Suggestions:
1. There are various steel structures with various ground resistances
2. Check industry standards, e.g.
http://www.ieee.org
Green Book, IEEE Std 142-1991
for standard lightning protections

You cannot use the steel structure as a lightning protection system. Strikes on steel structures have been known to go to ground through the steel doing no damage or blowing up and burning all kinds of things inside. There is now to predict what will happen.
Lightning protection systems do not protect buildings by conducting the stroke to ground.  There is no way a cable used in lightning can take 35 or 40 thousand amps to ground with out vaporizing.  
A well designed  lightining protection system discharges the static electricity in the structure and the earth around it into the air and lessen the potential for a strike.  If the charge in the building is blead off into the air there is no potential difference between the cloud and the ground - so no strike.
The small radii on the tips of lightning rods are a direct application of basic physics to allow the charge to bleed off into the air.  
If you want to see a system work go to the top  of a structure with a well designed lightning protection system during a thunder storm.  Laydown and watch the "points".  You can see a healthy flow of blue sparks radiating from the tips into the air. ( it's quite thrilling ) If they don't bleed the charge off fast fast enough the building may get a strike.  

Seriously don't go up there.  Watch it with binoculars or something.  If your structure is in a high Isokeraunic area get someone who knows how to design a system.  It dosen't matter if the building is steel framed, wood or mud and wattle with a thached roof. If it's valuable you have to protect it.

I trust this post helps Nandkumar.
Your consultant's advice is okay and is borne out by the codes as well. A metallic structure does not need any additional lightning conducting devices provided the structure is electrically continuous and is effectively grounded. What this means is that the joints are either welded or there are metallic straps across bolted joints to make the structure electrically continuous AND the base is grounded through an earth electrode. You can weld a lightning rod on the top but you donot need to connect it through another vertical conductor to the earth electrode. The structure acts as the vertical conductor.

rajeev krishan
 

Suggestions:
To the previous posting:
1. It appears to be presented the way it should be.
Generally,
2. It might be a good idea to go over the design and what the designer (perhaps the consultant) and the previous posting claim.
3. Grounding is much too much safety related to leave it based on a vague and non-engineering approach. This forum can chip in a little in this direction.

You can use the steel structure as the down conductor for the lightning protection system but you still need air terminals on top of the building to properly protect the structure.  As stated in a reply above the air terminals help to bleed off charge and reduce the potential and therefore the magnitude of a strike.  If the building recieved a full magnitude strike without a lightning protection system quite a bit of structural damage could be done despite the fact that it was a steel structure.  In a system that uses the steel structure as a downlead air terminals are connected together by cables and at a certain number of points on this cable/air terminal array it is bonded to the structure with cable.  In addition, ground rods are place around the perimeter of the building and also bonded to the structure.

In ANSI regions, NFPA Standard 780 may apply.

   www.nfpa.org/Catalog/TOC/780-00-toc.html
  

"BS 6651:1999 Code of practice for protection of structures against lightning" permits the use of the sructural steel

There's a key issue that has not been addressed here.  Is your roof metallic, heavy gauge, and electrically continuous with the structure?  If you have a membrane roof material, other non-conductive roof, or flimsy metal roof, expect lightning damage unless you install air terminals.  If you have a heavy-gauge metal roof continuous with the steel structure, you should be fine.

The steel columns can be used as down conductors in either case.

You should provide ground rods at grade around the perimeter of the building, bonded to your down conductors, to ensure that the lightning does not dissipate itself through the concrete footings.

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The rest of this post applies only to US/NFPA installations:

Get a copy of NFPA 780, it clearly describes all of this.

If you do opt to install a lightning protection system, your consultant's recommendations really have very little to do with it.  What concerns you is the UL Master Label that should be provided by your LP system installer.  Your consultant has nothing to do with the Master Label.  The only real point to having a consultant on an LP system is for coordination with the architectural systems.  Any other drawings/specs/advice from your consultant is a worthless waste of money as they will be overridden by the installer, he won't provide the Master Label unless HE is happy with the installation.

As someone who has spent a lot of time reading BS6651, I'm with rajivkrishen and seanmx on this one.