Weight of OWJ Structures inSupreme Command building

CONTENTS

1.Preface2.Load and Design3.Design Approval4.Quantities5. Inputs & Lesson Learnt6.Conclusion and Reasons for More Weight

1. Preface

a. There was no information about the OWJ Structures in our Design Drawings and Section 16 of QCS which is about Structural Steelwork was not included as a part of Design Documents. So it was obvious that the Structure has to be done on a Design and Built Basis. We had studied the Architectural and MEP Drawings of the Areas served by OWJ Structure to f inalize the Depth of Structure.

2. Loads and Design

a. In our case we had a Span of 20m. For the Ground floor OWJ The total load (DL (of Concrete) + LL) with the safety factors as per design drawings comes to 6.2 Tons / M (other than self-weight of truss) for the main girder which is a very high load. This load was very high for it to be designed as OWJ as per recommendations from Steel Joist Institute (SJI). The term OWJ is used. (All the special ist whom were invited for a Quote were giving quotations as per SJI code of Practice)

b. We started exploring other options. The most obvious choice was that with a Post tensioned Beam with Hollow core Slabs . Design was worked out and a safe option was arrived to. We were able to get an economic layout. But when the load imposed on the Supporting Columns was calculated it was too high and we were not able to assure the sufficiency of those columns. So that option was also ruled out.

c. So we had no option but to do it as a truss. Girder Structure was modeled as a Truss. The depth of a truss, or the height between the upper and lower chords, is what makes it an efficient structural form. So in order to Increase the depth of Girder we planned Truss in such a way that that Duct wil l pass thru the Truss and we wil l get the depth of Duct also to the girder.

3. Design approval

a. Our design drawing requires office area to be designed for 3 KN/SqM load and corridors to 5 KN/SqM load apart from other loads. We proposed GF OWJ design with 3 KN/SqM considering it as office area. That proposal went up to Head of Structural division of AEB and they rejected and we started debating. Later we negotiated for 4 KN/SqM load and they agreed . Remember that each KN Load (Non Factored) on one SqM area wil l add 0.5 T/m on each girder.

4. Quantities

a. The quantity of steel to be used was initial ly found out and too was much away from our budget. However the quantity of steel required as proposed by various special ist contractors l ike Zameel, was also reviewed and their quantity was also similar for the depth on acceptable range with load of 3 KN/SqM load apart from other loads. But their proposal was to increase the no’s of main girders to double so that load on each girder can be minimized so as to meet the recommendation from SJI as specified in Section 2-a. That proposal was never meeting the architectural requirements and not in l ine with the provisions allocated for the services.

5. Inputs & the Lesson Learnt

a. In a case where we need to Consider a Structural steel work for a Deck slab we have to consider various factors l ike Span, DL and LL to be considered, Depth Possible Other Imposed Weights etc. In a similar case we should al locate at least 100 kg for each SqM Of area to be on a conservative side as the table denotes

AreaWeight of

SteelTons

AreaSqM

Load consideredKN/ SqM

Steel Used Kg/SqM Remarks

Ground Floor

47 580 10.25 81

Roof Floor 53 790 10.25 67Additional 45 Tons

due to Sky light structure

6. Conclusion and Reasons for More Weight

a. The weight of structure was increased due to following reasons

i . I f the initial proposal with 3Kn/SqM Load would have been approved, we could have completed the Ground floor Structure with just a steel usage of 42 Tons and requirement from AEB for doing the design with 4 KN/SqM we were forced to Use 48 Tons

ii . In Ground floor we had too many restrictions for the Depth for Truss. If we could have increased the depth of Truss to 2m, things would have been totally different. But we were forced to restrict the depth to 1.4m

i i i . I had too much meeting with AEB for convincing them to lower load. I even discussed with provisions from BS 6399 Part 1: Loading for buildings - Code of practice for dead and imposed loads . But they were not at al l ready for this as this slab is the roof for the Operation room where too many developments are planned worth mil l ions

iv. The usage of Steel in roof is more due to additional span of more than 90 cm. This is because the Girders are resting on Columns directly and in the Ground Floor Level it was on a corbel.

v. The spacing of Main girders is also very high in Roof when compared to GF Structure. This wil l create more load in the Main Girder

vi. There was additional load (Factored) of 45 tons due to Sky l ight which is in the roof f loor which is very high and I am sure that was never even thought of.

vi i . Severe damage for the steel consumption was avoided in the roof by increasing the depth of Girder at roof since it was possible there