๊ฐ ๋ณด์˜ ํœจ๊ฐ•์„ฑ, ๋‹จ๋ฉด์ , ๋ฐ€๋„, ๊ธธ์ด๋ฅผ ๋‚˜ํƒ€๋‚ด๋ฉฐ, $q(x,\: t)$๋Š” ๋ณด์˜ ํŠน์ • ์˜์—ญ์— ์žฌํ•˜๋˜๋Š” ์ถฉ๊ฒฉํ•˜์ค‘์ด๋‹ค. ์ด๋Ÿฌํ•œ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์€ ์‹ค์ œ๋กœ๋Š” ์—ฐ์† ๊ตฌ์กฐ๋ฌผ์— ๊ฐ€๊นŒ์šฐ๋‚˜ ์ด ์—ฐ๊ตฌ์—์„œ๋Š” ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์‹ฑ์˜ ๋ถ„์ ˆ ์‹œ๊ณต์„ ๊ณ ๋ คํ•˜์—ฌ ์–‘ ๋‹จ์˜ ๊ฒฝ๊ณ„์กฐ๊ฑด์„ ๋‹จ์ˆœ์ง€์ง€๋กœ ์„ค์ •ํ•˜์˜€๋‹ค.

Fig. 1. Modeling of a Prestressed Lifeline Structure Embedded in Soil. (a) Prestressed Pipe Structure, (b) Schematic of a Beam Supported by an Elastic Foundation

../../Resources/KSCE/Ksce.2025.45.3.0313/fig1.png

์ด๋Ÿฌํ•œ ์„ค์ • ํ•˜์—์„œ ๊ธด์žฅ๋ ฅ์„ ๊ณ ๋ คํ•œ ๋ณด-์ง€๋ฐ˜ ์‹œ์Šคํ…œ์˜ ํœจํŒŒ๋™ ์ „ํŒŒ๋Š” Eq. (1)์˜ ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹์„ ํฌํ•จํ•˜๋Š” ์ดˆ๊ธฐ-๊ฒฝ๊ณ„๊ฐ’ ๋ฌธ์ œ๋กœ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ํ‘œํ˜„๋œ๋‹ค.

(1)

$\dfrac{\partial^{2}}{\partial x^{2}}\left(EI\dfrac{\partial^{2}w}{\partial x^{2}}\right)+P\dfrac{\partial^{2}w}{\partial x^{2}}+kw+\rho A\dfrac{\partial^{2}w}{\partial t^{2}}=q(x,\: t)$,

$0 < x < L,\: 0 <t\le T,\:$

(2)
$w(0,\: t)=0$, $0 < t\le T$,
(3)
$\dfrac{\partial^{2}w}{\partial x^{2}}(0,\: t)=0$, $0 < t\le T$,
(4)
$w(L,\: t)=0$, $0 < t\le T$,
(5)
$\dfrac{\partial^{2}w}{\partial x^{2}}(L,\: t)=0$, $0 < t\le T$,
(6)
$w(x,\: 0)=0$, $0\le x\le L$,
(7)
$\dfrac{\partial w}{\partial t}(x,\: 0)=0$, $0\le x\le L$.

์—ฌ๊ธฐ์„œ $w(x,\: t)$๋Š” ๋ณด์˜ ์ฒ˜์ง์ด๊ณ , Eqs. (2)~(5)๋Š” ๊ฒฝ๊ณ„์กฐ๊ฑด, Eqs. (6)~(7)์€ ์ดˆ๊ธฐ์กฐ๊ฑด์ด๋‹ค. ์œ„ ์ดˆ๊ธฐ-๊ฒฝ๊ณ„๊ฐ’ ๋ฌธ์ œ์˜ ๋ณ€๋ถ„์‹์€ ํŒŒ๋™๋ฐฉ์ •์‹ (1)์— ์‹œํ—˜ํ•จ์ˆ˜ $v(x)\in H_{0}^{2}(\omega)$๋ฅผ ๊ณฑํ•˜๊ณ  ์ด๋ฅผ ์ „ ๊ตฌ๊ฐ„ $\omega =\{x \vert 0\le x\le L\}$์— ๋Œ€ํ•ด ์ ๋ถ„ํ•˜์—ฌ ์–ป์„ ์ˆ˜ ์žˆ์œผ๋ฉฐ, ๊ทธ ๊ฒฐ๊ณผ๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™๋‹ค.

(8)

$\int_{0}^{L}EI\dfrac{\partial^{2}v}{\partial x^{2}}\dfrac{\partial^{2}w}{\partial x^{2}}dx +\int_{0}^{L}P\dfrac{\partial v}{\partial x}\dfrac{\partial w}{\partial x}dx +\int_{0}^{L}kvwdx$

$+\int_{0}^{L}\rho Av\dfrac{\partial^{2}w}{\partial t^{2}}dx =\int_{0}^{L}vq dx$.

์ด ์‹์—์„œ ๋ณด์˜ ์ฒ˜์ง $w(x,\: t)$์™€ ์‹œํ—˜ํ•จ์ˆ˜ $v(x)$๋Š” $-1\le\xi\le 1$์˜ ๋งˆ์Šคํ„ฐ ์ขŒํ‘œ๊ณ„์—์„œ ์ •์˜๋˜๋Š” 3์ฐจ Hermite ํ˜•์ƒํ•จ์ˆ˜๋ฅผ ์‚ฌ์šฉํ•˜์—ฌ ๊ทผ์‚ฌํ•  ์ˆ˜ ์žˆ๋‹ค. ์ด๋Ÿฌํ•œ ๊ทผ์‚ฌํ•จ์ˆ˜๋ฅผ Eq. (8)์— ๋Œ€์ž…ํ•˜๋ฉด ์š”์†Œ๋ณ„ ์ฒ˜์ง $w$์™€ ์ฒ˜์ง๊ฐ $w'$์œผ๋กœ ๊ตฌ์„ฑ๋˜๋Š” ๋ฏธ์ง€๋ฒกํ„ฐ ${u}_{e}\left(=\left[{w}_{1}{w}_{1}'{w}_{2}{w}_{2}'\right]^{{T}}\right)$์— ๋Œ€ํ•œ ์ค€์ด์‚ฐ ํ˜• ์šด๋™๋ฐฉ์ •์‹ ${M}_{e}\ddot{{u}}_{e}+{K}_{e}{u}_{e}={F}_{e}$๋ฅผ ์–ป์„ ์ˆ˜ ์žˆ๋‹ค.

(9)
$({M}_{e})_{ij}=\int_{-1}^{1}\rho A\psi_{i}\psi_{j}\dfrac{h}{2}d\xi$,
(10)

$({K}_{e})_{{ij}=}\int_{-1}^{1}EI\dfrac{d^{2}\psi_{i}}{d\xi^{2}}\dfrac{d^{2}\psi_{j}}{d\xi^{2}}\left(\dfrac{2}{h}\right)^{3}d\xi -\int_{-1}^{1}P\dfrac{d\psi_{i}}{d\xi}\dfrac{d\psi_{j}}{d\xi}\dfrac{2}{h}d\xi$

$+\int_{-1}^{1}k\psi_{i}\psi_{j}\dfrac{h}{2}d\xi$,

(11)
$({F}_{e})_{{i}}=\int_{-1}^{1}q\psi_{i}\dfrac{h}{2}d\xi$.

