You can use Dynamic Controls for this ::
protected void btnTraningAdd_Click(object sender, EventArgs e)
{
HtmlTable ht = new HtmlTable();
HtmlTableRow tr = new HtmlTableRow();
HtmlTableCell tc = new HtmlTableCell();
tc.InnerText = "hello";
ht.Rows.Add(tr);
tr.Cells.Add(tc);
PlaceHolder1.Controls.Add(ht);
}