请问下面的英语长句是什么从句，by which 在句中的作用是什么？困扰好久了，请各位大神指点下，不胜感激

去掉修饰部分，剩余主干看一下

Formulations are presented by which a secant stiffness-based finite element algorithm can be modified to provide analysis capability for arbitrary loading conditions, including reversed cyclic loads.

Formulations are presented by which a algorithm can be modified to provide capability。

从句部分恢复词序：

a algorithm can be modified by Formulations to provide capability

所以这里by which引导了定语从句，修饰 Formulations。

因为从句太长，因此没有紧跟在被修饰词的后面。

正常应该是这样：

(Someone presents Formulations, by which a ......)

本回答被提问者采纳

Formulations主 are presented （by which a secant stiffness-based finite element algorithm can be modified to provide analysis capability for arbitrary loading conditions定语从句）, including reversed cyclic loads状语.
by which a secant stiffness-based finite element algorithm主语 can be modified谓语 to provide analysis capability for arbitrary loading conditions不定式作状语
by which引导定语从句，修饰Formulations
结构：
Formulations are presented（主谓宾的被动语态）

一个学分体系可提供一种直接有效的标准，通过它（学分体系）来衡量你们的孩子在学校是如何进步的。
which引导的定语从句，且是【介词+引导词】的结构。which是关系代词，代表学分体系