์—ฌ๊ธฐ์„œ $\psi_{i}(\xi)$ ๋ฐ $\psi_{j}(\xi)$๋Š” 3์ฐจ Hermite ํ˜•์ƒํ•จ์ˆ˜์ด๊ณ  $i,\: j = 1,\: 2,\: 3,\: 4$์ด๋‹ค. ${M}_{e}$, ${K}_{e}$, ${F}_{e}$๋Š” ๊ฐ๊ฐ ๊ธธ์ด $h$์ธ ์š”์†Œ์˜ ์งˆ๋Ÿ‰ํ–‰๋ ฌ, ๊ฐ•์„ฑํ–‰๋ ฌ, ํž˜ ๋ฒกํ„ฐ์ด๋‹ค.

3. ์‹œ๊ฐ„์˜์—ญ ํƒ„์„ฑํŒŒ ์ „์ฒดํŒŒํ˜•์—ญํ•ด์„์˜ ์ •์‹ํ™”

3.1 ๋ฌผ์„ฑ์น˜ ์ถ”์ •์„ ์œ„ํ•œ ์—ญํ•ด์„ ๋ฌธ์ œ์˜ ์ •์˜

Fig. 1๊ณผ ๊ฐ™์€ ํƒ„์„ฑ ์ง€์ง€ ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ๋ณด์—์„œ ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ $E(x)$์™€ Winkler ์ง€๋ฐ˜๊ฐ•์„ฑ $k(x)$๋ฅผ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ์—ญํ•ด์„ ๋ฌธ์ œ๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๋ชฉ์ ํ•จ์ˆ˜ $J$๋ฅผ ์ตœ์†Œํ™”ํ•˜์—ฌ $E(x)$์™€ $k(x)$์˜ ์ตœ์ ํ•ด๋ฅผ ๊ตฌํ•˜๋Š” ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹ ๊ตฌ์† ์ตœ์ ํ™” ๋ฌธ์ œ๋กœ ์ •์‹ํ™”๋  ์ˆ˜ ์žˆ๋‹ค.

(12)

$\begin{align*} \begin{aligned}\min J(w,\: E,\: k)\\E,\: k\end{aligned}:= & F_{m}+R_{E}(E)+R_{k}(k)\\ \\ = &\dfrac{1}{2}\sum_{i=1}^{N_{r}}\int_{0}^{L}\int_{0}^{T}[w(x,\: t)-w_{m}(x,\: t)]^{2}\delta(x-x_{i})dtdx \end{align*}$

$+R_{E}(E)+R_{k}(k).$

์ด ์ตœ์ ํ™” ๋ฌธ์ œ๋Š” Eqs. (1)~(7)์˜ ํŒŒ๋™๋ฐฉ์ •์‹, ๊ฒฝ๊ณ„์กฐ๊ฑด, ์ดˆ๊ธฐ์กฐ๊ฑด์„ ๊ตฌ์†์กฐ๊ฑด์œผ๋กœ ํ•œ๋‹ค. $w_{m}(x,\: t)$๋Š” $N_{r}$๊ฐœ์˜ ์„ผ์„œ์—์„œ ์ธก์ •๋œ ๋ณด์˜ ์ฒ˜์ง ์‘๋‹ต์ด๋‹ค. $F_{m}$์€ ๊ณ„์‚ฐ์‘๋‹ต๊ณผ ์ธก์ •์‘๋‹ต ์ฐจ์ด์˜ ์ œ๊ณฑ์œผ๋กœ ํ‘œํ˜„๋˜๋Š” ๋ถˆํ•ฉ์น˜๋Ÿ‰์ด๊ณ , ๋ชฉ์ ํ•จ์ˆ˜ $J$๋Š” ์ด ๋ถˆํ•ฉ์น˜๋Ÿ‰๊ณผ ์ •๊ทœํ™”ํ•ญ $R_{E}(E)$, $R_{k}(k)$๋กœ ๊ตฌ์„ฑ๋œ๋‹ค. ์œ„์˜ ์ตœ์ ํ™” ๋ฌธ์ œ๋ฅผ ๋น„๊ตฌ์† ์ตœ์ ํ™” ๋ฌธ์ œ๋กœ ์ „ํ™˜ํ•˜๊ธฐ ์œ„ํ•˜์—ฌ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๋ผ๊ทธ๋ž‘์ฃผ ์Šน์ˆ˜ $\lambda_{w}$๋ฅผ ๋„์ž…ํ•ด ๋ผ๊ทธ๋ž‘์ง€์•ˆ ๋ฒ”ํ•จ์ˆ˜๋ฅผ ๊ตฌ์„ฑํ•œ๋‹ค.

(13)

$L(w,\: \lambda_{w},\: E,\: k)=J(w,\: E,\: k)$

$+\int_{0}^{L}\int_{0}^{T}\lambda_{w}\left[\dfrac{\partial^{2}}{\partial x^{2}}\left(EI\dfrac{\partial^{2}w}{\partial x^{2}}\right)+P\dfrac{\partial^{2}w}{\partial x^{2}}+kw+\rho A\dfrac{\partial^{2}w}{\partial t^{2}}\right]dtdx$.

์ด ์—ฐ๊ตฌ์—์„œ๋Š” Tikhonov (TN) ์ •๊ทœํ™” ๊ธฐ๋ฒ•์„ ์‚ฌ์šฉํ•˜์˜€์œผ๋ฉฐ, ์ •๊ทœํ™”ํ•ญ $R_{E}(E)$์™€ $R_{k}(k)$ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ์ •์˜๋œ๋‹ค.

(14)
$R_{E}(E):=\dfrac{R_{E}}{2}\int_{0}^{L}\left(\dfrac{d E}{dx}\right)^{2}dx$, $R_{k}(k):=\dfrac{R_{k}}{2}\int_{0}^{L}\left(\dfrac{dk}{dx}\right)^{2}dx$.

์—ฌ๊ธฐ์„œ $R_{E}$์™€ $R_{k}$๋Š” ์ •๊ทœํ™”๊ณ„์ˆ˜์ด๋ฉฐ, ์ •๊ทœํ™”๊ณ„์ˆ˜ ์—ฐ์†๊ธฐ๋ฒ•์„ ์‚ฌ์šฉํ•ด ๊ทธ ๊ฐ’์„ ๊ฒฐ์ •ํ•  ์ˆ˜ ์žˆ๋‹ค(Kim, 2023).

3.2 1์ฐจ ์ตœ์ ํ™” ์กฐ๊ฑด

๋ผ๊ทธ๋ž‘์ฃผ ์Šน์ˆ˜ $\lambda_{w}$, ์ƒํƒœ๋ณ€์ˆ˜ $w$, ์ œ์–ด๋ณ€์ˆ˜ $E$ ๋ฐ $k$์— ๋Œ€ํ•œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ $L$์˜ 1์ฐจ ๋ณ€๋ถ„์‹์ด 0์ด ๋˜๋Š” 1์ฐจ ์ตœ์ ํ™” ์กฐ๊ฑด์œผ๋กœ๋ถ€ํ„ฐ ๊ฐ๊ฐ ์ƒํƒœ๋ฌธ์ œ, ์ˆ˜๋ฐ˜๋ฌธ์ œ, ์ œ์–ด๋ฌธ์ œ๋ฅผ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค. ์ฒซ์งธ๋กœ, ๋ผ๊ทธ๋ž‘์ฃผ ์Šน์ˆ˜ $\lambda_{w}$์— ๋Œ€ํ•œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ์˜ 1์ฐจ ๋ณ€๋ถ„์‹์ด 0์ด ๋˜๋Š” ์กฐ๊ฑด $\left(\delta_{\lambda_{w}}L=0\right)$์œผ๋กœ๋ถ€ํ„ฐ ์ •ํ•ด์„ ๋ฌธ์ œ์˜ ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹์ธ Eq. (1)์ด ์œ ๋„๋˜๋ฉฐ, ์ด๋Š” ๊ฒฝ๊ณ„ ๋ฐ ์ดˆ๊ธฐ์กฐ๊ฑด (2)~(7)๊ณผ ํ•จ๊ป˜ ์ƒํƒœ๋ฌธ์ œ๋ฅผ ๊ตฌ์„ฑํ•œ๋‹ค.

