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True, there can be certain cases where UBC gives a higher base shear. However, without knowing basis of study, parameters involved and data points/ no of runs and sensitivity of each parameter quoted results, it hard to conclude. Keeping numbers apart, there are other differences too. IBC considers seismic design category which affects your calculated base shear whereas UBC considers your seismic zone only. While designing a hospital in a lower seismic category, you may end with a higher base shear than for a commercial building with a higher design category, both comparisons being done for IBC. As I said, it is not an apples to apples comparison, and your results can be different based on what parameters you consider. To me, the whole exercise is pointless, because basis are different. Number crunching.

The response of Umar Makhzuni is comprehensive, clear and correct. In addition, following should be noted. It is advisable that all designs in Pakistan adhere to IBC rather than UBC-97, which is now considered defunct. Use of UBC-97 with latest resistance codes for structural concrete (like ACI 318-05, 318-08, 318-11) and AISC codes with publication dates after 1997 is technically incorrect and must be avoided. I hope that structural design community should adapt to the IBC and the appropriate use of resistance codes.

All I can understand from your post is that you are doing a comparison between two codes for Saudi and trying to find out quantitative difference of base shear? Also, I believe you want someone to check your numbers? Before you do that you need to understand the differences between UBC and IBC Seismic Design Philosophy; you aren't comparing apples to apples if you just compare numbers. There is a lot different between two codes. Both codes design to a different Maximum Considered Earthquake (MCE). Considering everything is constant, IBC will give you a higher base shear. Read the attached article:IBC and UBC Comparison.PDF Use the method provided in code to identify your seismic response coefficients. Ss=Ground acceleration at short (0.2 second) period, which will allow you to determine Sms and subsequently Sds. Code outlines the procedure to calculate all these components. I don't know why are you calculating Ss like you did. Documents attached are result output, and calculation is not attached. Without a calculation, no one knows how you came up with the numbers. My best advice to you, to check your numbers is to compare them with a solved example. You can't go wrong.