Not a bridge guy so I may have misunderstood a little but how about something like:

"....my experience covers a variety of bridge projects ranging from the design of pre-tensioned integral and steel composite box girder bridges to the assessment of a post-tensioned box girder bridge."

Also, I'm not 100% clear on what you are trying to present but something like the folowing might assist:

"...... my experience covers a variety of bridge projects including the design of three pre-tensioned integral and steel composite box girder designs and the assessment of 1 post-tensioned box girder design."  

Assuming that this is a cover letter for a job application, focus on your skills, not the bridges.

"I have designed (or been a member of the design team) of several bridges.  I also assessed the condition (or design?) of a bridge to achieve some specific goal."

Leave the details of what types of bridges for the resume.

 

==> use of article
There are two articles, 'a' and 'the'.  Use 'a' when speaking in generalities and 'the' when being specific.

If you did a design of a pre-tensioned integral and steel composite box girder bridge, you would be saying that you did one design, not necessarily the only only design, and not necessarily the design chosen, of that type of bridge.

If you did the design of a pre-tensioned integral and steel composite box girder bridge, you would be saying that you did the actual design chosen for that type of bridge.

If you did a design of the pre-tensioned integral and steel composite box girder bridge, you would be saying that you did one design, not necessarily the only only design, and not necessarily the chosen design, of a very specific bridge of that type.  It would be assumed that the reader knows exactly which bridge you're talking about.

If you did the design of the pre-tensioned integral and steel composite box girder bridge, you would be saying that you did the actual design chosen for a very specific bridge of that type.  Again, it would be assumed that the reader knows exactly which bridge you're talking about.

I would suspect that you want to say that you did the design of a bridge.

That being said, I agree with MintJulep.  Let the cover letter speak to your skills and your resume or CV to specifics.  Another option to consider might be this:

... my experience has been concentrated in a variety of bridge projects which include both bridge design and bridge assessment.
 

Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein

Thank you guys for in-depth explanation on the use of article.

I understand a cover letter focuses on general skills rather than what I did on a particular project. However, when I drafted the cover, the issues of "article" come to my mind and I want to take this opportunity to make it clear.....
 

The only issue is one that we tend to forget...if the reader's command of the English language is a great as that of the writer.

For example some readers might believe the plural of fish is fishes and would mentally give you a black mark as a result.

So in my experience, it's better to keep it as simple, yet professional as possible and allow the resume to be read by those in the know and the cover letter to be read by those who simply read cover letters.

drawn to design, designed to draw

CC...there are actually three articles.  "An" is the other, albeit another form of "a".  I believe the articles are somewhat dictated, as you noted, by the singular or plural use of the noun.  In the noted example, I would consider Kenat's re-write to be a good presentation of the intended generalization.

==>CC...there are actually three articles.
I consider a and an the same article since a is used before words that begin with consonant sounds and an before those with vowel sounds.

==> I believe the articles are somewhat dictated, as you noted, by the singular or plural use of the noun.
That's not quite what I said.  It's true that a(n) is only used with the singular since it's generic in nature, i.e., not referring to specific noun or nouns.  However, 'the' may be used with both singular and plural nouns.  'The' is used when referring to a specific item or items.
The bridge is out.
The bridges are out.
 

Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein

I think your explanation was exceptionally clear, Cajun. The subject of the use of articles seems difficult for many non-native English speakers. It seems to me that the rules on the use of articles, in many other languages, are the opposite of those in English.  

Quote:

....my experience has been concentrated in a variety of bridge projects which ranged

Just to point out you either misused the word, "which," or you left out a comma prior to the word, "which." The word, "which" sets off an independent clause that can be excluded from the sentence and still preserve meaning.  As a result, it needs to be used in conjunction with a comma.

Carry on. . .

f-d

¡papá gordo ain't no madre flaca!