โ€ข ์ƒํƒœ๋ฌธ์ œ: Eq. (1)~(7)๋กœ ์ •์˜๋˜๋Š” ์ดˆ๊ธฐ-๊ฒฝ๊ณ„๊ฐ’ ๋ฌธ์ œ์™€ ๊ฐ™์Œ.

๋‘˜์งธ๋กœ, ์ƒํƒœ๋ณ€์ˆ˜ $w$์— ๋Œ€ํ•œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ์˜ 1์ฐจ ๋ณ€๋ถ„์‹์ด 0์ด ๋˜๋Š” ์กฐ๊ฑด $\left(\delta_{w}L=0\right)$์œผ๋กœ๋ถ€ํ„ฐ ์•„๋ž˜์™€ ๊ฐ™์ด $\lambda_{w}$์— ๊ด€ํ•œ ์ˆ˜๋ฐ˜๋ฌธ์ œ๋ฅผ ๊ตฌ์„ฑํ•  ์ˆ˜ ์žˆ๋‹ค.

โ€ข ์ˆ˜๋ฐ˜๋ฌธ์ œ:

(15)

$\dfrac{\partial^{2}}{\partial x^{2}}\left(EI\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}\right)+P\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}+k\lambda_{w}+\rho A\dfrac{\partial^{2}\lambda_{w}}{\partial t^{2}}=$

$-\sum_{i=1}^{N_{r}}[w(x,\: t)-w_{m}(x,\: t)]\delta(x-x_{i})$,

$0 < x < L,\: 0\le t < T$,

(16)
$\lambda_{w}(0,\: t)=0$, $0\le t < T$,
(17)
$\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}(0,\: t)=0$, $0\le t < T$,
(18)
$\lambda_{w}(L,\: t)=0$, $0\le t < T$,
(19)
$\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}(L,\: t)=0$, $0\le t < T$,
(20)
$\lambda_{w}(x,\: T)=0$, $0\le x\le L$,
(21)
$\dfrac{\partial\lambda_{w}}{\partial t}(x,\: T)=0$, $0\le x\le L$.

Eq. (15)๋Š” ์ˆ˜๋ฐ˜๋ฌธ์ œ์˜ ์ง€๋ฐฐ๋ฐฉ์ •์‹์ด๊ณ  Eqs. (16)~(19)๋Š” ๊ฒฝ๊ณ„์กฐ๊ฑด์ด๋ฉฐ Eqs. (20)~(21)์€ ์ตœ์ข… ์‹œ๊ฐ $T$์—์„œ์˜ ์ตœ์ข…์กฐ๊ฑด์ด๋‹ค.

์…‹์งธ๋กœ, ์ œ์–ด๋ณ€์ˆ˜ $E$ ๋ฐ $k$์— ๋Œ€ํ•œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ์˜ 1์ฐจ ๋ณ€๋ถ„์‹์ด 0์ด ๋˜๋Š” ์กฐ๊ฑด $\left(\delta_{E}L=0,\: \delta_{k}L=0\right)$์œผ๋กœ๋ถ€ํ„ฐ ๋‹ค์Œ๊ณผ ๊ฐ™์€ ์ œ์–ด๋ฌธ์ œ๋ฅผ ์–ป์„ ์ˆ˜ ์žˆ๋‹ค.

โ€ข ํƒ„์„ฑ๊ณ„์ˆ˜ $E$์˜ ์ œ์–ด๋ฌธ์ œ:

(22)
$-R_{E}\dfrac{d^{2}E}{dx^{2}}+\int_{0}^{T}I\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}\dfrac{\partial^{2}w}{\partial x^{2}}dt=0$, $0 < x < L$,
(23)
$\dfrac{d E}{dx}(0)=\dfrac{d E}{dx}(L)=0$.

โ€ข ์ง€๋ฐ˜๊ฐ•์„ฑ $k$์˜ ์ œ์–ด๋ฌธ์ œ:

(24)
$-R_{k}\dfrac{d^{2}k}{dx^{2}}+\int_{0}^{T}\lambda_{w}w dt=0$, $0 < x < L$,
(25)
$\dfrac{dk}{dx}(0)=\dfrac{dk}{dx}(L)=0$.

Eqs. (22), (23)๊ณผ Eqs. (24), (25)๋Š” ๊ฐ๊ฐ $E(x)$์™€ $k(x)$์˜ ์ตœ์ ํ•ด๋ฅผ ์œ„ํ•œ ๊ฒฝ๊ณ„๊ฐ’ ๋ฌธ์ œ์ด๋‹ค. ์œ„์˜ ์ƒํƒœ๋ฌธ์ œ, ์ˆ˜๋ฐ˜๋ฌธ์ œ, ์ œ์–ด๋ฌธ์ œ๋กœ ๊ตฌ์„ฑ๋œ ๋น„์„ ํ˜• ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹ ์‹œ์Šคํ…œ์€ ์ด ์ตœ์ ํ™” ๋ฌธ์ œ์— ๋Œ€ํ•œ ์—ฐ์†์  ํ˜•ํƒœ์˜ KKT (Karush-Kuhn-Tucker) ์กฐ๊ฑด์ด๋ฉฐ, ์ด ์กฐ๊ฑด์„ ๋งŒ์กฑํ•˜๋Š” $E(x)$์™€ $k(x)$๊ฐ€ ์ด ์—ญํ•ด์„ ๋ฌธ์ œ์˜ ์ตœ์ ํ•ด์ด๋‹ค. ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{E}$, $R_{k}$๋Š” ์ •๊ทœํ™”๊ณ„์ˆ˜ ์—ฐ์†๊ธฐ๋ฒ•์„ ์‚ฌ์šฉํ•˜์—ฌ ๊ฒฐ์ •ํ•  ์ˆ˜ ์žˆ๋‹ค(Kim, 2023). ์ด ์—ฐ์†๊ธฐ๋ฒ•์€ ์ •๊ทœํ™”๊ณ„์ˆ˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{E}$์™€ $\varepsilon_{k}$๋ฅผ ์‚ฌ์šฉํ•ด ์—ญํ•ด์„์˜ ๋ฐ˜๋ณต๊ณ„์‚ฐ๋งˆ๋‹ค ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{E}$์™€ $R_{k}$๋ฅผ ๊ณ„์‚ฐํ•˜๋ฉฐ, ์ด ๊ฐ€์ค‘์น˜๋Š” 0์—์„œ 1 ์‚ฌ์ด์˜ ๊ฐ’์„ ๊ฐ€์ง„๋‹ค.

