Agreggate Expression / Summary Rows

I have the next problem:

I have a radgridview with data, but i need sum rows of the same column and i use the next code: (vb.net)

 Dim summaryItem As New GridViewSummaryItem()
        summaryItem.Name = "column6"
        summaryItem.AggregateExpression = "(Sum(column6))"
        Dim summaryRowItem As New GridViewSummaryRowItem()
        summaryRowItem.Add(summaryItem)
        Me.RadGridView1.SummaryRowsTop.Add(summaryRowItem)

but i dont know in what event is correctly put it.

I put it in a function but the SummaryRowsTop is repeated as many times as entry records. 

i need only one SummaryRowsTop