3.3 ์ตœ์ ํ™” ์•Œ๊ณ ๋ฆฌ์ฆ˜

์ƒํƒœ, ์ˆ˜๋ฐ˜, ์ œ์–ด๋ฌธ์ œ๋ฅผ ์—ฐ๋ฆฝํ•˜์—ฌ ํ’€๋ฉด $w$, $\lambda_{w}$, $E$, $k$์˜ ์ตœ์ ํ•ด๋ฅผ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค. ๊ทธ๋Ÿฌ๋‚˜ ์ด ๊ณผ์ •์€ ๊ณ„์‚ฐ์ด ๋ณต์žกํ•˜๋ฏ€๋กœ ์ตœ์ ํ™” ๋ณ€์ˆ˜๋ฅผ ์ œ์–ด๋ณ€์ˆ˜ $E(x)$์™€ $k(x)$๋กœ ๊ตญํ•œํ•˜๊ณ  ์ƒํƒœ, ์ˆ˜๋ฐ˜, ์ œ์–ด๋ฌธ์ œ๋ฅผ ๋ฐ˜๋ณตํ•ด์„œ ํ’€์–ด $E(x)$์™€ $k(x)$์˜ ์ตœ์ ํ•ด๋ฅผ ๊ณ„์‚ฐํ•  ์ˆ˜ ์žˆ๋‹ค. ์ด๋ฅผ ์œ„ํ•ด ๋จผ์ € $E(x)$์™€ $k(x)$๋ฅผ ๊ฐ€์ •ํ•˜๊ณ  ๋™์  ํ•˜์ค‘์— ๋Œ€ํ•œ ์ƒํƒœ๋ฌธ์ œ (1)~(7)์˜ ํ•ด $w(x,\: t)$๋ฅผ ๊ตฌํ•œ๋‹ค. ์ด ์ƒํƒœ๋ฌธ์ œ์˜ ํ•ด๋ฅผ ์ด์šฉํ•˜์—ฌ ์ˆ˜๋ฐ˜๋ฌธ์ œ (15)~(21)์˜ ํ•ด $\lambda_{w}(x,\: t)$๋ฅผ ๊ณ„์‚ฐํ•œ๋‹ค. ์ด๋ ‡๊ฒŒ ๊ตฌํ•œ $w$์™€ $\lambda_{w}$๋ฅผ ์ด์šฉํ•ด ์ œ์–ด๋ณ€์ˆ˜ $E$, $k$์— ๋Œ€ํ•œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ์˜ ๊ทธ๋ž˜๋””์–ธํŠธ๋ฅผ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๊ณ„์‚ฐํ•œ๋‹ค.

(26)
$\nabla_{E}L=-R_{E}\dfrac{d^{2}E}{dx^{2}}+\int_{0}^{T}I\dfrac{\partial^{2}\lambda_{w}}{\partial x^{2}}\dfrac{\partial^{2}w}{\partial x^{2}}dt$,
(27)
$\nabla_{k}L=-R_{k}\dfrac{d^{2}k}{dx^{2}}+\int_{0}^{T}\lambda_{w}w dt$.

Eqs. (26)๊ณผ (27)์— ์ œ์‹œ๋œ ๊ทธ๋ž˜๋””์–ธํŠธ๋Š” ๊ฐ๊ฐ ์ œ์–ด๋ฌธ์ œ์˜ ์ง€๋ฐฐ๋ฐฉ์ •์‹์ธ Eqs. (22)์™€ (24)์˜ ์ขŒ๋ณ€๊ณผ ๊ฐ™๋‹ค. $E$์™€ $k$๋ฅผ ์—…๋ฐ์ดํŠธํ•˜๊ธฐ ์œ„ํ•œ $i$๋ฒˆ์งธ ์—ญํ•ด์„ ๋ฐ˜๋ณต๊ณ„์‚ฐ์—์„œ์˜ ์ด์‚ฐ ๊ทธ๋ž˜๋””์–ธํŠธ ๋ฒกํ„ฐ ${g}_{i}^{E}$ ๋ฐ ${g}_{i}^{k}$๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™์ด $E$์™€ $k$์˜ ๋…ธ๋“œ๋ณ„ ๊ทธ๋ž˜๋””์–ธํŠธ ๊ฐ’์œผ๋กœ ๊ตฌ์„ฑ๋œ๋‹ค.

(28)
${boldg}_{i}^{E}=(\nabla_{E}L)_{i}$,
(29)
${g}_{i}^{k}=(\nabla_{k}L)_{i}$.

์ด ๊ทธ๋ž˜๋””์–ธํŠธ ๋ฒกํ„ฐ๋ฅผ ์ด์šฉํ•˜์—ฌ $E(x)$์™€ $k(x)$์˜ ๋…ธ๋“œ ๊ฐ’์œผ๋กœ ๊ตฌ์„ฑ๋˜๋Š” $i$๋ฒˆ์งธ ๋ฐ˜๋ณต๊ณ„์‚ฐ์—์„œ์˜ ์ œ์–ด๋ณ€์ˆ˜ ๋ฒกํ„ฐ ${bold E}_{i}$ ๋ฐ ${k}_{i}$๋ฅผ ์—…๋ฐ์ดํŠธํ•  ์ˆ˜ ์žˆ๋‹ค. ์ด๋ฅผ ์œ„ํ•œ ํƒ์ƒ‰๋ฐฉํ–ฅ ๋ฒกํ„ฐ ${bold d}_{i}^{E}$, ${bold d}_{i}^{k}$๋Š” CG ๊ธฐ๋ฒ•์— ์˜ํ•ด ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๊ตฌํ•œ๋‹ค.

(30)
${d}_{i}^{E}=\begin{cases} -{g}_{i}^{E},\: &(i=0),\: \\ -{g}_{i}^{E}+\dfrac{{g}_{i}^{E}\bullet{g}_{i}^{E}}{{g}_{i-1}^{E}\bullet{g}_{i-1}^{E}}{d}_{i-1}^{E},\: &(i\ge 1),\: \end{cases}$
(31)
${d}_{i}^{k}=\begin{cases} -{g}_{i}^{k},\: &(i=0),\: \\ -{g}_{i}^{k}+\dfrac{{g}_{i}^{k}\bullet{g}_{i}^{k}}{{g}_{i-1}^{k}\bullet{g}_{i-1}^{k}}{d}_{i-1}^{k},\: &(i\ge 1). \end{cases}$

Eqs. (30)๊ณผ (31)๋กœ๋ถ€ํ„ฐ ํƒ์ƒ‰๋ฐฉํ–ฅ ๋ฒกํ„ฐ๊ฐ€ ๊ฒฐ์ •๋˜๋ฉด ๋‹ค์Œ๊ณผ ๊ฐ™์ด $i + 1$๋ฒˆ์งธ ๋ฐ˜๋ณต๊ณ„์‚ฐ์—์„œ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์™€ ์ง€๋ฐ˜๊ฐ•์„ฑ ๋ฒกํ„ฐ๋ฅผ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค.

(32)
${E}_{i+1}={E}_{i}+\alpha^{E}{d}_{i}^{E}$,
(33)
${k}_{i+1}={k}_{i}+\alpha^{k}{d}_{i}^{k}$.

์—ฌ๊ธฐ์„œ $\alpha^{E}$์™€ $\alpha^{k}$๋Š” ๊ฐ๊ฐ ${d}_{i}^{E}$์™€ ${d}_{i}^{k}$ ๋ฐฉํ–ฅ์œผ๋กœ์˜ ์Šคํ…๊ธธ์ด์ด๋‹ค. ์ด ์—ฐ๊ตฌ์—์„œ๋Š” $i$๋ฒˆ์งธ ์—ญํ•ด์„ ๋ฐ˜๋ณต๊ณ„์‚ฐ์—์„œ์˜ ์Šคํ…๊ธธ์ด๋ฅผ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๊ณ„์‚ฐํ•˜์˜€๋‹ค.

(34)
$\alpha^{E}=\dfrac{1}{s_{p}}\dfrac{\left |{E}_{i}\right |}{\left |{d}_{i}^{E}\right |}$,
(35)
$\alpha^{k}=\dfrac{1}{s_{p}}\dfrac{\left |{k}_{i}\right |}{\left |{d}_{i}^{k}\right |}$.

์œ„ ์‹์—์„œ $s_{p}$๋Š” $i$๋ฒˆ์งธ ๋ฐ˜๋ณต๊ณ„์‚ฐ์—์„œ ์žฌ๋ฃŒ ๋ฌผ์„ฑ ๋ฒกํ„ฐ์™€ ํƒ์ƒ‰๋ฐฉํ–ฅ ๋ฒกํ„ฐ์˜ ์œ ํด๋ฆฌ๋“œ๋†ˆ์˜ ๋น„์— ๊ณฑํ•ด์ง€๋Š” ์กฐ์ •๊ณ„์ˆ˜์ด๋ฉฐ, $s_{p}$์˜ ๊ฐ’์ด ํด์ˆ˜๋ก ์Šคํ…๊ธธ์ด๋Š” ๊ฐ์†Œํ•œ๋‹ค. ์ด ์—ฐ๊ตฌ์—์„œ๋Š” ์ œ์–ด๋ณ€์ˆ˜์˜ ์ˆ˜๋ ด ์†๋„๋ฅผ ์ ์ • ์ˆ˜์ค€์œผ๋กœ ์œ ์ง€ํ•˜๊ธฐ ์œ„ํ•˜์—ฌ 100์—์„œ 10,000 ์‚ฌ์ด์˜ $s_{p}$ ๊ฐ’์„ ์‚ฌ์šฉํ•˜์˜€๋‹ค.

4. ์ˆ˜์น˜์˜ˆ์ œ

๊ธด์žฅ๋ ฅ์ด ์ ์šฉ๋œ ์ง€์ค‘ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์™€ ์ง€๋ฐ˜๊ฐ•์„ฑ์„ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ์ˆ˜์น˜์˜ˆ์ œ๋กœ์„œ Fig. 2์™€ ๊ฐ™์ด Winkler ๊ธฐ์ดˆ๋กœ ์ง€์ง€๋˜๋Š” ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ๋‹จ์ˆœ์ง€์ง€๋ณด๋ฅผ ๊ณ ๋ คํ•˜์˜€๋‹ค. ๋ณด์˜ ๊ธธ์ด $L = 40{m}$์ด๊ณ  ๋ฐ€๋„ $\rho =2000{kg}/{m}^{3}$, ๋‹จ๋ฉด์  $A=0.1{m}^{2}$, ๋‹จ๋ฉด2์ฐจ๋ชจ๋ฉ˜ํŠธ $I=3.33\times 10^{-4}{m}^{4}$์ด๋ฉฐ, ๊ธด์žฅ๋ ฅ $P = 800{k N}$์œผ๋กœ ์„ค์ •ํ•˜์˜€๋‹ค. ๋ณด๋Š” ๊ธธ์ด๊ฐ€ 0.05 m์ธ $C^{1}$ ์š”์†Œ๋กœ ์ด์‚ฐํ™”ํ•˜์˜€์œผ๋ฉฐ, ๊ทธ ๊ฒฐ๊ณผ ์š”์†Œ์˜ ์ด ์ˆ˜๋Š” 800๊ฐœ์ด๋‹ค. ๋ณด์˜ ๋™์  ์ฒ˜์ง์„ ๊ณ„์ธกํ•˜๋Š” ์„ผ์„œ๋Š” ๋ณด์˜ ๋ชจ๋“  ๋…ธ๋“œ์— ์œ„์น˜ํ•˜๋Š” ๊ฒƒ์œผ๋กœ ์„ค์ •ํ•˜์˜€๋‹ค.

๋ณด์˜ ์ค‘์•™์— ๊ฐ€์šฐ์‹œ์•ˆ ํ˜•ํƒœ์˜ ์ถฉ๊ฒฉํ•˜์ค‘ $q(x,\: t)=$$p(t)\delta(x-20)$์„ ๊ฐ€ํ•ด ๊ตฌํ•œ ์„ผ์„œ์—์„œ์˜ ๋™์  ์ฒ˜์ง ์‘๋‹ต์„ ๊ณ„์ธก ๋ฐ์ดํ„ฐ๋กœ ํ™œ์šฉํ•˜์˜€๋‹ค. ์ด ๊ณ„์ธก ๋ฐ์ดํ„ฐ๋Š” $E(x)$์™€ $k(x)$๊ฐ€ ๋ชฉํ‘œ ํ”„๋กœํŒŒ์ผ๊ณผ ๊ฐ™๋‹ค๋Š” ๊ฐ€์ • ํ•˜์— ๊ตฌํ•œ ์ฒ˜์ง์˜ ์ˆ˜์ง€ํ•ด์ด๋‹ค. $\delta$๋Š” Dirac-delta ํ•จ์ˆ˜์ด๊ณ , ์ด ์ˆ˜์น˜์˜ˆ์ œ์— ์‚ฌ์šฉ๋œ ๊ฐ€์šฐ์‹œ์•ˆ ํ•˜์ค‘ $p(t)$๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™๋‹ค.

(36)
$p(t)= P_{0}e^{-\dfrac{(t-t_{0})^{2}}{2s^{2}}}$.

์œ„ ์‹์—์„œ $P_{0}= 1{k N}$, $t_{0}= 0.01{s}$, $s = 0.001{s}$์ด๊ณ  ์ด ์žฌํ•˜ ์‹œ๊ฐ„์€ 0.5์ดˆ, ์‹œ๊ฐ„ ๊ฐ„๊ฒฉ $\triangle t=10^{-4}{s}$๋กœ์„œ ์ด ํ•˜์ค‘ ์‹ ํ˜ธ์˜ ์ตœ๋Œ€ ์ฃผํŒŒ์ˆ˜๋Š” 600 Hz์ด๋‹ค. Fig. 3์€ ํ•˜์ค‘์˜ ์‹œ๊ฐ„์ด๋ ฅ, ์ฃผํŒŒ์ˆ˜ ์ŠคํŽ™ํŠธ๋Ÿผ, ์ƒ˜ํ”Œ๋ง ์œ„์น˜ SP1, SP2, SP3์—์„œ์˜ ์ธก์ •๋œ ๊ณ„์ธก์‘๋‹ต์„ ๋‚˜ํƒ€๋‚ธ๋‹ค.

์—ญํ•ด์„์„ ํ†ตํ•ด ์žฌ๊ตฌ์„ฑ๋œ ๋ฌผ์„ฑ๊ฐ’์˜ ์˜ค์ฐจ๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™์ด ์ •๊ทœํ™”๋œ $L^{2}$๋†ˆ ์˜ค์ฐจ๋ฅผ ์‚ฌ์šฉํ•˜์—ฌ ๊ณ„์‚ฐํ•  ์ˆ˜ ์žˆ๋‹ค.

(37)
${\vert \vert}e{\vert \vert}_{2}=\dfrac{\int_{0}^{{L}}\left({p}({x})-{p}_{{tg}}(x)\right)^{2}dx}{\int_{0}^{L}\left(p_{{tg}}(x)\right)^{2}dx},\: {p}={E}{or}k$.

์œ„ ์‹์—์„œ $p_{{tg}}(x)$๋Š” ํƒ„์„ฑ์ง€๋ฐ˜์œผ๋กœ ์ง€์ง€๋˜๋Š” ๋ณด์˜ ๋ชฉํ‘œ ํ”„๋กœํŒŒ์ผ์ด๊ณ , $p(x)$๋Š” ์žฌ๊ตฌ์„ฑ๋œ ํ”„๋กœํŒŒ์ผ์ด๋‹ค. ๋ณ€์ˆ˜ $p$๋Š” ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ $E$ ๋˜๋Š” ์ง€๋ฐ˜๊ฐ•์„ฑ $k$๋ฅผ ๋‚˜ํƒ€๋‚ธ๋‹ค.

Fig. 2. An Elastically Supported Prestressed Euler-Bernoulli Beam Subjected to a Concentrated Load at Its Center

../../Resources/KSCE/Ksce.2025.45.3.0313/fig2.png

Fig. 3. Gaussian Pulse Load $p(t)$ and the Beamโ€™s Deflection at Sampling Points. (a) Time History, (b) Frequency Spectrum, (c) Measured Deflections

../../Resources/KSCE/Ksce.2025.45.3.0313/fig3.png

4.1 ํƒ„์„ฑ๊ณ„์ˆ˜ E(x)์˜ ์žฌ๊ตฌ์„ฑ

์ฒซ ๋ฒˆ์งธ ์ผ€์ด์Šค๋Š” ์ง€๋ฐ˜๊ฐ•์„ฑ $k(x)$๋ฅผ ์•Œ ๋•Œ ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ $E(x)$๋ฅผ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ๋ฌธ์ œ์ด๋‹ค. ์ง€๋ฐ˜๊ฐ•์„ฑ $k(x)$๋Š” ์ผ๋ฐ˜์ ์ธ ์ ํ†  ์ง€๋ฐ˜์„ ๊ฐ€์ •ํ•ด $100{k N}/{m}^{2}$๋กœ ์„ค์ •ํ•˜์˜€๋‹ค. ๋ชฉํ‘œ ํƒ„์„ฑ๊ณ„์ˆ˜๋Š” ์•„๋ž˜์™€ ๊ฐ™์€ ๊ณ„๋‹จ์‹ ๋ถ„ํฌ๋กœ ์„ค์ •ํ–ˆ์œผ๋ฉฐ, ์ด ์ค‘ ๋‘ ๋ฒˆ์งธ์™€ ๋„ค ๋ฒˆ์งธ ์˜์—ญ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜๋Š” ๋‹ค๋ฅธ ์˜์—ญ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์ธ 15 GPa ๋Œ€๋น„ ๊ฐ๊ฐ 17 %์™€ 33 % ๋งŒํผ ์ž‘๋‹ค.

(38)
$E(x)=\begin{cases} 15{GPa},\: 0\le x< 10{m},\: \\ 12.5{GPa},\: 10\le x< 15{m},\: \\ 15{GPa},\: 15\le x< 25{m},\: \\ 10{GPa},\: 25\le x< 35{m},\: \\ 15{GPa},\: 35\le x\le 40{m}. \end{cases}$

Figs. 4~7์€ ๊ฐ๊ฐ ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{E}$์˜ ๊ฐ€์ค‘์น˜(Kim, 2023)๊ฐ€ $\varepsilon_{E}$ = 0.2, 0.4, 0.6, 0.8์ผ ๋•Œ์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์ด๋‹ค. ์ •๊ทœํ™”๊ณ„์ˆ˜์˜ ๊ฐ€์ค‘์น˜๊ฐ€ ๋†’์„์ˆ˜๋ก ์ •๊ทœํ™”๊ณ„์ˆ˜๊ฐ€ ํฌ๊ฒŒ ๊ณ„์‚ฐ๋˜๋ฉฐ, ์ด์— ๋”ฐ๋ผ ์ œ์–ด๋ณ€์ˆ˜์˜ ๋ถˆ๊ทœ์น™ ๋ณ€๋™์„ ์ค„์ด๋Š” ์ •๊ทœํ™” ํšจ๊ณผ๊ฐ€ ํฌ๊ฒŒ ์ž‘์šฉํ•œ๋‹ค. ํƒ„์„ฑ๊ณ„์ˆ˜์˜ ์ดˆ๊ธฐ ๊ฐ€์ •๊ฐ’์€ 10 GPa์ด๋ฉฐ, 500๋ฒˆ์˜ ๋ฐ˜๋ณต๊ณ„์‚ฐ์„ ํ†ตํ•ด ๋ชฉํ‘œ ํ”„๋กœํŒŒ์ผ์ด ์„ฑ๊ณต์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑ๋˜์—ˆ๋‹ค. ํƒ„์„ฑ๊ณ„์ˆ˜๊ฐ€ ๋‚ฎ์€ ๋‘ ์˜์—ญ์˜ ํญ, ๊ฒฝ๊ณ„์™€ ํƒ„์„ฑ๊ณ„์ˆ˜ ๊ฐ’์ด ํšจ๊ณผ์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑ๋˜์—ˆ์Œ์„ ์•Œ ์ˆ˜ ์žˆ๋‹ค. ์—ญํ•ด์„์˜ ๋ฐ˜๋ณต๊ณ„์‚ฐ์„ ๊ฑฐ์น˜๋ฉด์„œ ์ธก์ •์‘๋‹ต๊ณผ ๊ณ„์‚ฐ์‘๋‹ต์˜ ์˜ค์ฐจ์ธ $F_{m}$์€ ๋ชจ๋“  $\varepsilon_{E}$์˜ ๊ฒฝ์šฐ์— ๋Œ€ํ•ด ์ดˆ๊ธฐ๊ฐ’์˜ 0.07 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค.

Fig. 8(a)๋Š” ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{E}$์˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{E}$ ๊ฐ’์— ๋”ฐ๋ฅธ $F_{m}$์˜ ๋ณ€ํ™”๋ฅผ ๋‚˜ํƒ€๋‚ธ๋‹ค. ์ผ๋ฐ˜์ ์œผ๋กœ ์ •๊ทœํ™”๊ณ„์ˆ˜์˜ ๊ฐ€์ค‘์น˜๊ฐ€ ์ž‘์œผ๋ฉด ์—ญํ•ด์„์ด ๋‹ค์ˆ˜์˜ ์ง€์—ญํ•ด๋ฅผ ๊ฐ–๋Š” ๋ฌธ์ œ์ ์ด ๋ฐœ์ƒํ•  ์ˆ˜ ์žˆ๋‹ค. ์ด ๋ฌธ์ œ์—์„œ๋Š” $\varepsilon_{E}$์˜ ๊ฐ’์ด ์ž‘์„์ˆ˜๋ก ๋ถˆํ•ฉ์น˜๋Ÿ‰ $F_{m}$์˜ ๊ฐ์†Œ ์†๋„๋Š” ์ƒ๋Œ€์ ์œผ๋กœ ๋น ๋ฅธ ๊ฒƒ์œผ๋กœ ๋‚˜ํƒ€๋‚ฌ๋‹ค. Fig. 8(b)๋Š” Eq. (37)๋กœ ๊ณ„์‚ฐ๋œ $E(x)$ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์˜ $L^{2}$๋†ˆ ์˜ค์ฐจ๋ฅผ ๋‚˜ํƒ€๋‚ธ๋‹ค. 500ํšŒ ๋ฐ˜๋ณต๊ณ„์‚ฐ ํ›„ $\varepsilon_{E}$ = 0.2, 0.4, 0.6, 0.8์˜ ๊ฒฝ์šฐ์— ๋Œ€ํ•ด ${\vert \vert}e{\vert \vert}_{2}$๊ฐ€ ๊ฐ๊ฐ ์ดˆ๊ธฐ๊ฐ’์˜ ์•ฝ 0.11 %, 0.12 %, 0.09 %, 0.13 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค. ์ด๋กœ๋ถ€ํ„ฐ ์ด ๋ฌธ์ œ์—์„œ๋Š” ํƒ„์„ฑ๊ณ„์ˆ˜ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์˜ $L^{2}$๋†ˆ ์˜ค์ฐจ๊ฐ€ ์ •๊ทœํ™” ์ •๋„์— ๊ด€๊ณ„์—†์ด ์ถฉ๋ถ„ํžˆ ๊ฐ์†Œํ•จ์„ ์•Œ ์ˆ˜ ์žˆ๋‹ค.

Fig. 4. Inversion Results for the Step Profile of $E(x)$ Using the Regularization Weight Factor $\varepsilon_{E}$ = 0.2. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig4.png

Fig. 5. Inversion Results for the Step Profile of $E(x)$ Using the Regularization Weight Factor $\varepsilon_{E}$ = 0.4. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

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Fig. 6. Inversion Results for the Step Profile of $E(x)$ Using the Regularization Weight Factor $\varepsilon_{E}$ = 0.6. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig6.png

Fig. 7. Inversion Results for the Step Profile of $E(x)$ Using the Regularization Weight Factor $\varepsilon_{E}$ = 0.8. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig7.png

Fig. 8. Response Misfit during the Inversion for $E(x)$ and the Normalized $L^{2}$ Error of the Reconstructed $E(x)$ Profile. (a) $F_{m}$ Up to 200 Iterations, (b) ${\vert \vert}e{\vert \vert}_{2}$ for $E(x)$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig8.png

4.2 ์ง€๋ฐ˜๊ฐ•์„ฑ k(x) ์žฌ๊ตฌ์„ฑ

๋‘ ๋ฒˆ์งธ ์ผ€์ด์Šค๋Š” ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ $E(x)$๋ฅผ ์•Œ ๋•Œ Winkler ์ง€๋ฐ˜๊ฐ•์„ฑ $k(x)$๋ฅผ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ๋ฌธ์ œ์ด๋‹ค. ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜ $E(x)$๋Š” 10 GPa๋กœ ์„ค์ •ํ•˜์˜€๋‹ค. ๋ณด์˜ ์ค‘์•™๋ถ€์— ์ง€๋ฐ˜ ์นจ์‹์ด ๋ฐœ์ƒํ•˜์˜€๋‹ค๊ณ  ๊ฐ€์ •ํ•˜์˜€์œผ๋ฉฐ, ์ด๋ฅผ ๋ฐ˜์˜ํ•ด ๋ชฉํ‘œ $k(x)$ ํ”„๋กœํŒŒ์ผ์„ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๋ณด์˜ ์ค‘์•™๋ถ€๋กœ ๊ฐˆ์ˆ˜๋ก ์ ์ง„์ ์œผ๋กœ ๊ฐ์†Œํ•˜๋Š” ์„ ํ˜• ๊ฒฝ์‚ฌ ํ”„๋กœํŒŒ์ผ๋กœ ์„ค์ •ํ•˜์˜€๋‹ค.

(39)
$k(x)=\begin{cases} 100{k N}/{m}^{2},\: 0\le x\le 10{m},\: \\ 100 - 6(x-10){k N}/{m}^{2},\: 10\le x\le 15{m},\: \\ 70{k N}/{m}^{2},\: 15\le x\le 25{m},\: \\ 70 + 6(x-25){k N}/{m}^{2},\: 25\le x\le 30{m},\: \\ 100{k N}/{m}^{2},\: 30\le x\le 40{m}. \end{cases}$

Figs. 9~12๋Š” ๊ฐ๊ฐ ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{k}$์˜ ๊ฐ€์ค‘์น˜๊ฐ€ $\varepsilon_{k}$ = 0.2, 0.4, 0.6, 0.8์ผ ๋•Œ์˜ ์ง€๋ฐ˜๊ฐ•์„ฑ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์ด๋‹ค. $k(x)$์˜ ์ดˆ๊ธฐ ๊ฐ€์ •๊ฐ’์€ 50 kN/m2์ด๋‹ค. ์—ญํ•ด์„ ๊ฒฐ๊ณผ, ์นจ์‹์ด ์กด์žฌํ•˜๋Š” ์ง€๋ฐ˜์˜ $k(x)$ ํ”„๋กœํŒŒ์ผ์ด ์„ฑ๊ณต์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑ๋˜์—ˆ์œผ๋ฉฐ ํŠนํžˆ ์„ ํ˜• ๋ณ€ํ™” ๊ตฌ๊ฐ„์ด ํšจ๊ณผ์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑ๋˜์—ˆ์Œ์„ ์•Œ ์ˆ˜ ์žˆ๋‹ค. ๋ฐ˜๋ณต๊ณ„์‚ฐ ๊ณผ์ •์—์„œ ๋ถˆํ•ฉ์น˜๋Ÿ‰ $F_{m}$์€ ๋ชจ๋“  $\varepsilon_{k}$์˜ ๊ฒฝ์šฐ์— ๋Œ€ํ•ด ์ดˆ๊ธฐ๊ฐ’์˜ 0.01 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค.

Fig. 13(a)๋Š” ์ •๊ทœํ™”๊ณ„์ˆ˜ $R_{k}$์˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{k}$ ๊ฐ’์— ๋”ฐ๋ฅธ $F_{m}$์˜ ๋ณ€ํ™”๋ฅผ ๋‚˜ํƒ€๋‚ด๋ฉฐ, $F_{m}$์€ $\varepsilon_{k}$์˜ ๊ฐ’์— ๊ด€๊ณ„ ์—†์ด ์ถฉ๋ถ„ํžˆ ๊ฐ์†Œํ•˜์˜€๋‹ค. Fig. 13(b)๋Š” Eq. (33)์œผ๋กœ ๊ณ„์‚ฐ๋œ $k(x)$ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์˜ $L^{2}$ ์˜ค์ฐจ๋ฅผ ๋‚˜ํƒ€๋‚ธ๋‹ค. 1000ํšŒ ๋ฐ˜๋ณต๊ณ„์‚ฐ ํ›„ $\varepsilon_{k}$ = 0.2, 0.4, 0.6, 0.8์˜ ๊ฐ ๊ฒฝ์šฐ์— ๋Œ€ํ•ด ${\vert \vert}e{\vert \vert}_{2}$๊ฐ€ ์ดˆ๊ธฐ๊ฐ’์˜ ์•ฝ 0.06 %, 0.02 %, 0.05 %, 0.03 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค. $k(x)$ ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ์˜ $L^{2}$๋†ˆ ์˜ค์ฐจ๋Š” $\varepsilon_{k}=0.4$์ผ ๋•Œ ๊ฐ€์žฅ ์ž‘์•˜๋Š”๋ฐ, ์ด๋กœ๋ถ€ํ„ฐ ์ด ๋ฌธ์ œ์—์„œ ์ง€๋ฐ˜๊ฐ•์„ฑ ์žฌ๊ตฌ์„ฑ์„ ์œ„ํ•œ ์ •๊ทœํ™”๊ณ„์ˆ˜์˜ ๊ฐ€์ค‘์น˜๋Š” $\varepsilon_{k}=0.4$๊ฐ€ ์ ํ•ฉํ•จ์„ ์•Œ ์ˆ˜ ์žˆ๋‹ค.

Fig. 9. Inversion Results for the Ramp Profile of $k(x)$ Using the Regularization Weight Factor $\varepsilon_{k}$ = 0.2. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig9.png

Fig. 10. Inversion Results for the Ramp Profile of $k(x)$ Using the Regularization Weight Factor $\varepsilon_{k}$ = 0.4. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig10.png

Fig. 11. Inversion Results for the Ramp Profile of $k(x)$ Using the Regularization Weight Factor $\varepsilon_{k}$ = 0.6. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig11.png

Fig. 12. Inversion Results for the Ramp Profile of $k(x)$ Using the Regularization Weight Factor $\varepsilon_{k}$ = 0.8. (a) In\verted Profile, (b) Response Misfit, $F_{m}$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig12.png

Fig. 13. Response Misfit during the Inversion for $k(x)$ and the Normalized $L^{2}$ Error of the Reconstructed $k(x)$ Profile. (a) $F_{m}$ Up to 370 Iterations, (b) ${\vert \vert}e{\vert \vert}_{2}$ for $k(x)$

../../Resources/KSCE/Ksce.2025.45.3.0313/fig13.png

5. ๊ฒฐ ๋ก 

์ด ๋…ผ๋ฌธ์€ ์ง€์ค‘ ํ”„๋ฆฌ์ŠคํŠธ๋ ˆ์ŠคํŠธ ๋ผ์ดํ”„๋ผ์ธ ๊ตฌ์กฐ๋ฌผ์˜ ์žฌ๋ฃŒ ๋ฌผ์„ฑ์„ ์žฌ๊ตฌ์„ฑํ•˜๊ธฐ ์œ„ํ•ด ํœจํŒŒ๋™์˜ ๊ณ„์ธก ๋ฐ์ดํ„ฐ๋ฅผ ์ตœ์ ํ™”ํ•˜๋Š” ํƒ„์„ฑํŒŒ ์ „์ฒดํŒŒํ˜•์—ญํ•ด์„ ๊ธฐ๋ฒ•์„ ์ œ์•ˆํ•˜์˜€๋‹ค. Winkler ๊ธฐ์ดˆ๋ฅผ ์‚ฌ์šฉํ•˜์—ฌ ๋ณด์™€ ์ง€๋ฐ˜์˜ ์ƒํ˜ธ์ž‘์šฉ์„ ๋ชจ๋ธ๋งํ•˜์˜€์œผ๋ฉฐ, ๊ธด์žฅ๋ ฅ์ด ํฌํ•จ๋œ ์˜ค์ผ๋Ÿฌ-๋ฒ ๋ฅด๋ˆ„์ด ๋ณด ์ด๋ก ์„ ์ ์šฉํ•˜์—ฌ ํœจํŒŒ๋™ ์ „ํŒŒ๋ฅผ ๋ชจ๋ธ๋งํ•˜์˜€๋‹ค. ์ด ๋ฌธ์ œ์˜ FWI๋Š” ํœจํŒŒ๋™์˜ ์ง€๋ฐฐ๋ฐฉ์ •์‹์„ ๊ตฌ์†์กฐ๊ฑด์œผ๋กœ ํ•˜์—ฌ ์‘๋‹ต ๊ฐ„ ์˜ค์ฐจ์ธ ๋ชฉ์ ํ•จ์ˆ˜๋ฅผ ์ตœ์†Œํ™”ํ•˜๋Š” ํŽธ๋ฏธ๋ถ„๋ฐฉ์ •์‹ ๊ตฌ์† ์ตœ์ ํ™” ๋ฌธ์ œ๋กœ ์ •์‹ํ™”ํ•˜์˜€๋‹ค. ์ œ์–ด๋ณ€์ˆ˜์˜ ์ถ•์†Œ ๊ณต๊ฐ„์—์„œ ๋ผ๊ทธ๋ž‘์ง€์•ˆ์˜ KKT ์กฐ๊ฑด์„ ๋ฐ˜๋ณต์ ์œผ๋กœ ๊ณ„์‚ฐํ•˜์—ฌ ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜์™€ Winkler ์ง€๋ฐ˜๊ฐ•์„ฑ์„ ํšจ๊ณผ์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑํ•˜์˜€๋‹ค.

์ˆ˜์น˜์˜ˆ์ œ๋ฅผ ํ†ตํ•ด ์ •๊ทœํ™”๊ณ„์ˆ˜์˜ ๊ฐ€์ค‘์น˜๊ฐ€ ์—ญํ•ด์„ ๊ฒฐ๊ณผ์— ๋ฏธ์น˜๋Š” ์˜ํ–ฅ์„ ๋ถ„์„ํ•˜์˜€์œผ๋ฉฐ, ๋ชจ๋“  ๊ฐ€์ค‘์น˜ ์กฐ๊ฑด์—์„œ ๋ชฉํ‘œ ํ”„๋กœํŒŒ์ผ์ด ์„ฑ๊ณต์ ์œผ๋กœ ์žฌ๊ตฌ์„ฑ๋˜์—ˆ๋‹ค. ๋ณด์˜ ํƒ„์„ฑ๊ณ„์ˆ˜๋ฅผ ์žฌ๊ตฌ์„ฑํ•˜๋Š” ๊ฒฝ์šฐ์— ์ธก์ •์‘๋‹ต๊ณผ ๊ณ„์‚ฐ์‘๋‹ต ๊ฐ„์˜ ์˜ค์ฐจ์ธ $F_{m}$์€ ๋ชจ๋“  ์ •๊ทœํ™”๊ณ„์ˆ˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{E}$ ๊ฐ’์— ๋Œ€ํ•ด ์ดˆ๊ธฐ๊ฐ’ ๋Œ€๋น„ 0.07 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€์œผ๋ฉฐ, ํ”„๋กœํŒŒ์ผ์˜ ์žฌ๊ตฌ์„ฑ ์˜ค์ฐจ๋ฅผ ๋‚˜ํƒ€๋‚ด๋Š” ${\vert \vert}e{\vert \vert}_{2}$๋Š” ์ดˆ๊ธฐ๊ฐ’ ๋Œ€๋น„ 0.13 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค. Winkler ์ง€๋ฐ˜๊ฐ•์„ฑ ์žฌ๊ตฌ์„ฑ์˜ ๊ฒฝ์šฐ์— $F_{m}$์€ ๋ชจ๋“  ์ •๊ทœํ™”๊ณ„์ˆ˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{k}$์— ๋Œ€ํ•ด ์ดˆ๊ธฐ๊ฐ’ ๋Œ€๋น„ 0.1 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€์œผ๋ฉฐ, ${\vert \vert}e{\vert \vert}_{2}$๋Š” ์ดˆ๊ธฐ๊ฐ’ ๋Œ€๋น„ 0.06 % ์ˆ˜์ค€์œผ๋กœ ๊ฐ์†Œํ•˜์˜€๋‹ค. ํŠนํžˆ ์ •๊ทœํ™”๊ณ„์ˆ˜์˜ ๊ฐ€์ค‘์น˜ $\varepsilon_{k}$๊ฐ€ 0.4์ผ ๋•Œ ์žฌ๊ตฌ์„ฑ ์˜ค์ฐจ๊ฐ€ ๊ฐ€์žฅ ์ž‘์•˜๋‹ค. ์ด๋กœ๋ถ€ํ„ฐ ์ •๊ทœํ™”๊ณ„์ˆ˜๊ฐ€ ์—ญํ•ด์„์˜ ์ˆ˜๋ ด์„ฑ๊ณผ ์ •ํ™•๋„์— ์˜ํ–ฅ์„ ๋ฏธ์น˜๋ฉฐ ์ ์ ˆํ•œ ์ •๊ทœํ™”๊ณ„์ˆ˜ ๊ฐ’์„ ์„ ํƒํ•˜๋Š” ๊ฒƒ์ด ์—ญํ•ด์„ ๊ฒฐ๊ณผ๋ฅผ ๊ฐœ์„ ํ•˜๋Š”๋ฐ ์ค‘์š”ํ•จ์„ ์•Œ ์ˆ˜ ์žˆ๋‹ค.

Acknowledgements

This work was supported by Korea Hydro & Nuclear Power Co., Ltd. (No. 2024-Tech-05) and by the 2024 Hongik University Research Fund.

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